Ch. XIV.] OF THE STRATIFIED ROCKS. 301 



mass. The changes thus exhibited may be regarded as the 

 results of a series of experiments, made on a great scale, by 

 nature, under every variety of condition, both as relates to 

 the mineral ingredients of the rocks, the intensity of heat or 

 pressure, the celerity or slowness of the cooling process, and 

 other circumstances." * 



We need not enlarge our details by relating the well- 

 known experiments of Sir James Hall on the fusion of rocks, 

 by which we learn that a melted rock assumes various ap- 

 pearances on consolidation, according to the rate of cooling. 

 We would, however, gladly enumerate the conditions of 

 sedimentary rocks at their point of contact with currents and 

 dikes of recent lava, that is, of undoubted volcanic rocks ; but 

 we do not remember that such have been recorded, for 

 although the interference of lava of the modern epoch with 

 strata has been described, yet such lava is ancient, and cannot 

 furnish evidence so important as that which is now actually 

 produced by existing volcanos. 



The altered appearance of sedimentary deposits next trap 

 is of very common occurrence, and we shall therefore quote a 

 few examples, in order to show the precise nature of this 

 phenomenon. 



At Duntulm, in the north-western part of Skye, an ob- 

 scurely columnar trap covers beds of shelly limestone and of 

 sandstone, containing shells and carbonised wood, alternating 

 with shale : the junction of the trap and strata is, in many 

 places, attended with great confusion. The upper part of 

 these strata, about twelve or fifteen feet in thickness, is divided 

 into thin laminae, which have the appearance of shale alter- 

 nating with limestone : but when fragments are broken off 

 and closely examined, the schist is found to be of a black 

 colour, very hard and brittle, sharp in the fragments, and with 

 an obscurely rhomboidal fracture ; in short, it is a kind of sili- 

 ceous schist, a Lydian-stone : the sandstone is found to possess 



* Principles of Geology, vol. iii. p. 367. 



