Mineralogy of Sky. 179 



shale and sandstone, and, as we may conclude from the general 

 bearings of the strata already described, occupy the upper beds of 

 this formation. It is impossible to say that they do not exist at a 

 greater depth, since the inferior strata, as I have already shown, 

 can scarcely be considered sufficiently accessible to enable us to 

 determine on the absence of a substance of which the thickness does 

 not exceed a very few inches. Nevertheless, no indications of coal 

 can be perceived along the eastern line of cliffs, where the deeper 

 strata are exposed, and we may therefore for the present conclude 

 that they lie above the lias and its associated sandstone, or at least 

 among its uppermost beds. It can serve no purpose to enumerate 

 the places where these indications of coal have been observed, since 

 they cannot be verified on the map, and are indeed generally 

 nameless. As the strata rarely exceed an inch in thickness, it is 

 equally evident that those which are visible are worthless in an 

 economical view, while the certainty of a speedy interruption from 

 the intrusion of trap removes all temptation to penetrate to greater 

 depths, or to expend capital in a more effectual research. 



While on the subject of coal, I may add to the former account, 

 that I have observed portions of wood coal in more places than 

 those originally enumerated, but they are no where of sufficient 

 importance, or marked by any such peculiarities as to require 

 further description. 



In describing the seveial trap rocks of Sky, I am sensible of 

 having often spoken generally, when the circumstances might per- 

 haps have admitted of more accurate details. The cause of this 

 however is principally to be sought in the imperfect acquaintance 

 which geologists still possess with this infinitely varied and obscure 

 class of rocks, an obscurity which increased experience is daily 



