DEEP LEADS IN OTAGO.

Star (Christchurch), Issue 392, 16 August 1869, Page 2

DEEP LEADS IN OTAGO.

The .following letter has appeared in the Otago Daily Times:— I have read with great interest the observations of . Dr. Hector on deep leads, and as they support opinions so totally adverse to my own convictions on the subject so lately published by you, I feel bound to show that in anticipating their existence in Otago, I do not go on mere empirical grounds. I will first shortly recapitulate the opinions of the above geologist, for which I have the highest respect, though I differ from the same. 1. He does not anticipate that any deep leads, in the sense in which that term is applied in Victoria, can be ever found in Otago; but in 1862, at Wetherßtones, he urged that Government should aid endeavours to reach a deep lead in order to settle the point. 2. Depressions in Otago completely rockbound. ■ 3. Inequality in the ensuing subsidence of mountains. „ 4. Valleys occupied by lakes. Under this condition, detritus would be deposited in deep. water, and form deltas advancing into lakes. 5. The evidence of gradually increasing erosion, as found in the series of terraces which skirt the. more elevated basins. 6. Gold cannot be expected to occur in definite leads at a greater depth than the

level of the lowest notch' which occurs in the rocky rim which surrounds the basins. 7. Systems of action evidenced by the Reesand Dart rivers on Lake Wakatip at this present time. 8. Conclusion — not to recommend prospecting for the discovery of deep leads. I may observe that the principal cause of diversity of opinion between us lies in the theory of physical action — Dr Hector adhering to a system of great inland lakes (clause 4), while my convictions lead me to support a system of receding ocean estuaries. On the final determination of this difference will depend the true theory of the depositing of gold in Otago on the surface merely, or also in deep leads. To commence at the root in a discussion of this kind, I must remind your readers that this world is not exactly round, the polar axis being 26£ miles less than the equatorial diameter ; and on the surface of the world land rises several miles in height and water reaches several miles in depth. It has also been proved that ocean beds have several times been dry land, and dry lands several times been but beds of the ocean. Speculations as to the causes of this have nothing to do with our present subject. I only, from these facts, wish to have the rising and sinking of land on a general and not local principle admitted. Further, it is a well proved fact that the temperature of this world changes. This is a fact, patent not only to geologist?, but to historians. How great may be the range of temperature in a geological cycle, we need not inquire. All that need be admitted is, that the temperature here during the glacial age of Europe was sufficiently low to produce the same effects and extension. Glaciers, to have floated over the surface of this island — as their drifts prove them to have done — must have required that the. land was then 3000 to 4000 feet lower than it is at present; for icebergs, the offshoots of glaciers, require 300 to 1500 feet of water to float them, according to the quantity of ice above water; and our interior plains are 1000 to 1500 feet in elevation. Then how would our territory appear during the continuance of the most active part of the glacial age ? The western mountains would appear 3000 to 4000 feet above the level of the sea. The Dunstan and Hawkdun mountains would be large islands, and the Kakanui mountains would appear above the waters only as an islet, all covered with snow. Glaciers 1000 to 3000 feet in dt-pth would be seen lying on the flanks and valleys of the White Ranges, extending their spurs into the ocean. At this period, the Te Anau, Wakatip, Hawea, and Wanaka Lakes were solid ice, which, having the motion common to glaciers, ground down the sides and bottoms of their respective valleys, and whose lowest extremes stretched out into the ocean, annually de'tacbing and floating off icebergs, with their accumulated boulders and detritus. These would be spread and arranged by the oceanic and estuary currents, and also be driven by the prevailing winds. And what would be these ? Examine the remnants in the Valley of the Clutha, and the plains, and you will see. At the same time a very important action for collecting and distributing gold into deep leads would be going on, viz., the action of the under currents of the glaciers. These would be the first agents to effect this, and at a time when, as stated before, the western lakes and fiords were solid ice. Then, where would this first golden harvest be deposited ? at and beyond the extreme lower ends of the glaciers. Where would these reach ? At least 10, 15, and even 30 miles from the 4000 feet level of the present age. If this theory be correct, then the first and richest deposits ; and at the same time, the deepest, will be found in a curved and varying line, stretching from the eastern base of the Hunter mountains across the Five Rivers, Waimea, and Poniahaka plains ; across the bed of the Clutha above and below Balhousie ; thence by the eastern flanks of the Lammerlaw to Kyeburn. In this, let me be understood that I only attempt to indicate the principle, but I maintain that with this principle as a clv?, 12 months' observation and study, given solely to the action of the glacial moraines and drifts, would enable any competent enquirer to indicate with some accuracy, where and to what places the " prospecting " for deep leads by practical men should be directed. I will only make a passing remark on the processes of the age subsequent to the glacial, as these processes affect only the alluvial diggings. During this latter age the land rose gradually to its present level, and in bo doing ranged the drifts in terraces, which are higher as you approach the mountains, and lower as you approach the outlet of the estuaries and rivers, till they are bounded by the level sea beach. This process is wonderfully accurately imitated by an out-flow of water on a receding tide, and maybe actually observed on any day. The principles of nature are the same in small things as in great. — I am, &c, J. T. THOMBOK, ■•••.- .- F.R.G.S., F.R.S.S.A., &c.