GEOLOGY OP BUTE. 331 



The dike is composed of greenstone, is about fifteen feet 

 wide, and the prisms are mostly pentagons and hexagons. 



Fig. 41. 

 Side view of whin dike between Ascog and Kerrycroy. 



The cause of this peculiar structure has been already ex- 

 plained to be the parting of the heat of fusion laterally, per- 

 pendicular to the cooling surfaces, and the development 

 therefore of the prismatic structure in a horizontal direction. 



The Arctic Shell Beds. 



141. The north shore of Bute is classic ground in the history 

 of the glacial beds of Scotland. It was from an examination 

 of the shell beds found here that Mr. Smith of Jordanhill 

 was first led to recognize the arctic character of these deposits. 

 At Balnacaillie Bay, in the Kyles, opposite the Burnt Isles, 

 the beds occur first explored by him, still one of the richest 

 localities in the basin of the Clyde. A stream on the Bute 

 side of the Kyle has cut out the bed and exposed the shells; 

 the bed extends a considerable way up a hollow and along 

 the level shore. In such a locality great care must be taken 

 to distinguish the shells of the ancient beds from those of the 

 present sea, as well as those of one bed from those of another ; 

 since those successively newer overlap the older as we advance 



