[174] Trans. Edinburgh Geol. Soc., vol. v., 1888.
[175] Upham, one of the ablest and most experienced of the Glacial geologists in the United States, in a recent paper on the causes of the glacial period, states similar conclusions, and adduces the evidence of Gilbert, Andrews, Wright, Emerson and others in the same sense.
Still another question of great cosmic interest relates to the possible alternation of glacial conditions in the northern and southern hemispheres. There is evidence of drift in the southern part of South America, similar to that in the north; but was it deposited at the same time? If we could be sure that it was not, many difficulties would be removed. The southern hemisphere is at present emphatically the ocean hemisphere; the northern, the land hemisphere. Perhaps these conditions may be capable of being reversed, in which case the periods of depression in the south may have corresponded with those of elevation in the north. One thing which we know is, that there is a polar ice ring, not an ice cap, for we do not know what is within its edges at the South Pole, about 2,000 miles in diameter, and this in the only circumstances in which it can exist, namely, surrounded by a vast ocean furnishing it with abundant aqueous vapour. We also know that from this ice ring radiate glaciers, carrying débris, with which the sea bottom is strown half way to the equator. If continents were elevated out of the Southern Ocean, we should probably have on their surfaces glacial deposits more widespread and continuous than any remaining on the continents of the northern hemisphere, and like some of them thinning out to a terminal edge or border, instead of a terminal moraine like that of a glacier.[176] Thus we may say with some truth that the southern hemisphere is now passing through one phase of the Glacial period.
[176] This is now admitted by Chamberlain and others to be the case with the oldest boulder clay on the American continent.
I have often thought that in the southern hemisphere the condition of Kerguelen Island and Heard Island, as described in the reports of the Challenger,[177] must very nearly represent the state of some mountain ranges and peaks in North America in the Glacial age. Heard Island, in S. latitude 53° 2′, is a mountain peak 6,000 feet high, and 25 miles in length. It sends down large glaciers to the sea. In its larger neighbour, Kerguelen, the glaciers do not reach the sea; but there is evidence that at one time they did. It is still more curious that, in Kerguelen the modern ice overlies late tertiary deposits, holding remains of large trees, indicating a more continental condition and mild climate at no very remote period. The glaciers of Heard Island and Kerguelen have, no doubt, been carrying down moraine material into the sea, and this is certainly done on a still greater scale by those of the Antarctic continent. This sends off bergs which fill the whole ocean south of 60°, and float much farther north. Some of them have been seen 2,000 feet long and 200 high, and though most of the boulders they contain are necessarily concealed, yet masses of rock, supposed to weigh many tons, have been seen on them. The whole sea bottom off this continent, as far south as 64°, consists of blue mud, with boulders and pebbles, some of them glaciated, and farther north there is, as far as 47 degrees of latitude, a considerable percentage of drift material, and this sometimes in depths of 1,950 fathoms. It is evident that, if large areas of the southern hemisphere were elevated into land, we should have phenomena to deal with not much unlike those of North America at present.
[177] Vol. i. p. 370, etc.
Perhaps no discussion carries with it more of warning to geologists to exercise caution in framing theories than this of the great ice age; and if the collapse of extreme views on this subject shall have the effect of inducing geologists to keep within the limits of well-ascertained facts and sound induction, to adhere to the Lyellian doctrine of modern causes to explain ancient phenomena, and to bear in mind that most great effects involve not one cause, but many co-operating causes, it may lead to consequences beneficial to science; and so, emerging from the cold shadows of the continental glacier, we may find ourselves in the sunshine of truth.
References:—"Acadian Geology," 1st ed., 1855; 4th ed., 1892. Icebergs of Belle-Isle, and Glaciers of Mont Blanc, Canadian Naturalist, 1865. "Notes on Pleistocene of Canada," Montreal, 1871. Papers at various dates in the Canadian Naturalist and Canadian Record of Science. "The Ice Age in Canada," Montreal, 1893. Canadian Pleistocene, London Geological Magazine, March, 1883. Flora of the Pleistocene, Bulletin of Geological Society of America, vol. i., 1890, p. 311, Dawson and Penhallow.