Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/949

 6. Inferences from the foregoing Facts — I conceive that we are justified in concluding from the data which we at present possess that the following were the changes which Scotland underwent during the glacial epoch : —

(α.) That after the Tertiary period the country was covered with a great depth of snow and ice, very much as in Greenland at the present day ; but possibly some of the mountain-tops appeared as islands. During this and the subsequent period glaciers ploughed their way down from the inland ice, and icebergs broke off and reached the sea through the glens, then ice-fjords. This glacier- covering must, to a considerable extent, have extinguished the pre-existing fauna and flora, though I do not agree with Mr. Jamieson that the flora and fauna were wholly extinguished. To this period we owe the " till," though I consider that this till was forming also during the subsequent period, and, in fact, as long as the country was swathed in ice and snow. All this period also the laminated clays were beginning to form from the clay-laden subglacial rivers.

(β.) After this the country sank gradually, as Greenland is now sinking, to the depth of several hundred feet ; and daring this period most of the glacial laminated fossiliferous clays were formed. During this period boulders were deposited from the icebergs broken off from the glaciers of Scotland, as well as from the icebergs and other floating ice drifted both from the north and south, as was also the case during the former (a) period. I consider now that the greater portion of the boulders and other moraine was deposited from home bergs ; for the fact seems often to be lost sight of by some theorists that bergs broke off from glaciers of the country, as well as floated south from Scandinavia. What the extent of this submergence was is yet sub judice. The extent of submergence in Wales seems to have been 1800 feet or more ; but in Scotland fossil shells of that period have never been found much above 500 feet, though Mr. Jamieson thinks he saw marine beds as high as 1500 feet. However, until we have more positive evidence, we are justified in concluding that from 500 to 600 feet was the amount of subsidence. It is very suggestive that, on comparing Lyell's Map of Britain sunk 600 feet1, the very parts under the sea are almost identically those on which the greatest amount of fossiliferous Boulder-clay is now found.

(ψ.) The country seems then to have emerged from the water, but no doubt slowly, until the glaciers finally left the country, unless, perhaps, as in Norway, in the mountains, though it appears that the rivers, from the melting of the ice and the glaciers themselves, had disarranged the beds considerably, leaving behind them much debris of rocks &c. 2

1 ' Antiquity of Man,' p. 287, fig. 40.

2 Hitherto this has been argued on hypothetical grounds ; but since this paper was written, in a memoir read before the Edinburgh Geological Society (May 6, 1869.), 'Transactions,' vol. i. p. 330, and ' Geol. Mag.' vol. vii. p. 296), "On two River Channels buried under the drift," by Mr. James Croll, of the Geological Survey of Scotland, this rise has been lifted out of the range of hypothesis into