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Volume 14, 1881
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Art. LXXIII.—On Crystalline Rocks.

[Read before the Auckland Institute, 11th July, 1881.]

Crystalline rocks occur as altered sedimentary deposits, and comprise most of the eruptive rocks; the latter are to a great extent crystalline at the time of their formation, while the former were originally loose accumulations of various particles for the most part. Both kinds of rocks are subject to changes of condition which are termed metamorphism, by which the internal texture and composition have been altered gradually by chemical, electric and crystallographic action, by the withdrawal of, or addition, or substitution for some of the chemical elements, aided by heat and watery vapour acting under intense pressure. The changes in the sedimentary rocks are usually more apparent than in the eruptive, so that the term metamorphism has been more especially applied to these rocks.

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Table of Changes produced by Metamorphism.
Earthy. Indurated, and in which cleavage is shown by slaty rocks. Micro-crystalline, Schistose (foliated). Cryslline.
Foliated. Massive.
Mud, Clay, Shale Claystone, Clayslate or “Killas” Argillaceous Mica-Schist, Talc Schist
Sand Sandstone, Grit Quartzite, Quartz Schist, Felsite (Petrosilex) Gneiss Granite
Calcareous Mud Ghalk, Limestone Marble, Dolomite
Volcanic Ash and Tuff Slates Felstone Trachyte

The completeness of the change may vary from the incipient form shown by indurated clay-slate rocks in cleavage, which always occur in a direction other than the plane of bedding. Some slaty rocks are apparently volcanic ash deposits. This structure of cleavage is more mechanical than chemical, and caused by great lateral pressure by which the component particles were flattened, producing lines of weakness at right-angles to direction of pressure. This structure has been artificially produced in some soft substances, by Dr. Sorby and Prof. Tyndal, and also by Messrs. Fox and Hunt, by passing galvanic currents through masses of moistened pottery clay (Page's Geology, p. 154). The contorted condition of fossils that occur in the slates show also the disturbance of the particles forming the slate.

Limestones pass from an indurated into a compact and microcrystalline texture, becoming granular when highly metamorphosed. The latter generally occurs associated with schist, or in the proximity of eruptive rocks. The whole of these varieties are popularly termed marbles. Crystalline marble has been artificially produced by heating chalk under pressure sufficient to prevent the escape of carbonic acid gas. Many accessory minerals, such as zircon, spinel, corundum, lapis lazuli, are found in crystalline limestones.

Arenaceous rocks, such as sandstones, are composed of rounded particles of quartz; and grits of angular fragments and crystals, together with rounded particles of quartz; these are converted by metamorphosis into quartzites, the component particles being cemented with siliceous material; both kinds often contain felspar, which renders them capable of conversion into felstone rocks, and when mica is present, micaceous kinds are produced.

The alteration passes then into foliation, which is a segregation into crystalline layers of different mineral composition, the planes of separation being either along those of the bedding or cleavage. This structure is

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termed schistose. The various mineral layers blend into each other, and are composed chiefly of quartz, felspar, mica, and talc or chlorite, while veins of quartz ramifying through clay, slate, or other non-siliceous rock, render them convertible into argillaceous mica-schist or phyllite, mica-schist, granulite; or less siliceous kinds, forming talc, chlorite, hornblende, or actinolite-schist, or schorl rock.

Schistose structure has been found also to be occasionally produced in lavas, the vesicles in which have been compressed and attenuated in the direction of flow (Rutley, Q.J.G.S., vol. xxxvi., p. 285).

A further alteration of these rocks takes place into gneiss, which has a schistose structure, the quartz, felspar, and mica, and often hornblende, of which it is composed, being arranged in layers, the foliation constituting the chief difference from granite. Gneiss, and schistose rocks, with intercalated beds of crystalline limestone, form the laurentian rocks of Canada. The schist containing beds of graphite, or unoxidized carbon and apatite, (Dawson, Q.J.G.S., vol. xxxii., p. 285), denotes plant, and the limestone, animal life. Graphite occurs also at Pakawau, in Nelson, under similar condition in metamorphosed strata, and its presence denotes that no extreme temperature was attained during metamorphosis of the rock.

