242 ON THE DESTRUCTION OF ROCKS. 



der it incumbent on geologists to search for evidence 

 capable of determining with more precision what the 

 truth is. 



Such is the importance of this view, and so ex- 

 tensive the consequences to which it leads, that I 

 shall not be disappointed if geologists should refuse 

 their assent to it ; though the facts next to be re- 

 corded are equally satisfactory, and are at least free 

 from the suspicion of supporting any hypothesis. 



The chief part of the Orkney islands consists of an 

 alternating series of sandstone and shale, belonging 

 to the lowest, or old red sandstone of geologists. In 

 some places, the sandstone is red, but, like the shale, 

 it is more generally of a dark gray colour. But 

 there are also beds of a yellow and tender kind, 

 exactly resembling some of those which occur among 

 the upper secondary strata, and, at first, leading to a 

 belief that such a series exists here. It is only after 

 much examination that the true nature of this rock is 

 discovered ; when it is perceived to arise from a 

 change in the irqn of the blue strata, which thus be- 

 come tender as they acquire the yellow colour. Act- 

 ing on this hint, it will remain for geologists to in- 

 quire whether similar changes may not have taken 

 place in many other cases, where the tender and 

 yellow state is supposed to have been the original 

 condition of the strata. 



This deep decomposition is frequent in the trap 

 rocks ; and, in some of these, it leads to important 

 practical consequences, while it gives rise to geo- 

 logical suspicions of no small interest. The very 

 deep and rich soil of some parts of Scotland, which 

 lies above sandstone, is evidently derived from this 

 source, and, apparently, from an entire resolution of 

 the compact trap that has once covered it. In Sky, 



