66 
acid must be got rid of before any deposition can take place, 
and this would easily be effected by evaporation, for on 
boiling a solution of this character, or leaving it exposed for 
some time to the air, the gas escapes, and the carbonate 
having nothing to hold it in solution, is deposited. The 
question, then, as to the manner in which carbonic acid 
escaped from these carboniferous seas, is very easily solved. 
There can be no doubt that it was by the same process as I 
have just described, — namely, evaporation. The quantity of 
water evaporating from the sea is always proportional to the 
amount poured into it by rivers and rain ; and any volatile 
body, like carbonic acid, contained in such water, would 
evaporate along with it ; but fixed ingredients, like carbonate 
of lime, would be retained. It is, consequently, evident that 
the carbonic acid would be diminishing, and the carbonate of 
lime increasing, as this evaporation proceeded, and as fresh 
portions would be brought by the rivers year by year, a 
solution would at length be obtained exactly saturated, the 
removal of a little more carbonic acid would then cause 
crystallization to take place, and this would proceed as long 
as these seas existed and the supply of water to them was 
kept up. 
I stated before that silica was rendered soluble during the 
decomposition of these silicious minerals; this substance 
would, consequently, find its way into the waters along with 
carbonate of lime, and might separate in a similar manner. 
Phosphate and some other salts of lime are also soluble in 
waters containing carbonic acid ; hence we are able to 
account for the presence of these ingredients in limestone 
in general. Metallic salts of different kinds would be gene- 
rated in the process of decomposition, and these would be 
carried in a soluble condition, or in suspension, and embedded 
in minute portions with the general mass ; — hence they are 
always detected on analysis. 
