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hardening. But these limestones do not present the charac- 
ter of having been so washed and deposited ; they are all 
crystalline, and, consequently, must have been deposited from 
solution. Clay and sand, in all their forms, are instances of 
washed deposits. On examining them with a microscope, 
their particles always present a rounded and water-worn 
appearance, the true crystalline structure being destroyed. 
It is, therefore, evident that we must find some other ex- 
planation, more consistent with facts, to account for the 
formation of limestone ; and it so happens that a process is 
going on at the present day, of the same nature (though 
limited in extent) to the one by which these rocks were 
produced, namely, the solution of carbonate of lime by 
waters containing carbonic acid gas. This solution may 
easily be effected by passing a stream of carbonic acid 
through water in which chalk or limestone is suspended, 
as in the manufactory of Carrara water. 
I have shown before that the quantity of carbonic acid 
in the air, prior to the depositions of these rocks, must have 
been exceedingly great, and this gas being easily soluble in 
water, would be condensed by every shower. The rain which 
then fell would probably be a saturated solution of carbonic 
acid, and what could be the effect of this but solution — not 
only the formation, but the subsequent solution, of the car- 
bonate of lime produced as above. In this state it would 
ultimately find its way into the then existing seas, and the 
water of these seas would shortly become a strong solution 
of carbonate of lime in carbonic acid, but the quantity of the 
latter might be considerably greater than the amount neces- 
sary to dissolve the carbonate of lime. This, in fact, would 
undoubtedly be the case, if we may judge from the composi- 
tion of such waters as we now find them in nature, for in 
every instance with which I am acquainted the acid is in 
excess. It is, consequently, clear that a quantity of carbonic 
VOL. III. e 
