206 ON THE ORIGIN, MATERIALS, COMPOSITION, 



silica. But that it was deposited from water, originally, 

 in the state of sand and gravel, is rendered evident 

 from the rounded and foreign fragments of discordant 

 rocks which it often contains. At the same time, 

 there is no reason to deny that it may have been 

 exposed to the action of heat, as it is still capable of 

 undergoing that without suffering any change. That 

 it was consolidated by heat we cannot prove ; and are 

 scarcely in a condition to deny that it may have been 

 partly indebted for its constitution to that cause. We 

 are indeed almost certain that quartz sand can be 

 converted into solid and continuous quartz by heat ; 

 since, as elsewhere noticed, analogous effects take 

 place in the vicinity of trap, and as, in Fife, a bed of 

 such quartz is found among the coal strata, in the 

 neighbourhood of that substance, probably indurated 

 from the state of an ordinary sandstone. 



If shale could be indurated from water alone, there 

 would be no reason to deny that the same cause may 

 have operated in the primary argillaceous schists ; 

 while, that they have been deposited from water, is 

 proved by the fragments and the shells which they so 

 often contain. Here again, however, we are in the 

 same condition as with regard to quartz rock ; unable 

 to prove that it may not have experienced, in some 

 degree, the action of heat, as we know, from observa- 

 tions on the siliceous schists, that shells are not 

 necessarily obliterated in these circumstances. But 

 that action, if it existed, cannot have been very great ; 

 as we are certain, both from experiment and observa- 

 tion, that it is either fused or indurated to siliceous 

 schist by this cause. The very existence of that sub- 

 stance in the vicinity of trap and granite, produced by 

 the action of these rocks on shale and slate, not only 

 prove this fact, but show the verv limit where the 



