358 SKY. GEOLOGY, SILICEOUS SCHIST. 



satisfactory example occurs in the mass of rock under 

 Duntulm Castle ; in which also it is evident that the 

 influence of the trap extends through a considerable space 

 from the immediate contact.* That influence is however 

 palpably variable in extent; a circumstance not difficult 

 to explain when we consider the relatively variable bulks 

 of the stratified and of the interfering rock, and the other 

 numerous variations to which their connexion may have 

 been subject. 



The strata in question form a mass, of which the base is 

 covered by fragments, but the visible thickness is about 

 twelve or fifteen feet. It is surrounded on all sides by 

 the mass of obscurely columnar trap under which it lies, 

 the junction being in many places attended with great con- 

 fusion. The upper strata alternate with similarly thin beds 

 of an indurated sandstone, the whole precisely resembling 

 those alternations of shale and sandstone which occur 

 along this shore. It is not indeed till fragments of the 

 rock are examined in the hand, that the spectator can 

 imagine he sees any thing but a bed of shale alternating 

 with sandstone : but on thus examining the schist, it is 

 found to be an extremely brittle and hard substance, of a 

 black colour, giving fire freely with steel, sharp in the 

 fragments, and with an obscurely rhomboidal fracture com- 

 bined with the conchoidal ; this last character being the 

 only one by which it can be distinguished from the fine 

 grained basalts, particularly from those which occur in the 

 form of veins in the Cuchullin. The sandstone laminae 

 possess at the same time the hardness and jaspideous 

 aspect of that which I have described in the Geological 

 Transactions as lying in contact with the greenstone of 

 Stirling Castle. Considering therefore the analogy of these 

 two sandstones both in aspect and position, we may fairly 

 conclude that they have in both these instances been al- 

 tered from their original texture, in consequence of the 



* Plate XVI. fig. 2. 



