]866.] JAMIESOI^ CAITHNESS. 273 



rows of Miia Uddevallensis, entire in their bnrrows, are found in 

 the line marine sediment overlying the Boulder-clay, and many 

 other facts of a like nature might be cited. Now if the Caithness 

 Drift is the equivalent of the Old Boulder- clay of the rest of Scot- 

 land, why do we not find some fine laminated clay above it, as we do 

 in other places ? 



It may be said that the mollusca of the Caithness Drift are an 

 older group than those found in the marine clays of the other parts 

 of Scotland, and therefore, although these mollusca lived during the 

 Caithness Drift, it does not necessarily follow that the latter deposit 

 is more recent than the Old Boulder-clay of the rest of Scotland. 

 An inspection, however, of the list given in the Appendix to this 

 paper does not suggest an older age, but the contrary. In order to 

 place this in a clearer light I have drawn up Table 2, in which 

 the geographical relations of the Caithness shells may be compared 

 with those of groups from the glacial beds of other parts of Scotland. 

 Thanks to the efforts of Mr. Peach, we have a very good list from 

 Caithness; moreover, the specimens have been all examined and 

 named by Mr. Jeffreys, an eminent authority in these matters, so 

 that the list may be used with all confidence. 



I think it is a fair inference that the more nearly a group of 

 British fossil mollusca resembles the assemblage of species now living 

 upon the shores of Britain the more recent is the period to which 

 that group belongs. I have accordingly ranged the groups in a 

 series, those which show the lowest percentage of British forms 

 being reckoned oldest. From this Table it will be seen that the 

 Caithness group is the most modern, except that of Fort William, 

 which in a former paper I had referred to the very close of the 

 Glacial-marine period (Quart. Journ. Geol. Soc. voLxxi. p. 174, 1865), 

 while the proportion of Arctic species is less at Caithness than at 

 any of the other localities — less even than at Fort William. It is 

 not pretended that all these groups represent distinct stages in the 

 (ilacial period ; several of them I have no doubt were contempora- 

 neous, but I think we are entitled to suppose that the Errol, Elie, 

 and other groups at the beginning of the list are older than the 

 Caithness and Fort William ones. Here, then, we have further 

 evidence to show that the accumulation, or at least the final arrange- 

 ment, of the Caithness Drift was a comparatively late affair; it 

 therefore ought not to be confounded with the Old Boulder-clay. 

 It seems to me that it ought to be referred to the Glacial-marine 

 period. A set of marine beds containing Arctic shells were probably 

 deposited over the low part of Caithness ; and much drifting ice 

 seems to have passed over the district from the north-west, which 

 crushed and destroyed these marine beds, broke the shells, and mixed 

 them up with other superficial debris into that mass of rough pebbly 

 mud which now overspreads the surface. These marine beds were 

 probably of different ages, the older containing Arctic species, the 

 later containing chiefiy Boreal and southern forms. This would 

 account for that mixture of species which we observe in the Caith- 

 ness list. 



