of building stones generally, with special reference to the causes that lead to decay, and the means of preventing it. The principal bases of stones are silica, alumina, and lime. As can readily be inferred from the most superficial knowledge of these earths, the hardest and most durable stones are those in which the former predominates, many of them, such as granite and basalt, being indestructible. The building stones most subject to decay are sand and limestones. In the former, it is caused chiefly by the mechanical action of winds, rains, and frosts; and in the latter, by these and chemical agency combined. Sandstone is composed almost entirely of silica or quartz grains, or dust cemented together by lime, alumina, magnesia, or iron, and sometimes by a combination of two or more of these minerals. As the particles of quartz are, like the stones already mentioned, practically indestructible, the durability of sandstone depends entirely on the cementing material. When this is nothing but alumina or clayey matter, the stone is of an inferior quality, that base being deficient in adhesive properties, and generally soluble in water. The stone is therefore peculiarly susceptible to the action of the weather. The presence of an undue preponderance of clayey matter in sandstone may be detected by washing small pieces in water. If a large muddy residium is given, the specimen should be rejected as perishable. Craig Leith sandstone, the best in Great Britain, contains—

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Caversham stone, on the other hand, contains—

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The reddish sandstones generally contain iron in considerable quantities; when the iron is naturally in a low state of oxidation, the stone has a tendency to decay on exposure. Change from wet to dry seems to prevent rather than assist the cementing process. But when the iron is highly oxydized, and the whole a perfectly homogeneous and compact mass, the

stone is not affected by the changes of the weather, and may, therefore, be taken as durable.

Sandstone was deposited under water and hardened by pressure and drying, consequently it has a distinct natural bed. The stone is often of such a uniform colour and consistency that the lines of stratification are quite invisible, and as the stratum may not have retained its originally horizontal position, the mere inspection of a specimen in a museum or of a block in a quarry will not give the bed of the stone. It is, however, easily determined by the quarrymen, from the facility of working in a certain direction as compared with others.

As a general rule, sandstones are hardest and most compact when found at the lower side of a thick stratum, or in the vicinity of basaltic dykes, or other volcanic rocks that may have disturbed them. The facilities for drainage afforded by the lie of the adjoining land has also considerable influence on the consistency of the softer sedimentary rocks.

In building with stones from stratified rocks, it is absolutely necessary that they be laid on their natural bed. A disregard for this rule is the sole cause of decay in a large majority of cases where buildings have failed. When the stones are placed in an inclined position, they afford the greatest facility for absorbing moisture, and when vertical, the superincumbent weight has a tendency to split them. The latter evil is often greatly aggravated by a practice that exists among masons of working the beds slightly hollow, so as to ensure a neat joint.

The appearance of some of our soft stone buildings fully bears out the above remarks, as to the necessity of laying stones on their natural bed; some of them are smooth and solid after many years exposure, while others from the same quarry, and under exactly the same conditions, are in an advanced state of disintegration. This state of affairs could be prevented by simply marking the stones in the quarries where the lines of stratification are easily determined, and generally well known. Independent of the increased durability, it is advisable to lay all stones on their natural bed, for they are a fourth stronger in that position than in any other.

Calcareous stones are less subject to decay from the mechanical action of the weather than sandstone, but more susceptible to chemical agencies. As the cementing material is always the same, the durability depends entirely upon the aggregates, and the proportions in which they are mixed. The compact and crystalline limestones are believed to be unstratified, consequently they are not liable to exfoliation, and may be used in any position; but some of the softer kinds give indications of having been deposited in horizontal layers, in which case it is necessary to build with the stone on its natural bed. Although limestone is generally more

compact than sandstone, it absorbs more water; but, on the other hand, the water affects it much less than sandstone. The compactness of limestone seems to keep the water from freezing, and so neutralizes its most powerful disintegrating property. All the softer limestones are hardened by exposure to the atmosphere; at the same time the atmosphere contains the elements of their destruction. The indurating process is not, as is sometimes supposed, attributable to the absorption of carbonic acid from the atmosphere, like the setting of mortar. The lime in the stone, being already a carbonate, cannot in this way absorb more of the acid. The hardening on exposure is caused entirely by the evaporation or drainage of the moisture contained in the pores of the stone.

The ingredients in the atmosphere that have the most deleterious effect on stones are muriatic and sulphuric acid, both of which have an affinity for lime, and combine readily with it, thus rendering the stone soluble in water. The former acid is always present in the atmosphere near the sea, and the latter in manufacturing towns, where coal is burnt. All the softer limestones are more or less subject to the pernicious effects of both these acids, and when magnesia enters into their composition, they are particularly susceptible to the action of sulphuric acid. The English Houses of Parliament are built of magnesian limestone, from the Bolsover quarries in Derbyshire—its composition being as follows:—

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It is well known that this stone has been a decided failure; the buildings were not many years finished when they began to show symptoms of decay. This result is due entirely to the sulphuric acid with which the smoky atmosphere in London is impregnated. The selection of the Bolsover stone for such an important work is perhaps the most curious instance on record of the miscarriage of skill, experience, and good intention. The English Government, fully alive to the necessity of having the Houses of Parliament built of the best stone procurable, appointed a Scientific Commission for the purpose of enquiring into the qualities of the various building stones in Great Britain. The Commissioners were men of the highest standing, whether as regards their disinterestedness or scientific attainments; they had carte blanche to examine, enquire into, and experiment on every stone in the kingdom, in short their instructions appear to have

simply been “select the best.” After a long, laborious, and expensive investigation, and with the best possible intentions, the Commissioners selected “the magnesian limestone, or dolomite of Bolsover,” one of the most worthless stones for the purpose in Great Britain. The sole reason for this untoward conclusion is in the fact that, at that time, the peculiar affinity of magnesian lime for sulphur was unknown, and the Commissioners had the strongest possible proof of the durability of the stone in Southwell Minster, where it had withstood the action of the weather for 800 years. This was, however, in the pure air of a small country town—a condition that differs materially from that which the material occupied when exposed to the smoky and acidulous atmosphere of the metropolis.

Tests.—Except in rare cases, such as the arches of a long-spanned bridge, and the lower courses in a spire or chimney, the pressure on stones in a building never approaches their crushing weight; their cohesive properties may, therefore be disregarded in a popular investigation like the present one. I shall, however, consider shortly the proofs or tests of durability that should be observed in building with freestones.

Generally speaking the hardest, heaviest, and least absorbent stones in a class such as sand and limestone are the best; but this is no criterion when comparing classes. In sandstones the chemical test is the maximum amount of silica, and minimum of alumina; the proportions of the other ingredients being within certain limits apparently of no consequence. The best limestones are those that approach nearest the crystalline state; uniformity of tint and homogeneity of structure are also favourable indications. So far as strength and beauty, as well as durability under ordinary circumstances are concerned, the magnesian limestones are best when the lime and magnesia are in equal proportion. This, however, as already shown, seems the worst proportion for a smoky town.

The absorbent properties of stones can be tested by subjecting them to the action of water under a slight pressure. With 14 lbs. on the square inch English Sandstones absorb from one-seventh to one-fourth of their entire bulk; Limestones, one-ninth to one-fifth; Oolites and Dolomites one-fifth to one-fourth.

The resistance of stone to disintegration can be tested by what is called Brard's process; this consists in boiling specimens in a solution of sulphate of soda (Glauber's salts) and afterwards dipping them at intervals into the cold solution for a few days. The action of this salt closely resembles that of frost, and M. Vieat has calculated that the effect, after two days' application, is equal to the force exerted by frost at 21° Far. on wet stone. The hardest granite is segregated by Brard's process in thirty days.

