350 Proceedings of the Royal Physical Society. 



really no valid argument against tlie land-ice origin of this 

 deposit. 



It might be argued that the shelly drift is a product of 

 coast-ice driven along the shore by currents ; but the evidence 

 derived from the organic remains is quite at variance with 

 such a hypothesis. It has already been stated that the most 

 careful searching has only brought to light a few specimens 

 characteristic of the littoral zone, while the great majority of 

 the shells belong to deeper water. Moreover, such a supposi- 

 tion leaves quite unexplained the gradual deflection of the 

 local ice in its eastward course, neither does it account for the 

 actual inland limit of the grey shelly boulder clay. Another 

 formidable objection to this h^^othesis, which is also applic- 

 able to icebergs or floe-ice, is the entire absence of stratifica- 

 tion throughout the wide area occupied by this deposit. Dr 

 Croll long ago pointed out that if the grey shell}^ drift were 

 really due to floating ice, it would undoubtedly have shown 

 signs of stratification. We know that the finely laminated 

 shelly brick-clays round the coast of Scotland, wdiich occa- 

 sionally contain striated blocks, point to aqueous disposition. 

 But those who have examined the numerous sections of the 

 grey drift in Caithness unite in saying that in physical char- 

 acter it is indistinguishable from ordinary boulder clay. Mr 

 Jamieson states that it " resembles the Old Boulder Clay of 

 the middle of Scotland in regard to its physical arrangement, 

 but differs therefrom in the prevalence of marine organisms 

 scattered through it." And in order to account for the 

 occurrence of these organic remains, he imagines that " a set 

 of marine beds containing Arctic shells were probably depos- 

 ited 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 the superficial debris into 

 that mass of rough pebbly mud which now overspreads the 

 surface." 



Xow, there is nothing improbable in the supposition that 

 such marine beds were deposited in pre-glacial or inter- 

 glacial times on the low ground of Caithness, though none 

 has been chronicled by Mr C. W. Peach, Mr Jamieson, nor 



