64 
explain the manner in which these rocks were formed when 
the process of cooling had been accomplished. 
It is well known that when silicates of lime, potash, &c, 
are exposed for a long time to the action of water contain- 
ing carbonic acid gas in solution, the silicates are completely 
decomposed, giving rise to a carbonate of these bases and 
gelatinous or soluble silica. This is a process which is 
slowly going on at the present day in all our cultivated soils, 
and is the one by which alkalis and silica are rendered soluble 
for the supply of crops. The carbonate of lime found in the 
springs rising in sandstone and granite, has its origin in this 
decomposition ; and it would be no difficult matter for me to 
prove that at least two cwt. of silicates are thus decomposed 
in each acre per annum ; for the product of the decompo- 
sition of two cwt. is required for the growth of each acre of 
wheat, and a much larger amount probably escapes by drainage. 
If, then, decompositions to this extent take place in 
our own time, when the quantity of carbonic acid in the air 
and falling rain is so small, how much more rapid must have 
been such decomposition when the proportion of carbonic 
acid was so much greater, and the quantity of moisture 
condensed would bear no comparison to the showers of 
the present time ! We have sufficient data to warrant us 
in concluding that the atmospheric changes were then very 
extensive; consequently torrents of rain and mighty floods 
would be the result, and these combined would wear away 
the crystalline structure of the igneous rocks, all of which 
are found to consist of alkaline, earthy, and metallic silicates, 
to an extent beyond our conception. We have seen that 
the result of the decomposition of silicate of lime would be 
carbonate of lime and silica. Now, carbonate of lime is an 
insoluble salt, and might easily be washed away by the 
streams and rivers, and embedded as mud at the bottom 
of the then existing seas, and so form a mass of rock on 
