Review.

Otago Witness, Issue 1273, 22 April 1876, Page 7

Review.

Report an tlie Geology and Ooldfields of Otago (by F. W. Hu^ 0 ?. JfGKS., C.M.Z.S, Provincial Geologist, and G. ' H F. Ulhich, F.G.S.), with Appendices, &■?., &c. Published by order of the Provincial Council of Otago. A text-book of tho geology of Otago has long been a recognised want. Large sums of money have been expended by the Provincial Government with a view to the encouragement of geological science, but with Very meagre results, so far as the diffusion of looal knowledge has been concerned. Captain llutton'a " Keport" is, therefore, specially welcome ; and this not only be« cause of the importance of the subject whereof it treats, hub also as being indicative of a dea're to impart to the public the information obtained at the public expense. The easy colloquial style in which it Fa written adds considerably to its value. Technical phraseology has judiciously been, as far »s possible, avoided, so that the most Unlearned student of practical geology may •without; difficulty master its contents. This la ia itself no slight commendation, for the majority of scientific treatises are usually so t)h«cure'd by technicalities aa to be unintelligible to the general reader. The well* tsouoeivefl map which accompanies the •• Report" will also be approbated, and deservedly so. for it afForda a useful and frlost necessary guMe to the reader in ascertaining the appriocimtttn boundaries of the Variouß formations ; and it is worthy of note that this fa the only chart'of the kind that ban ever been published. A manuscript map was, indeed, prepared some years ago by Or Hector, with the aaßiataace of Mr J. M'Kerrow, tho original of which is in the Otago Museum } but it ie rather a physical than a geological map, Rhowiug the bush aad open o mntty, but without giving any geological details. It has never Been given to the public in any available form, and the map accompanying Captain Hutton'fl " Report" may 1 , therefore, strictly Bpeaking, be Regarded as the first; geological map of the Province. The only fault to be found with it >8 that jib is rather top (jmall— the scale

being 24 miles to the mch — and although it may be that, as remarked in the introduction, "a larger one would be unnecessary and inconvenient when bound up with tho •Report,'" it is certain that, for the purposes of practical explorers, a somewhat larger map would be of greater use. On this point, however, Captain Hutton says :—: — ' ' Before any detailed geological survey can be made of the Province, it will be necessary to prepare a topographical map on a scale of not less than one inch to the mile, showing all the physical features of the ground, for it is simply a waste of time and money attempting to put geological details on to a map that does not show the true position and shape of every hill." Such a map cannot be prepared a day too soon, and the attention of the Government may well be directed to the desirableness of providing for the work, not only in respect of Otago, but of the entire Colony — tho immense mineral wealth of which is as yet but very imperfectly known, and not even generally suspected.

The first part of the " Report," which is Captain Hutton's contribution, deals with the geology of Otago, and is divided into seven sections, headed respectively — " Physical Geography," " Previous Observers," "General Geological Structure," "Descriptive Geology," " Historical Geology," "Surface Geology," and "Economic Geology" — an orderly arrangement, which enables the reader to open tho book at those pages which he mostly desires to consult, an object in which he is further aided by a very copious "General Index;" whilst an "Index of Localities" facilitates reference to any particular part of the Provinse touching which he may require special information. The latter was, indeed, a necessary adjunct to the work, owing to the plan adopted of describing the forma tions in stratigraphical order, and not by districts.

