Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/106

 œconomy, 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 description 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 columnar, 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, (Pl. 2, fig. 3.) It is divided into thin laminæ, of which the upper ones alternate with similarly thin lamina 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 examined 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