154 ARCTIC SHELLS IN SCOTCH DRIFT. [Ch. XII. 



a group of marine shells, indicating a still greater excess of cold, has 

 been brought to light since 1860 by the Kev. 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 Perthshire ; 

 and has already afforded about 35 shells, all of living species, and 

 now inhabitants of arctic regions, such as Leda truncata, Tellina 

 proximo, .(see figures below), Pecten Groenlandicus, Crenella laevigata, 

 Gray, Crenella nigra, Gray, and others, some of them first brought by 

 Captain Sir E. Parry from the coast of Melville Island, latitude 76° 

 N. These were all identified in 1863 by Dr. Torell, who had just 

 returned from a survey of the seas around Spitsbergen, where he had 

 collected no less than 150 species of mollusca, living chiefly on a bot- 

 tom of fine mud derived from the moraines of melting glaciers which 



Fig. 135. Fig. 136. 



Leda truncata. Tellina proximo. 



a. Exterior of left valve. a. Outside of left valve. 



o. Interior of same. &. Interior of same. 



there protrude into the sea. He informed me that the fossil fauna 

 of this Scotch glacial deposit exhibits not only the species but also 

 the peculiar varieties of mollusca now characteristic of very high lati- 

 tudes. Their large size implies that they formerly enjoyed a colder, 

 or what was to them a more genial climate, than that now prevailing 

 in the latitude where they occur. Marine shells have also been found 

 in the glacial drift of Caithness and Aberdeenshire at heights of 250 

 feet, and in Banff of 350 feet, and stratified drift continuous with the 

 above ascends to heights of 500 feet. There are, likewise, othei 

 deposits in Scotland very similar in character but devoid of shells 

 more than 1000 feet high, resting on rocks grooved and polished by 

 ice-action. The want of marine shells in these last has naturally 

 inclined some geologists to suspect that they may have been de- 

 posited in glacier lakes, and this opinion may be correct, although on 

 this subject there is no small danger of drawing false conclusions 

 from negative evidence, so partially do organic remains occur in gla- 

 cial formations even in those of indubitably marine origin. When 

 the gravel and sand are of a porous nature, we can easily account for 

 the decomposition of the shells and their total disappearance in the 

 course of thousands of years, but a large part of the Scotch till is so 

 impervious to water that the absence of fossil testacea leads us rather 

 to suspect that it was originally the moraine of a terrestrial glacier, 

 and, therefore, from the first devoid of shells. 



I formerly suggested that the absence of all signs of organic life in 



