FOSSILIFEROUS BOULDER-CLAYS. 5 



Fort William/' and " the proportion of Arctic species is less than at any of the other 

 localities."^ 



Mr. Jamieson gives the following explanation of the probable origin of the Caithness 

 fossiliferous Boulder Clay : — " 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 crashed 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 

 chiefly Boreal and southern forms. This would account for that mixture of species 

 which we observe in the Caithness list.^'^ 



4. The fauna, also in some of the fossiliferous Boulder Clays, is somewhat mixed. 



Foraminifera, e.^., are found in the Caithness Boulder Clays, which have been 

 derived from the wearing down of the oolite, lias, and chalk blocks they contain. In the 

 same clays there is also a mixture of southern, British, northern, Arctic, and North-east 

 American species of Mollusca.^ 



While this Boulder Clay possesses the general appearance of the Boulder Clay first 

 described, containing the usual striated and polished stones and being compact and 

 unworkable, these characteristics may fairly be described as, in many instances, not quite 

 so intense in their development, although often their only distinguishing mark is the 

 presence of shell fragments. 



Its peculiar position in cliffs near the shore, the occurrence of fossils, and its general 

 composition, seem to sustain the theory that in some cases it marks the point where the 

 debris of great glaciers was pressed to the bottom of the sea at their final point of 

 descent; and it may be of the same age as the shell-beds, which, in other localities, rest in 

 hollows of the unfossiliferous Boulder Clay ; while in other cases it may be an accumula- 

 tion dropped from icebergs, and in others a wash from an older bed during the final 

 re-elevation of the land. 



Without reference, however, to the method of its formation, as a matter of fact there 

 exists a fossiliferous Boulder Clay (1) not necessarily qI the same age as the unfossiliferous 

 Boulder Clay ; (2) with an included fauna more or less Arctic, although mixed and 

 fragmentary ; (8) chiefly developed in the neighbourhood of the shore, in the form of sea 

 cliffs ; (4) physically, not always distinguishable from that which underlies the Shell Clay 

 of the Clyde district, although sometimes marked by a diminution in the intensity of its 

 ice-marks. 



1 " On the Glacial Phenomena of Caithness," by T. F. Jamieson, F.G.S., ' Quarterly Journal of Geol. 

 Soc.,' 1866, vol. xxii, pp. 272, 2/3. 



2 Jamieson, paper cited, p. 278. 



