64 GENERAL COMPARISON OF 



the knowledge of that rock acquired by English geologists, 

 in consequence of its predominance in the southern parts 

 of Britain, has not only served to ascertain its discrimi- 

 nating characters, and to determine its connexions and 

 extent in this country, but has enabled them to trace it 

 in other places, under circumstances of more geneial 

 interest. 



It will be remembered, that in the Shiant isles there is 

 found a shale containing belemnites and lying immediately 

 under the trap ; and that in the same islands, thick beds 

 of grey shale occur together with beds of siliceous schis- 

 tus ; the circumstances under which the latter rock is 

 found being the same as those under which it appears 

 in Sky. There is in this case as exact a correspondence 

 between the two sets of strata as could be expected under 

 similar circumstances; and it is therefore reasonable to 

 conclude, that the stratified parts of these islands are 

 either detached portions of the same strata which in Sky 

 accompany the lias, or are still connected with these 

 beneath the sea. 



There is even a nearer correspondence between the 

 spheroidal sandstone of Egg with its accompanying strata, 

 and the upper series of Sky ; a similar general succession 

 occurring, though under slight modifications; while the 

 organic remains of both, with the particular stratum in 

 which these lie, and which I have in Sky shown to be 

 the upper portions of the lias, are perfectly identical. The 

 rocks which ought to be found below these, are here also 

 invisible ; partly from the interference of the trap, and 

 partly from the position of the strata as they relate to the 

 level of the sea; for which reason the comparison must 

 be limited to the instances here adduced ; the proofs of 

 identity being sufficient to establish the continuity of the 

 lias formation at least. The same reasoning may be ex- 

 tended to Muck, although the portion of strata found 

 there is so limited in extent and accessible to so small 

 a depth ; the limestones of that island being in every 



