Art. XLVII.—Geology of Nelson.*
[Read before the Nelson Philosophical Society, 12th June, 1893.]
I am only about to attempt the barest outline of the geology of this district, and in doing so must acknowledge my indebtedness to the Geological Reports, issued by the Geological Department, and to the “Outline of New Zealand Geology,” prepared by Sir James Hector, Director of the Geological Department.
To describe the geology of Nelson it will be necessary to say a few words about the geology of New Zealand as a whole. New Zealand, there are good reasons for believing, is but the remains of what was once an extensive continent. Soundings made by the “Challenger,” on her famous expedition, brought to light the fact that a submerged plateau extends for many miles to the eastward of New Zealand. The depth of water over this plateau varies from 300 to 600 fathoms, while the water of the ocean beyond the plateau has a depth of 2,000 to
[Footnote] * The maps and diagrams referred to in this paper were enlargements of geological map and sections issued with “Outline of New Zealand Geology.”
2,600 fathoms. Owing to an insufficient number of soundings, the contour of the plateau has not been determined, but it is supposed to extend as far eastward as the Chatham Islands; while to the westward, especially in the south-west, the line of the plateau is almost identical with the coast-line. The coast-line itself also bears evidence of subsidence. Those who have travelled only from here to Wellington have probably noticed how the curve of the hills reaches almost to the water's edge. This shows that the land has had a downward movement in recent geological times. If the land had been stationary we should have had high cliffs, caused by the erosive action of the sea, presenting themselves; or, if the land had been rising, extensive sea-beaches would have fringed the coast. Small islands, near the coast, like Pepin Island, D'Urville Island, Arapawa Island, and Kapiti Island, also bear evidence of the subsidence of the land. These islands are but the tops of hills which once formed part of the mainland. A glance at the map of New Zealand also leads us to conclude that New Zealand had once a more extensive land-area. The coast-line, as you will see, is characterized by a few bold headlands with extensive bights lying between. These headlands are composed of hard rock which has been better able to resist the action of the sea, while the places into which the sea now flows, forming extensive bights, once formed part of the dry land. The remarks about the islands in Cook Strait will also apply to the Barrier Islands, White Island, and Stewart Island.
Let us suppose, then, that the submerged plateau of which we have spoken was once high above the water; that the contour of that plateau was the boundary of an extensive continent which extended from East Cape in the North Island to Shag Point in the South Island: then the place which we now call Cook Strait—I mean the narrow part—was merely a pass in the mountain-chain; and the place which we now know as Tasman Bay was a broad valley, through which, probably flowed a large river, the upper reaches of which are represented by the streams which at present drain into the bay.
The greatest depth of Tasman Bay does not exceed 50 fathoms, while a great part of it has a depth of less than 50ft. When the submerged plateau, then, stood above the sea some hundreds, or probably thousands, of feet, Tasman Bay was not merely a valley, but an elevated one; and the mountains by which we are surrounded, having a much greater altitude, were covered with perpetual snow, and glaciers filled our now smiling valleys. This fact is borne out by the extensive glacial deposits found in the Nelson District. The Moutere Hills, and part of the Port Hills, are of glacial origin.
The moraines of glaciers are also found in the neighbourhood of Lake Rotoiti, and in the Takaka district.
While we have good reasons for believing that the surrounding district was once much more elevated than it is at present, we have abundant evidence to prove that it was once far below the level of the sea. Beneath the glacial deposit to which reference has already been made there lies a series of stratified rocks, which, by their lithological character, and by the fossils which they contain, give unmistakable proof of a deep-sea origin. These rocks consist of sandstones and clays, and many of the fossils found embedded therein are those of species now extinct. Some of the lower members of this series are well developed in the Port Hills. When walking round the rocks one is obliged to tread on the upturned edges of the lower members of this series, known as the Lower Miocene formation; while in the cliffs above, especially near the basin, the rocks may be seen dipping into the hill at an angle of about 50°. These rocks, when formed, must have been laid down in a horizontal position; hence their present upturned condition must have been brought about by a considerable amount of upheaval in the earth's crust. Numerous fossils may be found in the above-mentioned rocks, but owing to decomposition it is very difficult to get perfect specimens. A few good ones were found in the tunnel driven by Mr. Brown in his search for coal.
Passing from the Port Hills to Richmond another set of rocks is met with, known as the Wairoa series. These rocks extend from the hills about Richmond to the Wairoa Gorge. They belong to what is known as the Triassic formation, and are older than the Miocene deposits of the Port Hills. Their relation to the rocks of the Port Hills is shown on the sketch, where the rocks of the Wairoa series are seen to dip below those of the Port Hills, while the Port Hills series forms what is known to geologists as a synclinal arrangement. The Wairoa formation is exceedingly rich in fossil remains, some of the hills above the gorge being literally masses of fossils. They may be found in the bed of the river just before entering the gorge, and on the hill-slopes this side of the river, as far north as Richmond. It is the study of these fossils that has led to the determination of the age of the rocks in which they are embedded. Monotis salinaria and Mytilus problematicus are the most common. Mention is made in the “Outline of New Zealand Geology” of teeth having labyrinthodont characters having been found in this formation. This being so, we may well suppose that, while these rocks were being deposited in the shallow seas of that age, amphibious creatures of considerable size disported in the lagoons, or basked upon the mud-flats.