Syenite, as you are aware, differs from true granite only in so far as it contains hornblende instead of mica. As mica and felspar are considered the perishable ingredients in these rocks, the durability of syenite can never be questioned; it is also on the whole tougher and more compact than ordinary granite. This stone is found in various localities on the West Coast and in Stewart Island, but the chief supply now available for industrial purposes is at the Bluff. Practically, the whole of the Bluff Hill consists of this material; it could, therefore, scarcely be in a more accessible situation. The Bluff syenite is hard and compact, and of a uniformly bluish-grey tint of great beauty, consequently it is suitable for kerbing, paving, and massive masonry, as well as monumental and architectural works, In my opinion, this stone is little, if anything, inferior to the famous Aberdeen granite, and I have no doubt the quarrying and dressing of it will ere long become an important industry. There is a curious variety of syenite found at Milford Sound, the body colour of which is a pure opaque white interspersed with oblong rectangular blotches of dark grey and black; these blotches are occasionally an inch long by three-eights of an inch in breadth.

Another vein of syenitic granite exists at Isthmus Sound; the grain is rather coarse, but the colour, which is of a uniformly grey tint, is good.

Pegmatite, or compact granite, is found at Milford Sound and Paterson Inlet. The former is of a grey tinge, with large spots of silvery white mica of great brilliancy. This is, perhaps, the most beautiful stone in Otago; but it is doubtful if its appearance would be permanent out of doors. The stone at Paterson Inlet has a pinkish ground, with grey spots, and is much coarser in the grain. When the utilitarian appetite of the colonist has been satisfied, and he has means and leisure to bestow on the ornamental, the beauties of the West Coast granites will be highly appreciated.

Although the stones above described vary much in appearance, there is little difference in their composition, and they are all embraced in the generic name of granite. All granite rocks are composed of felspar, quartz, mica, and hornblende, and the variety is due entirely to the number of the ingredients that it contains and the proportions in which they are mixed. An undue preponderance of mica and felspar in granite—particularly when the latter is alkaline—is supposed to render the stone liable to loss of colour and to decay; but, with that exception, granite of all kinds is practically imperishable.

I have compared Mr. Skey's analysis of the Otago granites with that of the Irish varieties given in “Juke's and Geikie's Geology,” and find that,

Bluestone, which is so largely used for road metal, and ordinary rubble masonry, is to be found in almost all districts that have been disturbed by volcanic agencies. Sometimes it exists only in combs and small columns fit for nothing but road metal and pitching, but at other times it occurs in large dykes that yield valuable building stone. The best quarries in the Province are those in the Dunedin Town Belt, the valley of the Leith, and Ross Creek. The most of the bluestone used in Dunedin comes from those quarries. It forms excellent rubble, with a little labour-picked ashlar, but it is altogether too hard for chiselled work. The basements of nine-tenths of the buildings in Dunedin are built of bluestone rubble, and many important edifices, such as St. Paul's Church; the Wesleyan Chapel, New Knox Church, Mercantile Agency Store, and the residence of the two Bishops, are built of coarse hammer-dressed rubble, with facing of lighter coloured materials, the effect of which is very pleasing.

Greenstone is simply bluestone in a more tractable form, and is used for much the same purposes. There is, however, no supply near the centres of population, so its use hitherto has been comparatively limited. Greenstone is found in the Mataura Valley, on the shores of Lake Wakatipu, and at Greenhills, in Southland; its colour varies from light green to dark grey.

Dolorite is a dark grey, or brownish stone, of vesicular texture, and harder, but more brittle, and easier worked than bluestone. It is usually found near volcanic centres associated with the other basaltic rocks. It is quarried for road metal. At Waihola and Tokomairiro a small vein that yielded building stone, now exhausted, was at one time worked near the top of York-place. The base of the University Building, one of the finest pieces of massive masonry in the Province is chiefly built of dolorite from this quarry.

Phonolite, or clinkstone and porphyry, are found in Bell Hill. As they do not exist in masses, they are comparatively valueless as building material. Some of them are remarkably beautiful in colour and fine in texture, capable of being used for ornamental purposes. A polished block of phonolite in the Museum shows an arrangement and blending of various shades of grey colours that excell the best efforts of the grainer. The Gaol and some of the other old buildings of Dunedin are built of clinkstone.

Timarite, an eruptive rock found on the Peninsula, resembles closely the Bluff syenite in colour and consistency, the only difference being that the latter has a slight tinge of green, intermixed with grey, instead of blue. It seems adapted for both useful and ornamental purposes, but has hitherto been little used.

The Breccias and Trachytes, with their connecting links, come next in order, and they are the most important class of hardstones in Otago. They

exist in large quantities in the vicinity of Port Chalmers, and throughout the Peninsula, and, in most cases, the quarries are easy of access by rail or water. The Port Chalmers stone, which was the first utilized, still holds the first place in point of strength and durability, and in the facility presented for getting it in large blocks. It is, however, inferior to some of the others in colour and smoothness of grain, which are essentials in architectural work. The Port Chalmers stone is a true breccia of a bluish-grey colour, with the rock fragments of all sizes, up to six inches. It is hard and tough, but yields readily to the pick. The Port Chalmers Graving Dock—one of the finest structures in New Zealand—is built entirely of this stone; the quarry, from which it is obtained, being within 200 yards of the work. All the kerbing used in Dunedin and Port Chalmers is from the same locality.

Most of the quarries now worked yield stone of a fine texture, easily dressed, and altogether well-suited for any architectural works of a substantial character. Although the labour of rubbing this stone to a perfectly smooth surface is greater, there is not much difference between it and the hardest sandstone, when worked with the chisel and fine axe. Some good specimens of this class of work can be seen at the Mercantile Agency Store, the Union Bank, and Messrs. Sargood's new warehouse.

After that used at the Dock, the next good building stone discovered was at Sawyer's Bay; with the exception of colour, this stone is, to all intents and purposes, the same as the former. The colour is a light grey, about the same shade as Portland cement, but with a slightly orange tinge. In consequence of its better colour, and the proximity of the quarry to the railway, this stone soon became a favourite in Dunedin. It has been extensively used, both as ordinary rubble and dressed ashlar-work; the facing of St. Matthew's Church, Messrs. Ross and Glendinning's warehouse, Messrs. Cargill's store, and a large number of private buildings, are of this material. It may be interesting to note that the railway now in progress through Wales' Quarry, at Sawyer Bay, has revealed the fact that the white stone is only on the outside of the cliff. On penetrating a distance of thirty yards, the colour gradually changes to blue, as found in the other quarries about Koputai Bay. On the other hand the Deborah Bay Tunnel, so far as it has been pierced, twenty-five chains at the south and ten chains at the north end, is almost entirely through Sawyer Bay stone, the same colour, but much softer than in the quarry. It should be noted that the Sawyers Bay stone does not retain its color when exposed to the weather. Although there is no symptom of decay, the stone in some of the older buildings is already considerably defaced by large stains.

The quarries and railway cuttings show that the breccia rock extends

from Sawyer Bay to the township of Mansford, a distance of nearly two miles, and from the sea level to the top of the range at Lane Rock, a height of 500 feet. The width inland is not known; but, were it only a crust on the mountain side a quarter of a mile thick, it could produce stones sufficient to make a Liverpool of Docks in Otago Harbour, with a Glasgow in each of the other provinces. The accessibility of the Port Chalmers stone is also worthy of notice. Two railways run through it at different levels, and the Harbour, with deep water at several places, skirts the foot of the rocks.

Breccia, similar to that at Sawyers Bay, is found at Broad Bay, Castle Larnach, and several other localities in the Peninsula, with the exception of Castle Larnach, which is chiefly built of this material, the Peninsula stone has not been much utilised.

