306 GENERAL COMPARISON OF 



rated by geologists. There is nevertheless sufficient evi- 

 dence to prove an enormous thickness ; since if the con- 

 secutive strata of Isla alone be considered, they will amount 

 to many miles ; their examination thus giving access to 

 considerable depths in the crust of the earth. 



But it is necessary to consider in greater detail the 

 nature of the changes from which the actual position of 

 these insular strata has resulted, and how far they are 

 connected with the present forms of the land : with the 

 elevations or the depressions which have produced those 

 hills that constitute the islands, and those vallies which 

 are now the beds of the sea. 



By taking a line transversely on the bearing of the 

 strata, which shall pass, for example, from the mainland 

 through Gigha to Isla, or through Shuna and Luing to 

 Lunga, a set of sections can be produced that may be 

 indicated by those lines in the accompanying diagram* 

 which lie above the black horizontal one representing the 

 sea. To produce such a series of mountains or islands, 

 four fractures of a set of horizontal strata may be imagined, 

 accompanied by as many subsidencies, or elevations ; 

 since the same effects will result in either way. In this 

 ease, those parts of the diagram which lie below the hori- 

 zontal line, will represent the original strata ; and the 

 directions according to which they must have moved to 

 produce the present appearances, will be indicated by the 

 circular arcs. It is plain, that in a similar manner, cases 

 more complicated may be represented, this being the most 

 simple example of a common dip. 



Should the preceding statement appear probable, it 

 follows, as just hinted, that the present thickness of the 

 collective strata of any number of hills or islands, laterally 

 placed, is not a certain measure of their original depth ; 

 since it may result from the repetition of fragments of a 

 much more limited number in a new position; and this 



* Plate X XIII. % 2. 



