OVERLYING AND TRAP ROCKS. 133 



already mentioned, to prove that origin for these 

 rocks, which I need not now repeat. I have else- 

 where noticed some minor appearances of an analo- 

 gous nature, such as the acquisition of a granular or 

 concretionary structure by the shales ; while I may 

 here add, that the sandstones sometimes undergo an 

 analogous change, but in which the spheroids are of 

 an enormous size ; a fact also occurring in the shales 

 and the argillaceous limestones : while in rare cases, 

 the sand of the former becomes locally arranged, yet 

 imbedded, in a form coincident with the faces of a 

 quartz crystal ; marking, what is important in the 

 history of silica, the power of heat to produce its 

 crystallization, as I have indeed elsewhere proved by 

 direct experiment. To these proofs of the action of 

 heat, are superadded marks of violence, often ex- 

 tremely conspicuous. If I formerly observed that 

 fragments of the including strata were often entangled 

 in the neighbouring and intersecting rock, they are 

 often of enormous size, while they also bear the 

 marks of further fractures, and of curvature, as in 

 Sky ; some splendid examples having been figured in 

 the account of the Western isles. That such effects 

 are the result of violence, is proved by the different 

 action exerted on yielding and on refractory materials; 

 the one, as I have formerly remarked, being bent, 

 while the other is broken. The same circumstances 

 occur in the veins ; the passage of which through 

 soft or through hard rocks, or in directions parallel 

 or tranverse to their natural divisions, are indicative 

 of a-force similarly exerted, but varying in its effects 

 according to the opposed resistance. In both these 

 cases, and in the veins more particularly, the dis- 

 placed and insulated parts could often be refitted if 

 we could restore the fluidity of the trap : the corre- 



