GEOLOGY OF BUTE. 339 



then have grounds for a safe generalisation. Meanwhile we 

 may note the great significancy of the contrasts we have 

 stated, in regard to any theory of its origin. It is persistent 

 in Clydesdale and the sheltered shores of the frith ; and it 

 exists at Errol, in Perthshire, an inland sheltered situation ; 

 but it is absent on the shores of Aberdeen; at Lochgilphead, 

 a situation once exposed, on an open frith ; and is not found 

 on the exposed shores of Arran, where also the state of the 

 shells in the bed that takes its place is very different from 

 that in which they are found in places where the laminated 

 clay occurs. From this clay to that above it the change is 

 abrupt everywhere. New conditions were suddenly intro- 

 duced all over the district. Animal life was rapidly and 

 richly developed on the sea bottom ; on this pecular horizon 

 the sediment became less fine, was more rapidly let down, 

 and it was charged in many places, but unequally, with 

 stones, dropped no doubt from floating ice. The life period, 

 as indicated by the fauna of the shell bed, is remarkable, 

 and proves clearly the prevalence of a severe climate; but 

 precise data are yet wanting in regard to successive stages in 

 this period. Yet are there indications of a greater severity 

 at first, and a gradual amelioration afterwards, during the 

 progress of which those species made their appearance which 

 still remain with us, though rarely found; and the conditions 

 became assimilated to those which now prevail; the place of 

 the Arctic species being gradually occupied by that assem- 

 blage which now fills our teeming waters. 