Leaving Wairoa Gorge, and travelling up Aniseed Valley, a still older series of rocks is met with. These rocks are known as the Maitai slates, and form part of a very extensive system, known to geologists as the Carboniferous system. These slates, in the lower part of the valley, consist of fine-grained clay-slates. Further up the valley the slates become calcareous; and finally a magnificent belt of mountain limestone is reached. This limestone, and the slates already mentioned, are the principal rocks of the Carboniferous system. These rocks are very extensive, and form the greater part of the mountain-chains of the district. They pass from end to end of the provincial district, forming part of the Spenser Mountains, St. Arnaud Mountains, and the low mountain-range running from the St. Arnaud to the Pelorus Sound. As offshoots from the main range they reach almost to the Town of Nelson—Fringe Hill and Botanical Hill being composed of this rock. Extending from Mount Franklyn to D'Urville Island, and running in the same direction as the mountain limestone and the Maitai slates, is a stretch of country known as the “mineral belt.” This formation presents a marked contrast to the limestone and slates of the Carboniferous formation. The latter are covered with dense bush, having a luxuriant undergrowth, but, when the mineral ground is reached, the bush terminates abruptly, and gives place to a succession of bare hills, whose rugged grandeur cannot fail to impress the least observant. The lithological character of the rocks is also strikingly different. Instead of regularly-stratified rocks, such as are found in the Port Hills, the Wairoa series, and the Maitai series, we have masses of dark horn-blendic rocks, diorite, serpentine, and dunite. Dunite, the typical rock of the Dun Mountain, is an olivine rock containing traces of chromium. The rock itself is crystalline, and of a yellowish-green colour; but where exposed to the weather its hue has changed to a rusty-brown; hence its appropriate name, dunite. It is this formation that contains the deposits of copper and chrome to which I shall refer more fully when dealing with the economic minerals of the district.
Reference has already been made to the mountain-forming character of the Maitai slates and the underlying Carboniferous limestone. I shall now attempt to describe how these rocks were formed, and how they came into their present position. Limestone, as most of you are aware, is of organic origin. Carbonate of lime exists in solution in sea-water. Certain marine animals have the power of extracting the carbonate of lime from the sea-water and of converting it into a solid substance, which they use as a protection or covering for their, bodies. When these animals die the shells in which they
lived are left behind, and by repeated accumulations of such shells vast deposits of calcareous rock are formed. All our great masses of limestone have been formed in this way, the coral polypi having had the greatest share in their formation. Try for a short time, then, to blot New Zealand as it is out of your memory, and conceive instead a coral reef in a tropical sea—something like the Great Barrier Reef off the coast of Australia. The sea-bottom is slowly sinking, but fresh layers of coral are formed, till several hundreds of feet have been produced. Then, owing to rapid subsidence or change of climate (or, perhaps, both), the work of coral-forming ceases, and layer upon layer of fine mud is deposited on the top of the coral-reef, till a thickness of several hundreds of feet of fine silt has been formed. This silt, of course, must have come into the ocean from the rivers of some adjacent land-area, and represents so much waste from that land. While this silt was being deposited the superincumbent pressure of its own weight, added to the weight of the ocean above, would consolidate it into a hard rock, and produce what is known as a clay-slate. Then an upward movement commences in the earth's crust beneath these rocks, and they are gradually raised till they stand thousands of feet above the level of the sea—not as we know them now, presenting an innumerable variety of landscape, made of valleys, and mountain-ridges, and mountain-peaks, but as a broad belt of elevated land. After a mountain-chain had been thus formed—or, probably, during the latter period of its formation—the pressure from beneath was so great that the overlying crust of limestone and slate gave way, and rock-matter, in a more or less plastic state, from the interior of the earth, was forced into the gap, thus giving rise to the mineral belt, which, as I have already stated, is composed almost entirely of crystalline rocks.