A breccia, of much the same consistency, but of a beautiful brown colour, exists on the northern slope of Puka Tapau. It seems capable of taking a fine polish, and will probably be used for monumental purposes. Another stone of the same colour, but finer in texture—possibly trachyte—is found in small quantities at Kakanui mouth.

The Trachytes proper, as a class, furnish softer and easier worked stones than the breccias; they are, therefore, more suitable for the ordinary purposes of the builder. There is a large assortment of trachytes on the Peninsula, and in the vicinity of Port Chalmers; many of the deepest cuttings on the Northern Railway, between Carey and Deborah Bay, are composed entirely of this material.

Tomahawk Valley produces a brown trachyte, with light orange spots; it is not much harder than some kinds of sandstone, and seems capable of being easily dressed; although rather dark for the whole front of a building, it might be introduced into some portions with grea effect.

The Port Chalmers trachytes are generally light in colour; one sample in the museum is a delicate fawn of uniform tint and soft even texture. Those in the railway cuttings, of which there seems to be an enormous quantity, are white, with a pale blue or greenish tinge. I am not aware that either of the latter two has been utilised; but there is no doubt this will be done on an extensive scale when better means of transit are provided. The white stone particularly should soon become popular in Dunedin, everything being so favourable to its use; it is easy of access, easily worked, and can be obtained in large blocks; it has, also, every appearance of durability.

There is a peculiar looking trachyte, or tuffa, at Harbour Cove on the Peninsula, which, so far as consistency is concerned, should be classed with the freestones. Its colour is a light brown, with white spots, and the texture is much the same as Oamaru stone, but with less grit in it. The

become one of our most popular building materials, when means of transit are provided. The deposit is in a very inaccessible situation, near Boat Harbour on the eastern side of the Peninsula, consequently the stone cannot be utilized at present.

A hard shelly white limestone has recently been discovered at Kakaunui, and used in some structures in that locality; it is of a uniform colour and consistency, nearly as hard as Sawyers Bay stone, but much easier worked; it should prove a valuable addition to our stock of building materials. A variety of this stone, from the same place, similar in colour and consistency, but full of large fossil shells, has been quarried for the foundations of the new road bridge; it is admirably adapted for work of that kind, but is altogether too rough for architectural purposes. These stones are both procurable in large blocks, and the supply is unlimited.

A coars grey limestone, of uniform colour and consistency, is found in large quantities on the Totara Station, near the Waireka Creek. With the exception of the foundations of the Waireka road bridge, it has hitherto been little used. Although more friable, the stone is about as hard as the Tasmanian sandstone; it has a beautiful warm tint of an agreeable shade, and seems capable of being dressed in any way, from hammered to polished work.

A valuable addition to the limestones has recently been worked at Waihola Gorge, in the shape of a beautiful grey stone, found on the western side of the main road, about 40 chains from the railway. The stone, when newly quarried, is harder than the Oamaru stone when dry, consequently it must be very much harder after being exposed to the air for some time. It can be dressed in any way is capable of taking a fine polish, and, being easy of access, it cannot fail to become popular as a building material, whenever the Southern Railway is open. A solid face of stone, 20 feet thick, is already exposed in the quarry, consequently the appliances for handling and transporting blocks must alone determine their size.

Both sides of Waihola Gorge contain large quantities of the limestone that is used for lime burning. This is a very hard compact stone, of a beautiful white or light cream colour, without a speck. So far as strength, appearance, and durability are concerned, it makes good building stone, but hitherto it has not been found in blocks of sufficient size. The whole rock is shattered into layers a few inches thick.

The blue and grey limestones of Pleasant Valley come next in order. Several varieties of them exist in large quantities, and they are all remarkable for beauty and uniformity of colour, fineness of texture, and the ease with which they can be dressed and carved. Unfortunately, however, they are too soft and friable for out-door work. This stone has been used in the

Bank of New Zealand, Waikouaiti, Mr. Hepburn's house, Brooklands, and other prominent buildings in that district.

I now proceed to the consideration of the most important building material that hitherto has been used in Otago, viz., the Oamaru Stone.

The use of this material is coeval with the settlement of the district in which it occurs, but it was little known beyond till 1866, when an export trade commenced with Dunedin. The first large building erected of this stone in the city was the University.

The Oamaru stone occupies that large tract of country in the northern parts of the province, extending northward from the Kakaunui to the Waitaki Plain, and outward from the coast to the Kawroo River. The same class of stone is also found from Riverton to the head of the Te Anau Lake in Southland, and at Castle Rock on the Taringtura Downs. Practically speaking, the supply of this material is inexhaustible. There are extensive quarries worked in the Oamaru district, from which a large quantity of stone is produced annually, both for local wants and export to other parts of the colony and Melbourne. The trade with the latter port is of recent birth, but it promises to be ultimately an important one. The principal quarries now at work in the Oamaru district are at Cave Valley and Kakanui. The town of Oamaru is chiefly supplied from Cave Valley, and Dunedin and other southern districts from Kakaunui. The trade to Dunedin alone is sufficient to keep one or two vessels constantly trading to Moeraki.

Much has been said as to the relative merits of the Oamaru stone from different localities, but I do not think that there is any practical difference in similar samples. The constituents of the stone are almost the same throughout the province, so any difference in colour or texture must be due to its proximity to foreign matter or facility of drainage.

The Oamaru stone, correctly speaking, is a white granular limestone. It has a remarkable uniformity of colour and texture; not only can large blocks be got of the same tint and consistency, but whole cities might be built, in which one stone could not be distinguished from another.

According to Mr. Skey, its component parts are—

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The ordinary English building stone which most resembles this is the Kelton Oolite, its analysis being—

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The weight of Oamaru stone, wet from the quarry, is 105 pounds per cubic foot, and, when perfectly dry, 92 pounds; that of the Kelton Oolite, when dry, 128 pounds. The lightest limestone in England is the Bath Oolite, which weighs 115 pounds per cubic foot. The New Zealand product is, therefore, the lightest by about 23 pounds per cubic foot.

Applying the chemical tests to the Oamaru stone, we place it on a par with the Oolites and common limestones of England and the Caen stone of France. According to Dr. Hector, the resistance it offers to the disintegrating action of Glauber Salts is comparatively feeble. Its inferiority to the above mentioned stones consists chiefly in its excessive porosity. I have made several experiments, with the view of measuring its absorbent powers, the results of which are worth recording; A block of Kakaunui stone, used as a footstool in my office since 1868, and consequently thoroughly dry and hard, furnished the best possible materials for the tests. A piece of this stone, seven inches square and one and a half inches thick, equal to 73.5 cubic inches, weighing, when dry, 56 ozs. 17 dwts. 11 grs. troy, was put in water; within 40 hours it had absorbed 12 ozs. 15 grs., equal to 31 per cent. of its entire bulk, and 21 per cent. of its weight. The specimen was allowed to remain in the water for sixteen days, when the quantity absorbed had increased to 14 ozs. 2 dwts. 19 grs., which gives 36 per cent. of the entire bulk, or 228 gallons of water in a cottage wall ten feet square and one foot thick.

