/. Croll. — Boulder-clay of Caithness. 275 



glaciers ; but, in addition, it projects in the form of a headland 

 across the very path of the ice. Unless Caithness could have pro- 

 tected itself by pushing into the sea glaciers of one or two thousand 

 feet in thickness, it could not possibly have escaped the inroads of 

 the ice of the North Sea. But Caithness itself could not have sup- 

 ported glaciers of this magnitude, neither could it have derived them 

 from the adjoining mountainous regions of Sutherland, for the ice of 

 this county found a more direct outlet than along the flat plains of 

 Caithness. 



The shells which the Boulder-clay of Caithness contains have thus 

 evidently been pushed out of the bed of the North Sea by the land- 

 ice which formed the clay itself. 



The fact that these shells are not so intensely Arctic as those found 

 in some other quarters of Scotland is no evidence that the clay was 

 not formed during the most severe part of the glacial epoch, for the 

 shells did not live in the North Sea at the time that it was filled 

 with land-ice. The shells must have belonged to a period prior to 

 the invasion of the ice, and consequently before the cold had reached 

 its greatest intensity. Neither is there any necessity for supposing 

 the shells to be pre-glacial. Of late a great many facts have been 

 observed in connection with the glacial drift of Scotland, a number 

 of which have not yet been made public, that conclusively prove, 

 from the character of the organic remains found, that the glacial 

 epoch must have been broken up by several periods of long duration, 

 when the climate was mild and equable, if not positively warm. In 

 so far as Scotland is concerned, it would be hazardous to conclude 

 that a plant or an animal is either pre-glacial or post-glacial simply 

 because it may happen not to be of an Arctic or of a Boreal type. 



The same remarks which apply to Caithness, apply to a certain 

 extent to the headland at Fraserburgh. It, too, lay in the path of 

 the ice, and from the direction of the striae on the rocks, and the 

 presence of shells in the clay, as described by Mr. Jamieson, it bears 

 evidence also of having been overridden by the land-ice of the 

 North Sea. 



In fact we have, in the invasion of Caithness and the headla,nd at 

 Fraserburgh by the land ice of the North Sea, a repetition of what 

 we have seen took place at Upsala, Kalmar, Christianstads, and other 

 flat tracts along the sides of the Baltic. 



The scarcity, or perhaps entire absence of Scandinavian boulders 

 in the Caithness clay, is not in any way unfavourable to the theory, 

 for it would only be the left edge of the North Sea glacier that could 

 possibly pass over Caithness ; and this edge, as we have seen, was 

 composed of the land-ice from Scotland. We might expect, how- 

 ever to find Scandinavian blocks on the Shetland and Faroe Islands. 



Tine STietland and Faroe Islands Glaciated hy Land-ice. 

 It is also worthy of notice that the striae on the rocks in the 

 Orkney, Shetland, and Faroe Islands, all point in the direction of 

 Scandinavia, and are what would be affected by land-ice moving in 

 the paths indicated in the diagram. And it is a fact of some signi- 

 ficance, that when we proceed north to Iceland, the striae, according 



