SKY. GEOLOGY. OVERLYING ROCKS. 377 



appearing indeed to be portions of clay which have 

 undergone changes, in consequence of their vicinity to 

 the basalt, resembling those which sandstones experience 

 in similar situations.* 



The succession of these several substances is often 

 found in the same place, but their order can never be 

 determined, as, from the cliffs rising to a height of 500 feet 

 or more, they are so far out of reach as to prevent us from 

 forming an accurate judgment respecting the individual 

 parts. It is only by examining the fallen specimens 

 that the varieties in any spot can be ascertained. They 

 seem more numerous from Loch Bracadale to Loch 

 Brittle than elsewhere, but possibly this may arise 

 from the greater facility experienced in examining 

 this line of the coast. The number of strata seem to 

 vary from eight or nine, to twelve, fifteen, or even more ; 

 but it is not easy to define their boundaries at the distance 

 from which they must be viewed. The manner in 

 which the several beds decompose often adds a very 

 remarkable feature to the cliffs : some become scoriform, 

 others moulder into large cavernous shapes, while a third 

 set fall to powder; and these various appearances com- 

 bined with the colours of the iron clay, give to the 

 whole that aspect of having undergone the action of fire 

 which strikes a common observer even more forcibly 

 than a geologist. Besides these, siliceous schists 

 of various kinds occur among the beds of stratified 

 trap, occasionally extending for a considerable space. 

 It is by them indeed that the divisions of the strata appear 

 often to be determined, and they perhaps offer the clue 

 to explain this disposition. It has been shown in de- 



* I have since received similar specimens from Guadaloupe, where 

 they occur among the lavas of that island, adding one more to the 

 numerous analogies already existing between the volcanic rocks and 

 jthose of the trap family. 