Gneiss has been found in many cases to merge into granite, so that the extreme of metamorphism may be regarded as granite, the fundamental rock throughout our earth; and its massive crystalline texture and its chemical combination of elements, namely, quartz, felspar and mica, must now be regarded as the ultimate crystalline condition, under great pressure, of sedimentary strata, either by slow consolidation after having been converted into a molten state, or by gradual chemical and structural change.

The quartz in granite often has cavities and enclosures of other minerals, principally rutile and chlorite; these cavities generally contain pure water, occasionally liquid carbonic acid, or a solution of chloride of sodium; they also contain bubbles, or rather vacuous spaces which show the contraction which the imprisoned fluid has undergone during the cooling of the rock. Dr. Sorby and others have endeavoured to calculate the amount of pressure shown by these contractions in volume. Spaces or beads of glassy or amorphous quartz, also occur, which denote that the quartz had first become viscous, and in consequence solidified without crystallizing. The liquefaction proved by the liquid cavities to have once been the condition of granite, has caused it in places to burst through adjacent rocks in an eruptive manner, when disturbed perhaps by an increased pressure, while other portions of the same mass may gradually blend into schistose sedimentary strata. Professor Judd has proved how granitic rocks in the Island of Mull, Scotland, and at Schemnitz, in Hungary, are directly connected with volcanic rocks, and both form portions of one and the same mass.

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“One of the arguments against the igneous origin of granite is that its quartz has a s.g. of 2.6, identical with that of silica, derived from aqueous solution, while the s.g. of fused silica is only 2.2″ (Rutley's “Petrology,” p. 207).

Professor Haughton, in his annual address to the Geological Society of Dublin in 1862, in alluding to a table of the specific gravities of natural and artificially fused rocks, remarks:—“It appears to me that the column of differences greatly strengthens the arguments of those chemists and geologists who believed that water played a much more important part in the formation of granites and trap rocks than it has done in the production of trachytes, basalts, and lavas, and that they owe their relatively high s.g. to its agency.”

The accompanying table from Dr. Page's “Geology” shows admirably the component parts of granite, the felspar occurring in two varieties, “orthoclase” and “oligoclase,” the former being associated with white and black mica (uniaxial and biaxial).

A table of felspars is also shown for the sake of reference. It shows the crystallographic relations with the chemical.

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Quartz Silica Silicon
Oxygen
Silica Silicon
Oxygen
Felspar Alumina Aluminium
Oxygen
Granite Potash Potassium
Oxygen
Silica Silicon
Oxygen
Magnesia Magnesium
Oxygen
Mica Potash Potassium
Oxygen
Lime Calcium
Oxygen
Peroxide of Iron Iron
Oxygen

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Divisions. Crystallographic Properties. Varieties. Chemical Character.
Orthoclase Felspar Oblique system Orthoclase Sanidin Potash acidic In Granite, Gneiss, Syenite and True Volcanic Rocks only
Albite Soda In Granite with Orthoclase and in many Diorites
In Granite with orthoclase
Plagioclase Felspar Doubly oblique system Oligoclase In Diabase and Diorite
Andesite In Trachytic Rock
Anorthite Lime basic In old Lavas
In Augitic Rocks (Dolerite Gabbro and Diallage)
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When granite and other rocks have penetrated superincumbent strata, the contact of the molten mass has usually produced a change in the neighbouring rocks, but of a different character to that widespread uniform character described under metamorphism. The difference has been brought about through its sudden character, and probably by the loss of a large portion of watery vapour, causing a vitrifying effect to be produced. Hornblende-slates are frequently formed along the contact margins of granite and clay-slates. (Q.J.G.S., vol. xxxii., p. 187, J. A. Phillips; Q.J.G.S., vol. xxxiv., p. 438).