After the formation of the mountain-chain as a broad belt of elevated land, the work of denudation went steadily forward. The heat of the sun by day, the cold of frosts by night, the storms of rain, and the never-ceasing chemical action of the atmosphere began to soften and wear away the rocks. The rain-water, in its endeavour to reach the sea, would form watercourses, which, by the erosive action of the water, would deepen and widen their channels till rivers were formed. The rivers, especially in time of flood, aided by innumerable fragments of rock loosened from the parent rock in the manner already described, would continue to wear down the country, thus forming the broad and deep valleys so characteristic of the hilly parts of this district. When looking down into any of these valleys from some elevated spot, and remembering at the same time that the whole of the valley has been scooped
out of the solid rock by the action of the river, two forcible questions present themselves to the mind: First, what has become of the material which once filled the valley from crest to crest of the existing hills? Second, how long did it take the river to remove that enormous mass of matter? The answer to the first question is comparatively easy. The matter thus eroded has been carried down to the sea, the lighter particles floating far out into deep water, and these settling down to form fresh deposits of sedimentary rock, while the heavier portion would settle at the mouth of the river, and there form a delta. Beneath our feet at the present moment lies the débris of the rocks which once formed an integral part of the mountains near us. The greater part of the Town of Nelson stands upon the delta of the Maitai, while the delta is still extending seaward by fresh accumulations brought down by every flood. In the same way the Waimea Plain has been reclaimed from the sea by materials brought down from the hills by the Rivers Wairoa and Wai-iti. The second question, “How long did it take the river to erode the valley?” cannot be answered definitely, but an approximation may be made. By calculating the amount of sediment held in suspension by several rivers, and by taking into account the rate at which they flow, it has been found that a river lowers the area of its basin about 1ft. in two thousand years. Not knowing the mean depth of our river-basins, I am unable to make any estimate upon this basis; but by making the most liberal allowances, in a rough guess, at least hundreds of thousands of years would be required for the formation of many of our valleys.
Thus far I have dealt almost exclusively with the geology of our own immediate neighbourhood. The fear of making this paper too long prevents me this evening from touching, even in barest outline, on the interesting geological facts connected with the Owen, the Wangapeka, the Baton, the Takaka, and the Collingwood districts. I shall therefore close this paper by a brief reference to the minerals of economic value found in the provincial district, mentioning, as I pass, the geological formations to which they belong. First in importance are the coal-deposits. The coal-deposits of New Zealand belong to the Cretaceo-tertiary formation. In this respect New Zealand differs widely from other countries, where coal is usually associated with rocks of Carboniferous age. The Cretaceo-tertiary formation comes between the Miocene rocks of the Port Hills and the Triassic rocks of the Wairoa Gorge. Its principal areas of development may be seen by a glance at the map, where the parts coloured green indicate the presence of Cretaceo-tertiary rocks. These coal-deposits must in the future prove a source of great wealth to the district.
The Buller Coalfield alone is estimated to contain 140,000,000 tons of good bituminous coal; the West Wanganui Coalfield, 25,000,000 tons of pitch-coal and 12,000,000 tons of brown coal; while the Brunner Mine contains about 4,000,000 tons. To the sum of these must be added the coal contained in the Collingwood Coalfield, which has an area of about twenty square miles. The output of coal for the Nelson District is about 200,000 tons per annum.
Gold, the next mineral of importance, is found in many parts of the Nelson District, Collingwood and the West Coast having so far yielded the largest quantities. The gold in the Nelson District is usually found in the gravels and drifts which have been derived from the older metamorphic rocks. These rocks stratigraphically underlie the Maitai formation, and are coloured sepia on the map. The gravels and drifts in which the gold is usually found are coloured red or yellow, red representing the older gravels, and yellow the more recent formations. On the map only the larger deposits of these formations have been represented. There are innumerable patches of recent gravels found in almost every river-valley of the district, which could not possibly be represented on a map of so small a scale. It must be borne in mind, however, that only the gravels from the older formations are auriferous. The Maitai slates, for example, as far as is known at present, are not gold-bearing; consequently, the gravels derived from them are not auriferous. The gold exported from the Nelson District last year was valued at £16,000, while since 1857 to the present time six million pounds' worth of gold has been obtained from the Collingwood district alone. Copper and chrome are found in many parts of the Dun Mountain mineral belt, but owing probably to the want of scientific prospecting these minerals have not yet been discovered in sufficient quantities to pay for working.
Argentiferous galena—that is, silver-bearing lead-ore—has been discovered at the Owen and at Wangapeka; and at Collingwood silver, lead, zinc, nickel, antimony, copper, bismuth, and iron are known to exist, in addition to the gold already referred to. Plumbago is also found in the Collingwood district, but not sufficiently pure for commercial purposes. The iron in the Collingwood district belongs to the class of ore known as limonite, or brown hæmatite. This substance is at present made into paint; but probably the time is not very far distant when iron will be smelted from these valuable deposits of ore. At the Hope Saddle an interesting iron compound is found; it is known as vivianite. It is an iron-phosphate, and is of a bluish colour. A specimen of it will be found upon the table, together with specimens of all the rocks and minerals mentioned in this paper.
Such, in brief, is the description of the geology of our district. It is very imperfect; but enough, I think, has been said to show how full of interest is the neighbourhood around us,—what food there is for the mind in the study of the rocks, the rivers, and the valleys by which we are surrounded. In the language of the poet, we may find “Books in the running brooks, sermons in stones.”