A bar of Oamaru stone, 13 inches long and 1.65 inches square, was next placed vertically in a glass of coloured water; it stood 3.2 inches into the liquid. In six hours the moisture was quite visible to the top of the bar, and in twelve hours the colouring matter had risen 7 ½ inches. As the stone in both these experiments was particularly dry, the maximum results are probably obtained; but, on the other hand, the vertical position of the bar, in the second experiment, was less favourable to the absorption of moisture than that usually occupied by stones in a building, particularly the horizontal parts of mouldings, cornices, copings, and window cills. It should be pointed out that the Oamaru stone absorbs 36 per cent. of its bulk without pressure, while the most porous English stone only absorbs

25 percent., under a pressure of fourteen pounds on the square inch. It is doubtful, however, if an increase of pressure in the former case would give corresponding results, the stone being so excessively porous, gets completely saturated at once. When the dry samples were first put into water, the air rushing from the pores of the stone, caused bubbles to rise to the surface for fully ten minutes. The first experiment shews that the stone is capable of absorbing ten pounds per cubic foot more water than it contains when in the quarry, a result to me quite unexpected, and not easily explained.

One of the most important points, in connection with the use of Oamaru stone, is the degree of induration it attains in drying, and the loss sustained by subsequent exposure to moisture. So far as the hardening is concerned, I am quite satisfied that the largest blocks used in ordinary masonry become equally hard throughout in a few months, and possibly in a few weeks, under the influence of a warm dry atmosphere. The hardness is not confined, as is sometimes supposed, to a thin crust on the surface of the stone, but penetrates to the centre, making the whole a perfectly homogeneous mass. In consequence of the time required, I have not been able to prove by direct experiment that a stone once hardened becomes soft on exposure to wet. I fear, however, that such is the case; the window sills and mouldings on the south side of the University building are now fully softer than when they left the quarry, and the chances are that these stones had acquired a considerable degree of hardness before being placed in the building. The cornice and parapet on Messrs. Dalgety, Nichols, and Co.'s warehouse, although in a much more favourable situation, on the sunny side of the street, is softer still; the stone can be scratched out in handfuls by the finger nails. This is, however, one of the oldest, if not actually the oldest piece of Oamaru stone masonry in Dunedin; it is, therefore, possible the material was bad to begin with.

Against these unfavourable examples, the bridge in Thames Street, Oamaru, built in 1860, and several other buildings of the same age, in that locality, are not decayed, nor unduly charged with moisture. The ultimate durability of our Oamaru stone buildings cannot of course be determined at this early stage of their existence, and any estimate, short of actual trial, is little more than conjecture. Professor Black might, however, give us his opinion as to whether it can long resist the action of the saline breezes from the Ocean Beach, the sulphurous fumes of the Green Island coals, and the other impurities that are now so rapidly accumulating in the atmosphere of Dunedin. I should be loth to prophecy evil, but if the durability of the Oamaru stone is to be measured by its power of resisting moisture, it is to be feared that the handsome spires and facades that now ornament the city

will not transmit the names of their architects to many succeeding generations.

Although the bad qualities of the Oamaru stone are quite apparent, there is, on the other hand, so much to recommend it, that it will always be a popular building, material. I shall, therefore consider the work for which it is well adapted, and the precautions necessary to ensure the best results from its use.

The stone is well suited for any ordinary work in a dry warm climate, like Victoria, and it is unexcelled for internal decorations of all kinds and in all situations. It is suitable for ecclesiastical architecture generally, and forms a beautiful contrast as facings to darker stone.

It should not be used in the southern side of buildings, particularly if they are recessed, and it is altogether unfitted for window-sills, parapets, and the upper side of large mouldings and similar projections. Buildings of this material should be designed to have as few of these as possible, and where unavoidable, the flat tops of the stones might be covered with some preservative; from an æsthetical point of view, this is the only part of a stone building where such should on plea be permitted. Dampness can be prevented, to a certain extent, by an impervious foundation and internal lining, hollow walls, and other expedients of a similar nature. I have made several experiments with Oamaru stone, to test the efficacy of certain appliances occasionally used to prevent damp. A bar of dry stone, after receiving two coats of ordinary oil paint, was deposited in water. In 40 hours it had absorbed 34 per cent. of its bulk, including the weight of the paint, against 31 per cent. absorbed by unprotected stone in the same time. Another sample, coated with soluble glass, the principal indurating ingredient in artificial stone, absorbed 27 per cent., exclusive of the weight of the solution, which was four per cent. more.

Although these experiments give an indication of the results to be derived from the application of the materials referred to, they are altogether too crude to be advanced as conclusive. The oil in the paint was absorbed to such an extent by the stone that the colouring matter, which remained on the, surface, could be washed off by water. It is, therefore, probable that much better results would be obtained by more coats, and the use of a heavier pigment like red lead. With reference to the use of soluble glass as a remedy for damp, I am not sure that this is a property to which it lays special claim. Although porosity is a primary cause of decay, it may be possible to increase the hardness and durability of stone, without removing the lesser evil. Besides, the method of applying the solution adopted by me, may not be exactly correct.

The following is a recapitualation of the results obtained by the various experiments on Oamaru stone.

The principal buildings entirely of Oamaru stone in Dunedin are:—The University, First Church and Manse, Union and New South Wales Banks, Fernhill, and the Pier Hotel. In Oamaru, nine-tenths of the buildings are of this material, and several of them, such as the National Bank and the Star and Garter Hotel, are worthy of a place with the architecture of the old world. The private residences in that district can also be classed along with the country houses of England, notably Windsor Park, Elderslie, Moa, and Totara. The stone has also been used in numerous road and railway bridges, many of them of considerable span.

The granular limestone found in Southland resembles closely the Oamaru variety in composition and colour; it is, however, a little coarser in the grain, and, if anything, harder and more compact. Large deposits are known to exist at Aparima Castle Rock, and several adjacent points, but hitherto it has been little utilized.

Sandstones.—The sandstones of Otago are as varied in consistency and more numerous than the limestones, but excel them in diversity of colour. The extremes in the latter are generally connected by gradations of blue and gray; but sandstones merge into all conceivable shades and hues.

As already stated, the Craigleith sandstone, the analysis of which has been given, is the best in Great Britain. It is, however, too hard for many purposes, so the Midland and Scotch stones, that have five or ten per cent. less silica, may be taken as the type of a good and useful building material. A corresponding type, in the colonial product, is found in the Tasmanian freestone, of which the High School, Custom House, and Cargill monument are built; it contains 86 per cent. of silica. Any Otago sandstone that has so much of this base, and has a hard compact texture, may be considered strong, durable, and dry.

The highest class of sandstones, as regards their relation with the hard stones, are grits. These abound throughout the Province, chiefly in the

form of large boulders, or erratic blocks, like the Sarsen or Druid stones of the South of England. Numbers of them exist on the ranges about Kaikorai, Tokomairiro, and Kaitangata. They yield stone of a red or brownish colour that varies in texture from coarse sandstone to conglomerate with large pebbles. The blocks are usually harder than ordinary sandstone; but are sometimes wanting in cementing material, so much so that the stone easily reverts, under pressure, to its original gravel.

The grits furnish good building material for massive coarse work; but are comparatively valueless for architectural purposes. The railway bridges at Chain Hills and Glenore are built of this kind of stone—that in the former work is comparatively fine in the grain, but the others are coarse and full of pebbles. They are both used in large blocks, which, along with the dark colour of the stone, tends to give the structures a massive appearance very appropriate to this clase of work.

Closely allied to the grits, and existing under much the same circumstances in the same localities, we have numerous freshwater sandstones. They are of various colours; but are all extremely hard and compact, apparently highly charged with silica. A very handsome stone of this kind, found in the Hillend district, has been used in the abutments of the Clutha Railway Bridge; it is of a silver-grey colour, and an even hard texture. Other samples found in the same locality, and at Chain Hills, are of a reddish-white colour, equally compact. Both varieties are too hard for dressing with the chisel. There is a good specimen of white sandstone in the Museum, from Murison's Gully, on the Rough Ridge; in all probability it belongs to this class. A connecting link between the grit and sandstone proper is found on the western side of the Waihola Lake, from Mary Hill to the Gorge. It has a tough granular texture, capable of being easily dressed with the pick or chisel, but too hard for smooth work; its colour is a light warm brown, very suitable for architectural purposes. The stone is supposed to exist in large quantities; but has hitherto been little used. Mr. Duff's house is the only building of it that I know.

