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1]\TK01>UCT0RY ESSAY. 



13 



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which they derive their impregnation, are subjacent 

 to those niinerals which appear as the external 

 crust of Scotland. The waters have not been 

 carefully analyzed: in taste, and other qualities, 

 they resemble those of Cheltenham, and they are 

 occasionally employed with effect to answer the 

 same medicinal purposes. 



From what has been said, it will appear that 



the great mass of the'island consists of calcareous 

 matter combined with carbonic acid ; but that the 

 hilly district is composed principally of argil, or 

 of argil and silcx, frequently blended with ferru- 

 ginous or bituminous matter, and containing frag- 

 ments of rocks of apparently much older formation. 

 After careful inspection, I am quite satisfied that 

 the argillaceous substances rest on the calcareous 



4 



or coralline mass, which constitutes the exterior 

 crust of the greater part of the island. 



It cannot be doubted that the calcareous forma- 

 tion, which has so frequently been mentioned, has 

 originated in the submarine operations of insects 

 belonging to the order of Zoophytes; and that the 

 various modifications of carbonate of lime, by 

 which the corallines are cemented, have been de- 

 rived from these substances acted on by water. 

 This island, which was once undeniably under the 

 surface of the ocean, now rises considerably above 

 it, the coralline rock, of which its highest elevation 

 consists, reaching between eleven and twelve hun- 



