THE SCHISTOSE ISLES. 297 



found. Beds of limestone are also to be observed, but 

 always of very limited extent, and the summit level of 

 the Crinan canal presents one accessible example of this 

 nature. 



HAVING thus finished the examination of the only 

 subordinate group which required further illustration, 

 it is necessaiy to proceed to a more general comparison 

 of the whole with each other, and with those parts of the 

 neighbouring mainland to which they seem to belong no less 

 in geological identity than in geographical position. It is 

 not necessary to notice the causes commonly assigned for 

 these and similar geographic distinctions. Whatever 

 expedients may be adopted for the explanation of the 

 common features of hill and dale, of mountain and lake, 

 the same are, with some exceptions, sufficient to explain 

 the relations of islands to each other and to the neigh- 

 bouring continents. It is in this case no ground for 

 distinction that the valley is so much deeper as to admit 

 the sea, and that the mountain summit is insulated among 

 the surrounding waters. 



The comparisons necessary for ascertaining the sequence 

 of rocks throughout such a district as would exist were 

 the bottom of the sea now exposed to view, must be 

 drawn from the correspondence in position of the several 

 rocks as they occur in the different islands, here to be 

 considered as interrupted portions of one mass ; and 

 from tracing analogous or identical successions of rocks 

 in those parts where the visible continuity is more 

 extensive, and where less frequent interruptions take 

 place. The latter source of comparison is so open and 

 so little subject to error, that it requires no explana- 

 tion. The former is less capable of rigid examination, 

 as the proofs must be taken with some little latitude, 

 arising from imperfections which do not at present admit 