The sandstones proper, which embrace all sedimentary rocks in situ, are found in immense quantities throughout the Province. Unfortunately the more accessible supplies are of an inferior quality, consequently this stone has hitherto been little used for building purposes.

One of the hardest sandstones in Otago is that at the Falls, Gore Township, and at other places on the Mataura River, it is of a dark green or blueish tint, almost as hard as bluestone and equally unworkable. It is found in large blocks, with natural joints and beds, and so is very suitable for massive coarse work; the two bridges over the Mataura are built of this material.

A sandstone, of much the same quality though scarcely so hard, is

found on the northern slope of the Puketapu; its colour is generally a light blueish grey, like Portland cement, but occasionally merging into yellow. The new road bridge at Palmerston is being built of this stone. Although found in large blocks, the deposit is supposed to be limited.

There is a hard yellow sandstone associated with the limestone at the Twelve-Mile Creek, Lake Wakatipu. The rock is very much shattered, consequently the stone is not procurable in large blocks; but it can be got of a sufficient size for housebuilding. This material has been used to a small extent in Queenstown.

Waikara, as might be expected from the geological character of the district, produces a compact hard sandstone, suitable for building. The only sample in the Museum is rather dark for architectural purposes; but I have no doubt there is an abundant supply of all kinds between the Clutha and Mataura. In 1865, Dr. Hector said of this stone: “It has a disagreeable colour; but its texture and stability is superior to any of the sandstones in the Province which have, as yet, been examined, although others have been seen that will probably prove of quite as good quality.” The Waikara sandstone contains 80 per cent. of silica, which is a near approximation to the Tasmanian stone in its essential constituents. Mount Hamilton, in Southland, produces an excellent sandstone of much the same character as that at Waikara, but firmer in the grain, and of a bluish colour. Altogether this is a first-class building material; but I have no information as to the extent of the deposit, or the facilities presented for working the stone.

The district between Palmerston and Moeraki contains an immense assortment of sandstones, many of them, like a portion of the cliffs in Trotter's Gorge are too soft and friable for building stones; but there are a number of isolated blocks and veins that yield good materials. A fine yellow stone, of much the same texture as the one from Waikara, has recently been worked near Puketuitai, and there is a smooth-grained dark red ferruginous sample from the Upper Horse Range, in the Museum. Both of these would make excellent building stones. The former, having a beautiful colour, should be particularly sought after when the means of transit are provided. These are only quoted as examples of what the district can produce; there are at least five places on the railway line between Pleasant Valley and Trotter's Creek, where good sandstones can be obtained.

Proceeding further up the Waihemo Valley, we find the accommodation house at Coal Creek built of a coarse-grained yellow sandstone, found in the neighbourhood. It is also said that a large deposit of fine white stone exists in the same locality.

A hard brown sandstone has recently been discovered and worked on the north side of the Otepopo Hill. It is being used in lining the tunnel now

in course of construction through that range. Although hard, the stone dresses readily with the axe. It is found in large blocks, with regular vertical cleavages, in both directions, at right angles to each other, which gives the stones two natural faces, as true as can be worked artificially, thereby presenting great facilities for quarrying and dressing. The brown sandstone of Otepopo is too dull in colour for ordinary architectural work in large surfaces; but seems well adapted for basements, facings, and massive masonry.

The class of sandstones that comes next under our notice is the rusty-yellow varieties found at Anderson Bay, Arden Bay, Kaikorai, Saddle Hill, and Greytown. In my opinion these rocks are simply ordinary soft sandstone, like that at Caversham, dried, consolidated, and baked by volcanic fires. In the early days of the settlement, this stone was used to some extent in Dunedin and its vicinity. In the Juror's Reports of the New Zealand Exhibition, analyses are given of several varieties, which show them to have, to a moderate extent, the essentials of a good building material. The reporters, however, say that, in consequence of the excess of impalpable cement contained in the Arden Bay stone, “it will not be durable if much exposed to the weather;” and that the Anderson Bay stone” breaks up rapidly when tested with sulphate of soda, so it will not resist the action of frost.” These two stones happen to have been used in the New Zealand Clothing Factory, built about the year 1861. From its enclosed position, the southern wall of this building never gets the sun, so the stone has been subjected to the severest meteorological test that can be applied in Dunedin. The predictions with reference to the Arden Bay stone have been realized, as the lintels and sills are beginning to decay, but three lintels of the Anderson Bay stone are as fresh as when erected.

The class of sandstone that comes next in order of hardness, I shall call the “Otepopo Free Stone,” as that district furnishes the greatest number and variety of specimens. They, however, occur at other places throughout the province, notably on Mr. Larnach's property, near Broad Bay.

In the Otepopo Valley, the stone is of all shades, from clear white to dark yellows and reds; that at Mr. Larnach's is bluish-grey, like Portland cement. Although it abounds in great quantities, and often in accessible situations, the distances of the deposits from centres of population or a shipping port, has hitherto prevented its use; neither has the stone, to my knowledge, been analyzed. It seems to have most of the attributes of a good building material. The only objectionable feature I can discern is an apparent deficiency of cohesion between the particles of sand. As the cementing ingredient does not appear to be clay or lime, it is possible that this defect does not exist in stone from the bed rock. If the objection just

Like many other native products, colonial bricks were for a long time held in great disrepute, and it was even thought impossible to produce a good article from the materials at command. There is not the slightest ground for this impression; on the contrary, the clays of Otago are, so far as I am able to judge, superior in quality to the English ones. In one respect their superiority is very marked, that is, in their freedom from stones and gravel, which is such a drawback to the English brick maker. The inferiority of the colonial article was caused entirely by want of care in selecting and preparing the clay, and insufficient burning. It is simply the old question of dear labour against large profits. In the early days of the settlement, when discomforts of any kind were accepted as a matter of course, or considered a charm of colonial life, so little attention was paid to our dwellings that everything connected with them became second rate; materials and workmanship were alike defective; but a radical change has within the last few years come over our ideas. Our houses are growing larger every year, and we are not satisfied with anything short of the comforts, if not the luxuries, of the old country. It is this change in the demand that is improving the quality of the colonial bricks as well as other building materials, and those who provide them must cater to the public taste.

Clay, for ordinary bricks, should not be too stiff and plastic on the one hand, or too friable and sandy on the other; neither should it contain an excess of lime, iron, or alkaline earths, although small quantities of these ingredients are in certain circumstances desirable. Bricks made of stiff rich clay shrink in drying, and crack and twist in burning; but this can be prevented by an artificial mixture of sand. If the clay is quite free from sand to begin with, about 20 per cent. will be required to reduce its strength. When this proportion is exceeded, the bricks become brittle, soft, and fusible at a moderately high temperature. The presence of lime in such quantities as to effervesce with acids, increases their softness, and causes disintegration in the bricks. The red colour of ordinary bricks is due to an oxide of iron; within certain limits, this improves their quality, but more than ten or fifteen per cent, of the metal gives an almost black colour, objectionable in architectural works.

According to Dr. Ure, the following is an analysis of clay that will make good red bricks:—

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Professor Black has kindly analysed a sample of ordinary brick clay, taken at random from a heap at Caversham. It contains—

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This is such a close approximation to the English product in its essential constituents, that we may safely conclude there would be no difficulty in finding any quantity of clay in Otago identical in every respect with the English type.

