Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 2?9 



admitting this example to be correctly described, it cannot 

 prove that all igneous rocks, including granite, have operated 

 in the same manner, in opposition to the numerous instances 

 which have been adduced, to show the possibility of the 

 inclined position of the strata having had another mode of 

 origin. But we think it not improbable that the supposed 

 metamorphic schist is only a foliated variety of the lava ; for 

 we know that this configuration occurs both in the masses 

 and veins of basalt, greenstone, syenite, porphyry, and other 

 intrusive igneous rocks. " It is occasionally combined with 

 the columnar form," says Macculloch, " and takes place 

 either transversely or parallel to the axis of the prism. If 

 often obscure and limited to small portions of an amorphous 

 mass, it sometimes occupies a considerable space, and is so 

 distinct, that the dark blue claystones, thus constituted, might 

 be almost mistaken for clay-slate ; though, as formerly 

 remarked, it is seldom perceptible but on surfaces under the 

 influence of the weather. " * " The foliated structure of the 

 lava-rocks," says Poulett Scrope, " is often accompanied by 

 the columnar, of which it is wholly independent." " A re- 

 markable example of this united structure occurs in the 

 Roche Tuiliere, in the Mont Dor. The clinkstone of which 

 this rock is composed (an insulated fragment of the vast 

 current descending from the Puy Gros) is regularly divided 

 into nearly vertical columns. It is so extremely schistose (as 

 the vulgar name of the rock implies), that its laminae are 

 used as slates for roofing, "f If our conjecture has any 

 foundation, then we can readily account for the position of 

 the strata, by the existence of the highly inclined igneous 

 rock before their deposition. 



Now, the first part of these details, concerning the trap- 

 rocks, accords well with the phenomena of existing volcanos; 

 and it may therefore be concluded that the analogy between 

 these igneous rocks is so far complete : but the second part of 



* System of Geology, vol. ii. p. 138. f On Volcanoes, p. 143. 



T 4 