Metalliferous veins are usually found to have been formed by the occurrence of intrusive rocks in the vicinity, the latter having been usually decomposed by acids and vapours, the introduced metals, or the metals from them, being deposited in veins.

Mr. J. A. Phillips in Q.J.G.S., vol. xxxv., p. 391, describes the district of Steamboat Springs, Nevada, where “fissures are being lined with siliceous incrustations which are being constantly deposited, while a central longitudinal opening allows the escape of gases, steam and boiling water; the water is slightly alkaline and contains carbonate of sodium, sulphate of sodium, common salt, etc.” These springs have deposited cinnabar (ore of mercury) with the silica (both amorphous and crystalline, the latter containing the usual liquid cavities and ordinary optical and other characters of ordinary quartz). At other springs in the same district silver and gold have been found enclosed in sinter-like deposits. In Australia gold occurs in pyrites contained in diorites and granite, and gold mines are worked in these rocks. Mineral veins often show by their structure that the fissures (Q.J.G.S., vol. xxxii., p. 169) they fill have been widened repeatedly, probably by the force of crystallization, successive infiltration having filled the fissure with siliceous and other substances forming a banded structure. The metals when they occur may either have been deposited from solution or by sublimation. The tin-bearing bands of schorl rock in granite of Cornwall, have been proved to have been formed through the decomposition of the granite along the sides of leaders or veins. Granites vary from coarsely porphyritic granites to the fine grained elvans (quartziferous porphyry) in which mica is present. The porphyritic texture is due to the inequality in the crystallizing power of the various minerals, felspar and mica crystallizing more readily than quartz, the latter always occurring in consequence in more irregular forms than the former.

Hornblende and schorl are sometimes found replacing mica to a great extent, forming syenite and schorlaceous-granite, and when only a small proportion of quartz occurs the rock passes into syenite and schorlrock. Granite frequently passes into felstone, micaceous felstone differing

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only from granite in texture. Trachytes are volcanic rocks, possessing precisely the same chemical constitution as felstone, and form their modern representatives; they occur largely in the central portions of this island. Both belong to the acidic group, and form far more extensive deposits than the basic, which are represented by melaphyres and basalts, and to these belong the Auckland Isthmus volcanic rocks, while intermediate forms termed trachydolerites (Scrope) predominate in some areas. This preponderance of siliceous kinds has caused some geologists to consider that they predominated in the older, and the basic in the more modern rocks. The eruptions from the greater number of the active volcanoes of the present day have apparently a basic character, but the recent investigations of the nature of the bed of the ocean show that while Globigerina ooze covers the ridges and plateaux down to 2,000 fathoms, lower deposits are covered with a red clay, formed of decomposed felsitic minerals with particles of highly vesicular felspar and pumice, and concretionary nodules of manganese, a large proportion of which must be derived from submarine eruptions; thus while comparatively circumscribed deposits of augitic lava are accumulated around the volcanoes, the more siliceous portions, comprising the ash and vesicular felsitic scoria, are accumulated separately on the bottom of the ocean. Lyell, in his “Elements of Geology,” mentions that it can by no means be inferred that trachytes predominated at one period of the earth's history and basalt at another, for we know that trachyte lavas have been formed at many successive periods, and are still emitted from many active craters; but it seems to me that felspathic lavas have generally preceded augitic when a volcanic action has extended over long periods. Professor Judd has shown that in the extinct volcanic district of Schemnitz, in Hungary, lavas of an intermediate (acidic and basic) character preceded outbursts of extremely acid, and then of extremely basic character; the tertiary andesitic eruptions of Hungary forming an exact counterpart to those in the palæozoic in the British Isles.

