the Formation of Rocks. 189 



by other circumstances. They regard such formations, how- 

 ever, merely as a mechanical sediment, and not as a chemical 

 precipitate from water. But, so far as I know, they do not ex- 

 plain whence they came. In order to be consistent, they must 

 admit that the transition and secondary limestones were at first 

 formed by fire, just as, in their opinion, the primitive limestone 

 was so produced, that they were then destroyed, reduced to a 

 powder by water, driven about for a certain period, and at last 

 deposited. But in this manner, we cannot account for the con- 

 stant increase of the masses in the newer members of this series, 

 without mentioning other difficulties. Here also we perceive 

 that the volcanic theofy leads from one perplexity into another. 



There thus remains no other opinion for our adoption, but 

 that which maintains that all carbonate of lime was at the be- 

 ginning dissolved in water, by the assistance of an excess of car- 

 bonic acid, and that, as the excess of acid was subsequently 

 separated, the carbonate was precipitated more slowly, and in a 

 more decidedly crystalline state, in the more ancient, but more 

 rapidly, and less perfectly developed in the newer period. 



It must here be remarked, that when carbonate of lime is se- 

 parated from a solution, it appears at first as a very bulky, 

 mud-like, and amorphous mass, that it remains for a time in 

 that condition, and only afterwards passes into a crystalline 

 powder, at which time it becomes contracted into smaller space. 

 But on the great scale, it could remain much longer in an amor- 

 phous condition, than on a small ; and, as a pasty mass, it could 

 bear along with it the substances mixed with it (chiefly sili- 

 cates), and these could crystallize freely in it. 



The occurrence also of clay, and the equal distribution of 

 that substance and of petrifactions in certain beds of secondary 

 limestone, become in this matter capable of explanation. Such 

 phenomena could not be accounted for if the carbonate of lime 

 had passed directly from the liquid condition to the crystalline, 

 and had at the same time been rapidly precipitated. 



It will, however, be asked, whence was derived the great 

 quantity of carbonic acid which served for the solution of the 

 neutral carbonate of lime ? This question does not embarrass 

 me, as I shall soon shew, when treating of the carbonaceous 

 series, to which I next proceed. 