The Section on Physical Geography conveys in brief space a surprising quantity of information respecting the chief natural features of the Province, especially indicating their many striking points of divergence from those of coi responding features in other countries. Thus, the lakes of Otago, unlike those ot Switzerland, "do not lie outside the principal mountain masses, but wind themselves close around their feet ;" the sharp serrated summits of the New Zealand Alps are exceptional in the European Alps ; whilst the numerous waterfalls which one sees at every turn in the latter, are rarely met with in the former. In Otago, also, the courses of the rivers conform to the curve of the ellipse, contrary to the generally accepted opinion of geographers, which is that " the parabola is the curve which rivers tend to assume from their sources to the sea." If, however, Mr Thomson's theory be correct, that the ellipse is the curve assumed when rivers are hollowing out their beds, and the parabola that to which they tend when raising them, it would seem that the former operation is still in progress in our rivers. Another remarkable fact is that the larger rivers do not run in what must be considered their natural channels, but constantly cross abruptly from one channel into another. Exemplifications of this are afforded by she courses of the Clutha River, the Taieri, the Pomahaka, and the outlet of tho waters of Lake Wakatipu. In reference to the first-named of these, Captain Hutton tells us that "the Clutha, after arriving at Cromwell, ought to have continued straight'on and joined the Frazer, instead of cutting through the Dunstan Range." There is a slight error in this. The Frazer itself cuts through a mountain range instead of following its natural valley, and if the country at the head of the Bannockburn valley were examined, it is believed that the olu course of the Clutha River would there be discovered in a low Baddle communicating with what is now a dry channel known as Deadman's Gully. But this is a small matter.

These and other anomalies are dealt with at greater length in the section on Surface Geology, in which Captain HuttoD explains his views on the Physiography of the Province. He appears to doubt whether it would not have been wiser to have kept back some of the ideas put forth in this Section until he was possessed of more facts whereby to teat their accuracy. But herein we mußt differ from him ; for, in the firat place, their publication will, as he suggests, have the beneficial effect of stimulating research, and,, moreover, we incline to the opinion that he does himself an injustice. Anyone who is at all acquainted with the physical features of the country, and who has given the slightest attention to its geological structure, must admit the correctness of the data upon which Captain Hutton bases his deductions, and with theae deductions most scioutific observere will find it neees. sary to aglee. The gkcial origin of the lakes ia now very generally admitted, and the existence of the ancient lake-beds that occupy bo considerable a proportion of the interior must be attributed to the same active cavses. A passage in which is de. scribed the formation of the rock basins, wherein these lakes and lake- beds lie, affords a good illustration of the simplicity of the language employed throughout t.he " Report" — a point to which reference has previously been made : "A glacier hollows out a rock-basin in this way. The terminal moraine at the end of a glaciar covers up the rocks all across^ the valley, and prevents further denudation taking place at that point. But above it the moving ice still keeps wearing away the rocks below it, and if this action is sufficiently long continued, the elope of the valley down which the flUoier is raoving may in time be reversed j the cohesion between the upper and lower portions of the ico enabling the upper portions, which stiil retain their original slope, to drag the lower portions up hill along with thorn, And when the glacier melts away, a rook basin is left behind which forms a lake. 5 ' Having illustrated this theory by reference to the physiography of the interior, Captain Hutton proceeds to account for the changes in the system of drainage that took place after the disappearance of the glaciers -such, for instanoe, as caused the Maniototo Lake to overflow into Strath Taieri, the confluent waters of the Hawea and the Wanaka to force a channel through tho Dunstan Gorge, and the Wakatipu to seek an outlet through the Arrow Bluff — by supposing the natural channels to have been dammed up with ice until the lakes found other outlets upon their, lowest margins, by the long con.