The clays of this province are so varied in colour and consistency that, independent of their industrial importance, they form an interesting study. Two years ago I made a collection of about 40 distinct varieties from the volcanic deposits around Dunedin. Many of them were of the most beautiful colours, bright red, yellow, and blue being quite common. When separated from the sombre tints of the surrounding earths they resembled artificial dyes or paints more than natural products in a crude state. The pottery, fire, and pipe clays also demand special notice. They too exist in an endless variety throughout the province. Although their colours are seldom very bright, they are extremely fine in texture, and unctuous to the touch, like fancy soap.

Like everything else in this mechanical age, the manufacture of bricks is now done wholesale; machinery is applied in almost every stage of the process, and in many cases there is only a few minutes from the time the clay is dug till the bricks are in the kiln. I question if this is an advantage, so far as quality is concerned. The old-fashioned way of digging the clay in autumn, leaving it exposed to the action of the weather throughout the winter, and working it up in spring, is more conducive to the production of a good article.

Tempering, the next step in the manufacture, is also done in an imperfect manner; there is often no attempt made to reduce the clay to a perfectly homogeneous mass, consequently the bricks are full of cavities and other flaws, which make them twist and crack in the kiln, and impair their cohesive strength. With reference to the burning, a few years since it was

scarcely possible to get a well burnt brick in Otago., but latterly a consider able improvement has been made in this respect. If the preparation and tempering of the materials were only brought to the same standard, there would be little cause for complaint.

The Hoffenan, or German perpetual kiln, of which there is a sample at Hillside, burns bricks, lime, or cement, in a very effectual manner, at a fabulously small outlay for fuel. The principle is simply the utilization of all the heat produced, which is done in a most ingenious manner. The air that feeds the fire passes through the cooling bricks, in doing which it cools them, and in exchange becomes heated, so as to act like a hot blast on the burning mass. Then the heated gases from the furnace are carried through successive stacks of unburnt bricks, by which means they are dried and rendered fit for the fire. The fuel used is dross, or dust from the Green Island lignite, and it is put into the furnace in homœpathic doses with a trowel.

An ordinary English brick, when perfectly dry, absorbs seven per cent. of its weight of water in fifteen minutes. I have made experiments to determine the absorbent property of the colonial article, and find that a hard red brick absorbs fourteen per cent., and a soft one thirteen and a-half per cent, of its weight in the same time.

The fact of the soft brick having absorbed nearly as much as the hard one, is a clear proof that the inferiority of the colonial product is attributable more to the imperfect manner in which the raw materials have been prepared than to deficient burning in the kiln.

The establishment of the pottery works at Tokomairiro, and the success which has attended them, proves in the most conclusive manner the existence and practical utility of fire and pottery clays throughout the Province. The articles manufactured there require raw materials of the most varied kind, from refractory fire clays that resist the fiercest heat to the mixed varieties that melt at ordinary temperatures. Nearly all these clays are found in the railway cuttings between Tokomairiro and Clutha, and the establishment of the pottery may be traced directly to the construction of the railway, which revealed the existence of the raw materials in that neighbourhood.

Although the bulk of the articles manufactured at an ordinary pottery have no connection with the building arts, there are many of its products that can be utilised. In addition to the common drain-pipes, chimney-pots, and tiles, we will soon want tesselated pavements for halls and hearths, and terra-cotta goods of all kinds for ornamental purposes.

The raw materials for these articles exist in considerable quantities

Durability is a property to which concrete lays special claim, and, I think, with good reason, for it increases in strength with age, while most other materials commence to deteriorate from the moment they are put into the building. Lime, which is of a perishable nature, enters into the composition of cement concrete, but, as the proportion is so small, seldom exceeding ten per cent., and as the lime is protected by the silicates and other durable ingredients that are in combination with it, the deleterious acids of the ocean, or atmosphere, can have little effect on the mass.

The advantages of cheapness, strength, dryness, and many other good qualities to which concrete lays special claim are not like those already mentioned “constant quantities.” They depend so much on locality, cost of ingredients, and skill in construction that no general comparison can be established between it and other materials for which it is a substitute.

The chief drawback to the use of concrete is the difficulty of ensuring good materials and workmanship, and the risk thereby incurred. From the peculiar nature of the work, the margin of safety is very small. There is only one step from absolute security to utter failure, and that step may consist of a simple act of carelessness in selecting or mixing the materials. It is popularly supposed that any ordinary labourer can build a concrete wall; but, such is not the case, the amount of skill and attention required, particularly in house-building, is equal, if not greater, than that demanded from the tradesman.

In addition to marine works, for which it is pre-eminently suited, concrete has, within the last few years been applied to an infinitude of purposes ashore. In England it has been used for pavements, causewaying, and water-pipes, as well as bridge-building and ordinary architectural and ornamental works. Paris has thirty-two miles of sewers, and thirty-seven miles of an aqueduct in concrete. The latter is the most extensive work of its kind in existence. There are nearly three miles of arches, some of them being fifty feet in height and forty feet span.

The village of Vescuit, near Paris, has a Gothic Church entirely of concrete, in one piece from foundation to spire, and the lighthouse at Port Said, eighty feet high, is of the same character.

Concrete is either built in blocks previously moulded, and laid like stones or bricks, or in what is called the monolithic system, which consists in laying the soft ingredient between frames in the position they are ultimately intended to occupy. The former is undoubtedly the better, as it does away with the risk of using faulty materials; but the latter is much cheaper, and on that account is more generally adopted. The simplest form of blocks is that of common bricks; in England these are manufactured in large quantities by machinery, and form excellent building materials. A compressed

concrete brick, composed of one of cement to six of sand, will, when six days old, resist a pressure of eighteen tons, which is about double the strength of ordinary red bricks.

Concrete is cast into blocks for arch-stones, quoins, sills, lintells, steps, and mouldings of all kinds.

In view of the interest taken in concrete as a building material, I shall devote a few remarks to the consideration of its properties. The cementing ingredient in concrete is generally hydraulic lime, or cement, or a mixture of the two. The former has not yet been discovered, or, at least, used as such in Otago, neither has the latter been manufactured, so they cannot be called native; but, as the raw materials for making cement exist in large quantities, there is no doubt its manufacture will become a colonial industry at no distant day. The proportion of cement to the aggregates varies from a fourth to a tenth, according to the nature of the work, the strength of the cement, and the character of the other materials; for house-building 1 to 6 is weak enough, particularly here, where the cement may have deteriorated by exposure on the voyage. The best aggregate is one in which the pieces are of all sizes, from two inch metal to fine sand, adjusted in such regular gradations that the cement will exactly fill the vacuities. Large metal and fine sand, with other materials of an intermediate size, does not make good concrete. The ingredients should be mixed dry, and water added in infinitesimal quantities, through a fine rose, or otherwise in the shape of spray. This is an important point, for a wash of water enriches one portion of the mass at the expense of another. No more water should be put in than sufficient to damp the cement; as a certain limited quantity only is required in setting, the excess evaporates, and leaves cavities for the reception and retention of moisture. Mixing, the next operation, is also equally important; it must be done in a thorough systematic manner, so that every piece of stone, or particle of sand, is completely coated with cement. It is almost impossible to get this work done properly by manual labour, and although machinery is constantly employed on large works, the necessity for it in ordinary house-building is not yet fully recognised.

The manner of depositing the material in the moulds, or frames, has given rise to a difference of opinion; some authorities hold that the concrete should be placed loosely, as pressure impairs the setting properties of cement, while others advocate excessive ramming. If Roman, or any other quick setting cement is used, pressure will undoubtedly do harm; but, with ordinary heavy Portland Cement, or hydraulic limes, in the proportions usually adopted, there is no risk in ramming, and the quality of the concrete is so much improved by it, that, if necessary, it would be better to

retard the setting of cement by a mixture of ordinary lime, or by prolonged, mixing, than omit the operation.

