KAMBLES OF A GEOLOGIST. 315 



much larger proportion than the weaker ones, may Cyprina 

 itself have been preserved in much larger proportion than its 

 more fragile neighbours. Occasionally, however, escaped, 

 as if by accident, characteristic fragments are found of shells 

 by no means very strong, such as Hytilus, Tdlina, and 

 Astarte. Among the univalves I can distinguish Dentalium 

 entale, Purpura lapillus, Turritella terebra, and lAttorina lit- 

 torea, all existing shells, but all common also to at least the 

 later deposits of the Crag. And among the bivalves Mr Dick 

 enumerates, besides the prevailing Cyprina islandica, Ve- 

 nus casina, Cardium edule, Cardium echinatum, Mytilus 

 edule, Astarte danmoniensis (sulcata), and Astarte compressa, 

 with a Mactra, Artemis, and Tellina* All the determined 

 species here, with the exception of Mytilus edule, have, with 

 many others, been found by the Rev. Mr Gumming in the 

 boulder-clays of the Isle of Man ; and all of them are living 

 shells at the present day on our Scottish coasts. It seems 

 scarce possible to fix the age of a deposit so broken in its or- 

 ganisms, on the principle that would first seek to determine 

 its per centage of extinct shells as the data on which to found. 

 One has to search sedulously and long ere a fragment turns 

 up sufficiently entire for the purpose of specific identification, 

 even when it belongs to a well-known living shell ; and did 

 the clay contain some six or eight per cent of the extinct in 

 a similarly broken condition (and there is no evidence that it 

 contains a single per cent of extinct shells), I know not how, 

 in the circumstances, the fact could ever be determined. A 

 lifetime might be devoted to the task of fixing their real pro- 

 portion, and yet be devoted to it in vain. All that at pre- 

 sent can be said is, that, judging from what appears, the boul- 

 der-clays of Caithness, and with them the boulder-clays of 



* Mr Dick has since disinterred from out the houlder-clays of the Burn 

 of Freswick, Patella vulgata, Buccinum undatum, Fesus antiquus, Hostel- 

 laria, Pes pelicana, a Natica, Lutraria, and Ealanus. 



