Composition y and Analogies of Rocks. 37 



cessary for the support of that theory by which it was so anxiously 

 defended. 



Thus there have been produced two distinct sets of causes for 

 the formation of rocks ; the first chiefly applicable to the un- 

 stratified substances, and the last to the formation or consolida- 

 tion of strata. 



Mr. Playfair has objected to the possibility of aqueous consoli- 

 dation on these grounds ; that a liquid solvent could not exclude 

 itself from the pores of the rock after depositing the consolida- 

 ting matter, and that it should, therefore, remain within the 

 stone, or else leave the body pervious to water ; *' neither of 

 which is" said to be ** the fact." On the contrary, both of these 

 propositions are true. The presence of water in stones is so 

 universal, that I have never yet found any rock in which it did 

 not exist, when that could be procured quickly from a sufficient 

 depth. It is contained even in granite, and in the trap rocks ,* 

 and the great change of colour and hardness which many of the 

 latter undergo after being formed into specimens, is owing to its 

 evaporation. Thus specimens of augit rock, which have the 

 waxy soft look and green colour of serpentine, when fresh 

 broken, become black in a few days. It is also kno^vn, that small 

 granite veins are sometimes found perfectly soft in the quarry ; 

 and these harden in a few days, apparently by the evaporation of 

 their water, and the consequent precipitation of silica, or else 

 by the nearer approximation of their parts. In Sky, I have found 

 masses of granular quartz or sandstone, which could be moulded 

 by the hand when first found, but which, in the same manner, 

 became solid in a few days. In all these cases, the loss of 

 weight proves the presence of water, as it does the porosity of the 

 stones. Even the common quartz of veins contains water, under 

 the same circumstances ; losing both weight and transparency on 

 drying. The porosity of stones, as well as the presence of Avater, 

 are thus both proved by the same facts. But the former property 

 ought never to have admitted a doubt ; since the compactness 

 of flint and agate are apparently far greater than that of any 

 rock, compound or simple, and since these not only give pas- 