The large French works that I have mentioned are all built of a concrete invented by Mon. F. Coiquet, and known as “betou agglomere;” the composition being as follows:—

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This has been the most successful application of concrete to ordinary building purposes hitherto recorded, and the result is due almost entirely to careful manipulation.

In addition to the essentials of a proper adjustment of the ingredients, and thorough mixing with the minimum quantity of water, great stress is laid on the necessity for heavy ramming. The concrete is spread in thin layers, and hammered with iron-faced beaters till each layer is compressed to a third of its original thickness. The surface is then raked to form a bond with the next layer, and so the work is carried on continuously to the end. The result of this careful treatment is that “betou agglomere” is one of the most compact, impervious, and durable building materials at present in ordinary use.

General Gillmore, of the United States Army, in reporting to his Government on the question, made some experiments to determine the relative strength of concrete prepared in the usual way, and in the method adopted by Mon. Coiquet. I give a few of the results:—

Compressive strength.—Crushing weight of Portland cement pure and mixed with sand, in pounds per square inch, on blocks seven days' old:—

Tensile strength under the same conditions—

Independent of these experiments, the defects of the loose method of depositing concrete in buildings is apparent to any observer. The cavities occasionally amount to a third of the whole, consequently a nine inch wall is no stronger than one of six inches in which the materials are compressed into a solid mass, and the porosity of the structure must be proportionately

great. If it is possible to rain “betou agglomerè” into a third of its original bulk, it is quite obvious that the voids in the impressed article must equal or exceed the solid parts, or that the whole mass lacks the density essential to strength and impermeability. A coat of plaster on the outside of a building will not, as is sometimes supposed, effectually keep out damp. At the most, Portland cement and its mixtures are only limestone or calcareous sandstones or grits, and, as such, are more or less absorbent; it is, therefore, necessary to convert them into the compact state by pressure, if we want it to resist moisture, and it is impossible to do so in plastering.

Although some thousands of experiments have within the last few years been made to determine the strength of Portland cement and its mixtures, under every conceivable circumstance, there is no record of any regular experiments having been made to test its powers of resisting damp. General Gillmore made one or two trials of “betou agglomerè,” and he pronounces it to be practically impervious; the amount of moisture absorbed in four days was immeasurably small.

An Indian engineer, Mr. Horace Bell, found that neat Portland cement absorbed 20 per cent. of its weight in an hour, and 25 per cent. in three hours. In view of the paucity of our information on this subject, I made a few experiments with samples in my possession. The specimens were not prepared for this purpose, so the proportions of the various ingredients and mode of mixing them were not recorded with the exactness necessary in a thorough investigation; the results are, therefore, not advanced as absolutely conclusive.

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Another experiment was made with a specimen block of concrete, made by the proprietors of the Logan Point Quarry. It was composed of one of

cement to seven of fine road metal and small stuff from the stone-breaker; none of the metal was larger than an inch each way, and the other ingredients were well adjusted. The concrete was not heavily rammed like “betou agglomerè,” but it seems to have been very firmly pressed. Altogether, it was a first-class piece of concrete, and the greatest difficulty was experienced in breaking it up with a wedge and heavy hammer. The block measured 24 inches long, 12.25 inches high, and 10.08 inches broad, and weighed, when dry, 240 lbs., the outside being covered with a thick coat of rich cement plaster, as it is intended to have in a building.

The first experiment was to determine the impermeability of the plaster. A wall of clay was put round the edge, leaving a square foot exposed; water was poured on, and in three hours about three-quarters of a pint had penetrated the surface.

The whole block being then immersed, it instantly absorbed two and a half lbs. more, and in sixteen hours the quantity had further increased to four lbs.

On breaking, it was found that the moisture had permeated every portion of the block, and the centre was as wet as the outside.

The two samples of concrete thus experimented on were of a very superior quality; I have never seen anything to compare with them in ordinary work. Although these experiments are very crude, and the results much higher than would be obtained from less carefully prepared specimens, they go a long way to prove that the property of perfect immunity from damp, to which concrete houses lay claim, is not secured by the mode of building usually adopted in Otago, and, I believe, the experience already acquired in actual practice, fully supports this assertion.

I shall now consider the strength of concrete, in order to compare the cost of the various materials under description, which I intend to do further on. The properties of brickwork being so well known, it has from time immemorial been selected by municipal authorities as the standard from which to determine the strength of buildings, and there are regulations in every town fixing the thickness of brick walls in whatever position they occupy. I shall, therefore, adhere to the same standard.

The following table gives the crushing strength of various kinds of bricks and concrete; but, for the purposes of a more general comparison, a few examples of other materials are added.

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Smooth dressed ashlar, in large blocks, with cement mortar, is practically as strong as the stone of which it is built, but rubble masonry is three-fifths weaker. We may, therefore, assume the crushing strength of this class of work, built from our native bluestones and hard breccias, at 4000 lbs. per square inch.

From the above data the relative thickness of walls of equal strength in the ordinary building materials would be approximately as follows:—

Although, in theory correct, it is practically impossible to adopt this standard, for we all know that nine inch brick walls are sufficient for a one storey house; but the idea of reducing them to one inch is altogether too absurd to be entertained, no matter how strong the material may be. The objection, also, holds good with the thickest walls; for weight and breadth of bearing are as much required as cohesive strength. Those who maintain that concrete is superior to every other building material advocate thin walls, or at least, affirm that they are permissable, and many houses have been erected, the sides of which resemble monumental slabs more than

agglomerè sewers in Paris are calculated to have cost 20 per cent. less than any other material procurable of the same quality. It must, however, be borne in mind that in both those places the circumstances are very much in favour of this result; the cementing ingredients are manufactured on the spot, consequently concrete is on a par with brickwork, and has an advantage over stone, which comes from a distance. In Otago the conditions are exactly reversed, brick and stone being in the locality can be produced at a moderate rate, while cement has to bear the heavy charges inseparable from the importation of a low priced article.

When the manufacture of Portland cement is established in New Zealand the relative costs of the three building materials will in all probability, approach nearer the European proportion. The price of concrete in plain walls, near London, with cement at 8s. per cask, is from 9s. to 11s. 6d. per cubic yard. Betou agglomerè in Paris, for the same work, with cement 8s. per cask, hydraulic lime about half that price, and labour 3s. per day, costs from 20s. to 24s. per cubic yard.

The greater cost of the latter is a proof that there is more labour and care bestowed on its preparation than is done with concrete in England. The price of cement concrete for ordinary engineering purposes in Dunedin is about 35s. per cubic yard, and M. Petre, who has had considerable practice in building with concrete, informs me that its price in a plain wall is about 41s. 6d.; if to this is added, the cost of outside plastering, which is indispensable in any class of dwelling house, we bring it up to 50s.

