330 GEOLOGY OF BUTE. 



The silica present is iu a state of mechanical mixture. 



These analyses are confirmatory of the main points of the 

 views already stated, and seem clearly to establish the new 

 and remarkable fact, that by the igneous action in these 

 instances the magnesia has been driven off from the lime- 

 stone. The unaltered rock is a dolomite containing nearly 

 70 per cent, of carbonate of lime, and nearly 20 per 

 cent, of carbonate of magnesia; while the altered rock 

 contains but from 1 to 2 per cent, of he latter in- 

 gredient. To what cause are we to assign the changes that 

 have taken place? Has the magnesia been sublimed by 

 heat? or has it been withdrawn by the solvent power of 

 free carbonic acid? On the nature of these and the other 

 chemical changes that have been induced, it is difficult to 

 express an opinion; and from such limited premises it 

 would be unphilosophical to draw any general conclusions. 

 The subject is one, however, of great interest both to the 

 geologist and chemist, as the facts are directly opposed to 

 the received views ; and as no instance of similar changes on 

 dolomitic rocks has, so far as we are aware, ever been put on 

 record. 



140. A remarkable dike intersects the sandstone between 

 Ascog and Kerrycroy, well worthy of attention on account of 

 the striking illustration it affords of the mode of cooling of 

 basaltic rocks from igneous fusion. It runs parallel to the 

 shore for some distance, and then retires from it towards 

 the south-west, and striking the inland cliff already mentioned, 

 whose direction is here the same as that of the dike, it forms 

 the perpendicular face of the cliff in front of the sandstone, 

 rising like a wall to the height of twenty or thirty feet. The 

 direction of the cliff soon changes, however, and the dike 

 then enters the hill behind, and is lost. The sandstone 

 having been completely worn away from the seaward face 

 of the dike, a very large surface of the side is laid bare, and 

 thus the structure is well exhibited, as in the annexed cut 

 (li*. 41). 



