Volcanoes. 



167 



the valley, as its excavation has destroyed the continuity of 

 the beds. Now, it is generally acknowledged that these val- 

 leys were formed by the last great diluvian action ; therefore 

 we have a convenient and natural division of extinct volcanoes 

 into ante and post diluvian. 



For the sake of illustration, let us suppose Jig, 59. to 



represent a chain of hills, in which the outliers «, h, and c 

 are composed of the same volcanic rock. It is evident, we 

 think, from their situation and dip, that they were once con- 

 tinuous, and that the catastrophe which excavated the valleys 

 destroyed the intervening portions of the bed : therefore the 

 valley is antediluvian. But if we find the cone d and its con- 

 tiguous valley to be covered with a bed of lava, there will be 

 no danger in ascribing to it a postdiluvian origin. 



It is not uncommon to find volcanic rocks, or, at least, 

 rocks which so much resemble those which are now produced 

 by volcanoes, that few geologists, reasoning by analogy, doubt 

 their igneous origin, alternating with, or interposed between, 

 stratified deposits. The rocks of this class are generally 

 known by the name of trap, and consist of greenstone, basalt, 

 porphyry, &c. The rocks of the Pic du Midi de Bigorre 

 "(Schubert), for instance, particularly the limestones, are gene- 

 rally superposed by trap. At Christiania, in Norway, slate 

 and grauwacke are covered by a bed of porphyry, not less 

 than 1600 ft. in thickness; " and in another part of the coun- 

 try, at Holmestrand, the same mass of porphyry, covering 

 beds of sandstone, is seen to pass in the lower part, by almost 

 insensible gradations, into a hard, fine-grained, black basalt, 

 containing brilliant crystals of augite." (Bakewell's Geology.) 

 In the Island of Skye, red sandstone is traversed by a great 

 number of trap veins, and in some places is superposed by 

 this rock. The alternation of rocks observed in Lamlash is 

 sandstone, trap, conglomerate, and clinkstone, the whole of 

 which is intersected by a vein of spheroidal trap, (Macculloch's 

 Western Isles.) 



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