I am not aware of any building having been erected in Otago in strict accordance with the method adopted in France, so there is no way of getting at the cost from actual experience; but the data at command are sufficient to fix 60s. as a close approximate. In America the price of betou is estimated at from 36s. to 44s. for labour and materials alone, and these are much cheaper than with us. The following statement gives the comparative cost of building in London and Dunedin at the present day:—

At those prices, and the standard thickness of wall formerly established,

the relative cost of building in Dunedin, with the various materials at command is as follows:—

This proportion is not, however, applicable to the whole building, for the value of the masonry is generally less than half the total cost; furthermore, the high priced materials are seldom used in large quantities; the front of a business place in a street, or the facings in an isolated dwelling-house are all that is required to be of this class. Mr. Lawson estimates the difference in the cost of brick over timber in an ordinary dwelling-house, at from 33 to 50 per cent. Taking it at a mean of those rates, a wooden house worth £1,000 would cost £1,400 in brick; the cost of the walls being respectively £300 and £700. The interest of the amount saved is sufficient to rebuild the walls every ten years, which is oftener than required, but it is not sufficient to renew the whole house when the walls decay—a very probable contingency, for the renewal of the walls entails, practically, the entire reconstruction of the building. Beside, the interior of a wooden house is more subject to deterioration and injury than that of a brick or stone one, and the permanent charges, such as repairs, painting, and insurance are always much higher. Independent of the increased comfort and security obtained, I believe that even now it is true economy to build our houses with the more durable materials; and when the railways are in full working order, north and south, the matter will be placed beyond doubt.

At present Oamaru stone costs 5d. per cubic foot in blocks at the quarries, and 3s. 6d. in the same state here. When the railway is opened, it should be bought in Dunedin at 1s. 6d., the price when laid being 2s. 6d., which is a saving of 44 per cent. on current rates. The brown and grey freestones of Waihola are already within reach of railway carriage, and will be conveyed to town for about 4d. per cubic foot, so that they can be sold for 1s. 6d. As already stated, the former is too hard for fine work, but the latter is an admirable substitute for the Oamaru stone; it is a compact limestone of the proper consistency, soft enough to be easily worked, but sufficiently hard to stand the weather.

I trust, therefore, that one of the first benefits our city will derive from

This result is to be expected, for the rocks on that side of the island are too old and crystalline to produce a good article.

The exact locality of the Otepopo slate reef is about half a mile west from Charles Peak, at the confluence of a small tributary of the Kauru with the main stream. The distance from the township of Herbert in a direct line being about eight miles. The land has been taken up by a party of Dunedin gentlemen, who have opened out several faces to test the quality of the rock; and about 100,000 slates of all sizes have been split already. I visited the locality in February last, and although no work had then been done, I could see indications of an abundant supply of the material; and I felt satisfied that the discovery was one of the most important ever made in Otago. Of course it still remains to be seen whether the quarries will be commercially a success. They are in a very inaccessible situation, consequently a large outlay will be incurred in making a road or tramway to bring the slates to a market; and the refuse, which is very great in the best quarries, may be so out of proportion to the good slates, that they cannot be produced at a reasonable price. I understand that the proprietors intend to test the quality of the quarries in a thorough manner first; and if it is proved that the rock exists in sufficiently large blocks and faces to admit of being profitably worked, operations will be at once commenced on an extensive scale.

The locality has been named Ballachulish, after the famous quarries in Argyleshire. I trust that, like their great prototype, the Otago quarries will become so extensive and important as to prove a mine of wealth to their proprietors, and a boon to the country generally.

Roofing slate is found of all colours, from a creamy white to black, and there is also a considerable difference in the texture.

It has been found that the best slates are those of a bluish-grey colour, which is the exact tint of the Otago ones. The other essentials are, compactness of texture, impermeability, and the facility with which they can be split parallel and without twist.

The Otepopo slate possesses all these properties in a pre-eminent degree. I placed a Welsh and an Otago slate side by side in water for 48 hours, and found that, while the moisture rose from three-eighths to one-half an inch in the imported article, it did not rise at all in the colonial, which proves that the latter is the more compact and impervious of the two. The facility of splitting is also fully established, for the many samples to hand are of all thicknesses, and perfectly true to shape; and I have seen the slates split well with a common pick, instead of the broad knives used by the quarriers.

Shortly, I believe the Otago slate is little, if anything, inferior to the

best “blue Bangor;” and when similarly grained specimens of the two kinds are placed together, the best judge can scarcely distinguish them.

Captain Hutton informs me that there is a considerable difference between the cleavage of the Otago and English slate; instead of being at an angle to the strata, it is parallel to them. He points this out as a probable defect in the colonial article, but at the same time states that the property of splitting readily is not due to lamination but cleavage, consequently the pressure that gave this property must have been applied in a vertical rather than a horizontal direction. Without venturing to express an opinion on such an important geological question, it seems to me that the idea of a regular vertical pressure, induced or aided by attraction of gravity, is more natural than a horizontal one; not only is the pressure abnormal, but we must pre-suppose the existence of a solid mould which prevented the lateral extension of the material.

The roofing slates in England are all extracted from beds with inclined cleavage; and those taken from a horizontal stratum, where the angle of the cleavage planes is greatest, are supposed to be the readiest split, and otherwise the best; but I do not know that there is a sound reason for this conclusion; and although roofing slate has not hitherto been obtained from strata with a parallel cleavage, the existence of a cleavage of this kind in the clay-state formation is well known. Professor Geikie says, “Cleavage may either coincide with the original lamination of the rock, or cut across it at an angle;” it is, therefore, possible that the exception in the old country is the rule at the Antipodes.

Under any circumstance, the question cannot affect the industrial importance of the Otago slate; while we are satisfied that it splits freely, and is durable and impervious, its geological peculiarities may be disregarded.

In addition to roofing material, slate quarries yield slabs for paving, hearths, mantel-pieces, and other works of a similar kind; the finer sorts are usually too smooth and soft for street pavements, but I have no doubt varieties suitable for this purpose will be found in the same locality.

Following the plan adopted with the other materials, I shall devote a few remarks to the consideration of the comparative cost of slate and its principal sustitute, corrugated iron.

It is popularly supposed that there is a great difference in the cost; but such is not the case. Having occasion lately to decide on a covering for my own house, I calculated the difference carefully, and found that with Countess slates at £215 per thousand, and galvanised iron at £37 per ton, which are about the current retail prices, the cost of the two materials was identical for the same space of roof. There is, however, a difference in favour of the

iron in cartage, timber work, and labour, amounting to 10s. per square, or 16 per cent. on materials and labour combined.

This is a large proportion, as such; but when we consider that it only amounts to about £10 on a house forty feet square, the wonder is that any iron is used. Whether regarded as a matter of appearance, freedom from sound, and extremes of temperature, or durability, the superiority of slate over iron is undoubted, and were the difference in cost twice as great, the balance of advantages would be still on the same side.

From the Customs returns I find that, in 1874, there were imported into Dunedin alone—

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Assuming that £12,039 worth of iron is used for the walls of houses, fencing, and similar purposes, we leave a balance of £30,000, as sent out of the Province for roofing materials, which, in all probability, we have at our doors. I question the wisdom of fostering, or encouraging, at this early stage of our history, every industry that may ultimately be required, or that may succeed in the colony at some future time; but, in the case of a low-priced article like slates, the value of which is doubled by freight, and the other charges of importation, there is little wanted to turn the scale in favour of the native production.

I believe the enterprise that establishes and carries on the industry, and the individual support it receives, is sufficient to do so; we may, therefore, hope to see the imported roofing materials fairly supplanted by the colonial article at no distant day.

In concluding this division of my subject, I must repeat what I said at the outset as to the paucity of our information on the building materials of Otago, and the importance of the question.

Although I hope these papers will reveal a number of new facts, the researches that I have made in compiling them enable me to say, with greater emphasis than at the beginning, that our resources are still practically unknown.

The importance that is attached to the collection and diffusion of knowledge of this kind throughout the Colony was forcibly brought under my notice a few months since, by seeing in the papers that it was proposed to build the Auckland Docks of Aberdeen granite. Undoubtedly granite is the best building stone in existence; but it is also the dearest, and for this purpose it is no better than the stone of which the Port Chalmers Dock is