tinuance of which process the gorges through which they now find their way to the ocean were eventually formed as we see them. The structural formation of the Blue Spur, and the probcible deposition of its auriferous gravels and conglomerates during the eocene period, are thus referred to : — The Blue Spur, says Captain Hutton, is " a deep cup-shaped hollow in the schist-rocks. . , . On close inspection this hollow is seen to have been an old mountain tarn, with smooth polished sides. . . . I have no doubt but that it was excavated by a glacier. This old rock basin is filled up with beds of conglomerate that dip to the east, and as a rule the stones in the conglomerates get smaller towards the east, which, together with the direction of the dip, prove that the old tarn was filled up from the west. The conglomerates consist in great part of pebbles and sub-angular rocks of green quarlzite, and a dark purple jasperoid slate, with quartz veins, which rocks do not exist nearer than the Tapanui mountains, west of the Clutha River." He then points out that patches of these conglomerates occur at Waitahuna and Manuka ©reek, &c, thus indicating the former existence of a valley extending from the Tapanui mountains, through the Blue Spur, to Kaitangata. "No appearance of such a valley can, however, be seen at present, and its supposed course is now crossed at right angles by the Clutha, and by the Tuapeka and Waitahuna rivers." Hence he argues that " this old river must date back to a time previous to the formation of the present valleys," and that, as the Clutha "received the drainage of the pliocene glaciers of Maniototo, Idaburn, and the Manuherikia, we must place the existence of the Blue Spur glacier during a still earlier period of upheaval — that is to say, we must refer it to the eocene period." The auriferous conglomerates of the Blue Spur have therefore, he thinks, " been poured by the river into the old mountain tarn after the retreat of the glaciers, caused by the subsidence that took place previous to the deposition of the Oamaru formation." That there is nothing extravagant in this supposition is shown by facts observed in other localities. "In the valley of the Waitaki we have collateral proof that eocene glaciers existed in New Zealand, and that gold existed in the rocks during the eocene period, we have proofs in the auriferous sandstones of the Shag River, and in the auriferous conglomerates over the coal at Coal Point." We may add that similar auriferous deposits have been found overlying the coal formation at Evans Flat in the Tuapeka district, and in other localities.

We have devoted so much space to this interesting and attractive branch of the subject that our further remarks must neces sarily be of the briefest. The Sections dealing with Historic Geology, however, demand special notice. In the preceding Sections, which treat of the general geological structure of the Province, and the descriptive geology of the several formations found within its limits, Captain Hutton has massed together all the facts at present known, that in any way bear upon the subject ; and from their consideration he deduces " the causes to which they owe their origin," or, in other words, the geological history of the Province. Told by his own graphic pen, this history may be briefly epitomised as follows :—

During the Eozoic and Palaeozoic eras — our knowledge of the rocks rpertaining to which is confessedly limited — it is believed that New Zealand formed a very subordinate part of a large continent probably stretching far away to the northward. At the commencement of the Triaasic period of the Mesozoio era this continent began to subside below the sea. It is probable that this subsidence was not very great, and that t«e southern portion of the continent was not quite submerged, when another slight elevation, accompanied by an out-burst of volcanic action, took place. This again was followed by subsidence. Towards perhaps the middle of the Jurassic period the whole of the country was again elevated, and the chain of the New Zealand Alps was formed. This elevation was part of a large movement, which probably resulted in the upheaval of a continent, how far extended we have no means of knowing, but that it probably extended to South America is shown by all the formations later than this upheaval containing fossils related to, or identical with, those of Patagonia or Chili. Since this upheaval the New Zealand Alps have never been submerged.

Again this continent subsided, and at the olose of the Meaozoio em the north part of this island was 6000 feet, and Otago was 3000 foet lower than at present. New Zealand was then reduced to a narrow chain of islands, and from that time to the present it has always remained isolated from any large continental area. At quite tbe com mencement of the tertiary era, New Zealand was once more upheaved, and it was this lower eocene upheaval that gave the final form to the internal geological structure of the Alps. But at the close of the eocene period Otago was again depreosed to about 1600 or 2000 feet below its present level, so that the sea then penetrated as far as the Te Anau and Wakatipu Lakes. It was at this period of depression that volcanic action burst out with such energy in the neighbourhood of Dunedin and other places. During the middle miocene period Otago was again raised slightly above its present level, and in the upper miocene period it was once more considerably depressed, and Otago must have then stood at least 3246 feet lower than at precent.

