Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments / by Sir Charles Lyell.

  • Lyell, Charles, Sir, 1797-1875.
Date:
1865
Catalogue details

Licence: Public Domain Mark

Credit: Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments / by Sir Charles Lyell. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    of marine shells is at once accounted for if we assume it to be of glacier origin. The rarity of angular stones, those in the till being usually rounded or sub-angular, and the number of fragments polished and striated on one or several sides, may also be explained by supposing the till to have been shoved along under a heavy mass of ice, like that of Greenland, instead of forming parts of superficial mo- raines, carried down without trituration on the top of the ice. If, in accordance with the views above set forth, we admit a second glacial period, when the land was re-elevated after the great sub- mergence, the action of ice at this later date may well be supposed to have obliterated almost all signs of the sojourn of the sea upon the land in the highest regions, where the cold was most intense ; but in the lower country, some patches of marine strata with arctic shells might more easily escape destruction. The greatest height to which marine shells have yet been traced in Scotch drift is only 524 feet above the level of the sea, at which eleva- tion they have been observed at Airdrie, fourteen miles south-east of Glasgow. At that spot they were found embedded in stratified clays with till above and below them. There appears no doubt that the overlying deposit was true glacial till, as some boulders of granite were observed in it, which must have come from distances of sixty miles at the least.* Fig.129. Astarte borealis. Fig. 130. Leda oblonga. Fig. 131. Saxicava rugosa. Fig. 132. Pecten tslandicus. Fig. 133. Fig. 134. Natica clausa. Trophon clathratum. Northern shells common in the drift of the Clyde, in Scotland. I The shells here figured are only a few out of a large assemblage of living species, which, taken as a whole, bear testimony to condi- tions far more arctic than those now prevailing in the Scottish seas. But a group of marine shells, indicating a still greater excess of cold, has been brought to light since 1860 by the Rev. Thomas Brown, from glacial drift or clay on the borders of the estuaries of the Forth and Tay. This clay occurs at Elie in Fife, and at Errol in Perth- shire ; and has already afforded about 35 shells, all of living species, and now inhabitants of arctic regions, such as Leda truncata, Tel- lina proxima (see figures, page 154.), Pecten Grcenlandicus, Crenella Icevigata, Gray, Crenella nigra, Gray, and others, some of them first * Smith, of Jordan Hill, Geol. Quart. Journ., vol. vi. p. 387. 1850.