Most of the older eruptive rocks have been affected by metamorphic action, many intensely so; the vesicular kinds have had their cavities filled with minerals, often of extraneous origin, forming zeolites and geodes of agate, or by segregation, zeolites forming often constituent portions of basalts. Chlorite, which always appears to have accompanied mineral changes, is generally present in considerable quantities in the older members of these rocks. There is also generally more lime, the potash and soda having been more readily dissolved out than the lime. The rock termed serpentine occurs with schists, and also as an intrusive rock, and apparently is usually the result of decomposition of olivine rocks—dimagnesian (ferrous, etc.) silicates—similar to the New Zealand dunite, or of materials derived from their disintegration.

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One of the most interesting illustrations of the change produced by hydrothermal agency has been described by Professor Daubrée, who found that the water of the springs of Plombieres, in the Vosges, which have a temperature of 160° Fahr., had formed zeolites in the concrete of the Roman aqueduct built for conveying the water, the concrete being composed of lime, fragments of brick and sandstone. The minerals found include apophyllite, chabazite, and opal.

In the following table the relations of the various eruptive rocks forming dykes, lavas, and scoria are shown:—

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Compact. Crystalline, granular. Glassy, scoriaceous.
Acidic Old Felstone Quartziferous Porphyry, Elvan Pitchstone, Perlite
Modern Trachyte Trachyte Porphyry Obsidian, Pitchstone, Perlite, Pumice
Intermediate Trachy-dolrite
Basic Old Aphanite Diorite Tachylite
Modern Basalt Dolerite Tachylite, Pumice

The characteristic ingredients of these leading varieties may be stated thus:—felstones have orthoclase felspar and quartz, the glassy conditions are pitchstones and perlites. Trachytes, their modern representatives, are composed almost wholly of a confused mass of crystals of sanidin without perceptible free quartz; they are often porphyritic, the glassy form is obsidian. Hornblende is frequently present in these acidic rocks. The diorites comprise the hornblendic basic rock with orthoclase and oligoclase felspars. The dolerites, their modern representatives, have augite with sanidin and Labradorite felspars, tachylite forming the glassy condition; it closely resembles obsidian. The ashy and tufaceous kinds are found consolidated into felstones and aphanite-slates; microscopic examination shows these slates to contain crystals with fused surfaces, or with vitreous coatings, and isolated shreds of glassy matter in strings or bands (Rutley, Q.J.G.S., vol. xxxv., p. 338.)

The glassy varieties have been formed by rapid cooling of the molten mass, for when basaltic rocks have been experimentally melted, and cooled slowly, a state very similar to the original has been attained; but when cooled rapidly, they have assumed a dark brittle glassy condition, resembling obsidian. The perfectly amorphous condition of common glass is seldom attained in the natural rock, minute crystals of pyroxene and felspar being generally more or less scattered through the glassy matrix. The glassy condition being a particularly unstable one, the obsidians and rocks with allied glassy structures like perlite have been altered into pitchstones,

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which sometimes show under the microscope a base of homogeneous glass without a trace of double refraction; in some cases the glassy structure is destroyed by crystallization, and a micro-crystalline base formed, possessing the double refraction characteristic of felstones, so that what was once glassy lava is now a felsite with a crystalline structure. (See also Rutley's Petrology, p. 169.)

It may be mentioned as illustrative of the changes which glassy forms undergo that water extracts potash and soda from glass, together with portions of silica, the decomposition taking place with greater ease in proportion as the glass is richer in these alkalies and more minutely divided, and the temperature of the water higher. The pearly stratum with which specimens of antique glass found buried in the earth are covered, consists almost wholly of silica.

In thus briefly reviewing the general relations of the various kinds of crystalline rocks, the chief leading characters of the more important groups have been referred to only, the numerous varieties diverging from these groups forming intermediate forms of more or less subordinate interest. The consideration of the changes that rocks undergo, leads us to a certain extent into speculative ground, where different interpretations of facts are tenable. The chief differences of opinion occur with reference to the relations and formation of granite. Though these uncertainties may encircle the subject at the present time, we may expect before long to have a clearer knowledge, as petrographic research has been making rapid strides in the last few years.