From the absence of marine rooks of the older pliocene age, and the existence of marine shells in claya of the new«r pliocene and the pleistocene periods, Captain Hutton draws the inference that "thete can have been no gonrral elevation of the land extending over a long period since pliocene times." The subsidence which followed the last elevation was, he tells us, "continued until Otago stood at a much lower level than at present, and it Was subsequently followed by an elevation which may probably still be going on." In Bupport oi thio opinion numberless corroborative facta are adduced. Here he joins issuo with DrHeotor, who, in hie " Geological Exploration of the West Coast of Otago " argues that a gradual subsidence of the Sounds ia taking place. After carefully weighing tho evidence respectively urged in favour q

these antagonistic views, we incline to the opinion that the balance rests with Captain Hutton, and that the data upon which Dr Hector relies are insufficient to establish the theory of subsidence.

For obvious reasons, we cannot now more fully examine this question, and indeed, we have throughout purposely omitted the arguments advanced by Captain Hutton in support of the views which he advances, for the reason that it is not possible to give to them the consideration which they merit within the narrow limit of a general review. It is but right, however, to say that ample justice has been rendered in the " Report," to the works of previous observers, whose contributions to local geological science are collated and recorded in the second Section.

It only remains to make mention of the Section relating to Economic Geology, which deals briefly with agricultural geology, and enlarges on the coaljdeposits, building stones, and other economic products of the Province. Herein he deals with facts rather than theories, and the reader must be referred to the "Report" itself for fuller information. Of the goldfields Captain Hutton says but little, an omission which may be accounted for by the publication, in the same volume, of Mr Ulrich's valuable report. The latter has been for some time before the public, so that any extended reference to it here is unnecessary. The points upon wh ; ch Captain Hutton principally dilates in connection with this subject are consistently of a geological character. Inasmuch as he has adopted the theory that the old lake basins are referable to glacial action, he is consequently of opinion "that 720 derp leads exist along the bottom of them, and that the auri'erous gravels are confined to those parts of the margins of the old lakes immediately adjoin* ing the mouths of the streams that ran into them. It must, however, be remembered that in many cases these streams did not fall into the lake at right angles to it, but often very obliquely, and the gravels, therefore, would be carried in the direction of the streams, and not straight across the lake. We must also remember that during the course of the gravels down the river the gold gets separated from its matrix, and falls to the bottom, where it gets collected together, but it is very gradually pushed down the stream during floods by the violent movement of the gravel over it. But when these gravels and gold get pushed into the lake they get scattered over the fan- shaped deposit forming at the mouth of the stream, some being pushed one way and some another, and behig now in still water there is no movement to bring the gold together ; consequently the gold in these deposits is not collected in such definite leads as in the beds of rivers, and it is seldom that these deposits would be rich enough to pay except by hydraulic sluicing, consequently all these deposits, when situated below the surface drainage of the country are useless. " How true this is those who are acquainted with the results of the costly experiments involved in searching for imaginary " deep leads " at Wetherstones and Waitahuna can testify.

In juxta-position with those views Capt. Hutton places those of Dr Hector, whose theory it was that the old lakes had been formed principally by the subsidence of the mountains, and who held that riyerß play a very minor part in liberating tbe gold from the matrix. Upon these points we have no hesitation in expressing our entire concurrence with Captain Hutton, and so, alf-o, we venture te say, will most practical miaers who have ever copsidered the subject. Indeed all the information acquired by the experience of the last fifteen years points to the same conclusion, and we cannot but think that were Dr Hector to investigate the discoveries that have been made in Otago since he first pronounced his opinion, he would himself arrive at the like results.

The Appendices to the "Report" contain much information of a useful character, respecting the Bibliography of Local Geology, and the Minerals and Fauna of Ofcago. A Table of Altitudes is furnished by Mr M'Kerrow, and Professor Black contributes Analyses of Rocks and Minerals Some of the Plates are also fairly meritorious -and may be regarded as of greater utility than mere embellishments. Amongst these, two clever akefcehes of Milford Sound and Mount Aspiring, from the pencil of Mr W. M. Hodgkins, and a very good one of the marvellous terrace formation of the Upper Clutha. There are also a number of sectional diagrams elucidatory of the text, and indeed the desire to render the "Report" that whio'd it really is — a valuable work of reference — is very perceptible throughout.