ARUAN. GEOLOGY. SECONDARY STRATA. 391 



of eartliy matters, or lastly, by unknown causes. It 

 is easy to comprehend how the varying form of the 

 bottom, the irregularity with which the substances were 

 deposited, the flowing of rivers from mountains of dif- 

 ferent materials, and the consequent deposition of dif- 

 ferent substances on different parts of one common bot- 

 tom, should have produced the diversities of composition 

 now visible, and occasioned the want of lateral uniformity 

 in the composition of any one stratum. As the magni- 

 tude of any one deposit is thus regulated by the shape 

 and extent of the cavity in which it has been formed, 

 so will any given collection of strata be more or less 

 extensive; while its lateral uniformity will depend on the 

 identity of the substances which have been deposited 

 over the whole of such a supposed surface. It is easy, 

 for example, to conceive that if a continuous stratum were 

 to be formed along the southern coast of England under 

 the present sea, the western parts would consist of sand- 

 stone and shale, from the ruins of granite ; while in 

 another place it might be of a calcareous nature, and 

 in a third of a still different composition. 



The last subject to be considered respecting these 

 secondary strata, is their analogy to those that correspond 

 with them in geological position on the continent of 

 Scotland. It will hereafter be shown in considering the. 

 estuary of the Clyde, that the sandstone of Arran is 

 a portion of the great mass which occupies the middle 

 district of Scotland and terminates on the west side of the 

 Mull of Can tyre. The conspicuous differences between the 

 two, consist in the greater variety of substances presented 

 by the former, and in the great proportion which these bear 

 to the simpler rocks that give the prevailing character 

 to the whole. Many of them however occur, although 

 in less proportion, elsewhere, as, for example, in Bute ; 

 and will perhaps be considered by some geologists as 

 mere variations of character, while by others they are 

 viewed as accidental and subordinate substances. With 



