98 Dr. Mac Culloch's Sketch of the 



oeconomy, since the perpetual interference of the trap rocks and 

 the disturbance occasioned by them, would render it too expensive 

 an undertaking 



Among other substances I have reserved to this place the descrip- 

 tion of the siliceous schistus which is to be seen at Duntulm, on 

 account of its partial nature and because its history would have 

 interrupted the connection of the geological details. I have already 

 mentioned that the trap found in this vicinity is obscurely colum- 

 nar, forming high and picturesque ranges of cliffs surmounting the 

 hills and extending to the sea shore. I have also noticed in general 

 that beds of shelly limestone and of sandstone, containing shells 

 and carbonized wood, alternating with shale, are seen under it. But 

 the most interesting appearance is that of a disrupted portion 

 of a thick stratum of the schist, known by the name of siliceous 

 schist, and of that particularly hard and black variety which has 

 been called Lydian stone. This rock forms a portion of a bed the 

 base of which is covered by the fragments of the shore, but its visible 

 thickness is about twelve or fifteen feet. It is surrounded on all sides 

 by, and lies under, a mass of obscurely columnar trap, the junction 

 being in many places attended with great confusion, (PI. 2, fig. 3.) 

 It is divided into thin laminae, of which the upper ones alternate 

 with similarly thin laminae of sandstone, precisely resembling those 

 alternations of shale and sandstone which are so common and 

 so well known. It is not indeed till fragments of the rock are ex- 

 amined in the hand that the spectator can discover that 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 ; this last 

 character being the only one by which it can be distinguished from 



