248 
THE GEOLOGIST. 
entiy in Nature's accounts satisfies iis of the correctness of our con- 
elusions ; and we must always find the double-entry if we wish to 
verify our speculations — they are of no reliance without. 
Now the springs issuing from clay strata are characterized by a 
predominence of bi-carbonate of soda with the bi-carbonate of lime 
and magnesia, for the atmospheric waters charged with carbonic 
acid gas, in percolating the rock-masses, remove the soda, the lime, and 
the magTicsia, leaving behind the silica, alumina, and potash ; and as 
the clays become more and more sandy and permeable, the action of 
the filtering waters will be gi'eater, while the finer and compact clays 
resisting the penetration of water, will retain their soda, lime, and 
magnesia. On these principles the chemical composition of the 
ancient sediments must have been throughout all geological periods 
constantly changing, and hence it is when we examine the consti- 
tuents of the ancient rocks, we find them, where preserved under 
favourable circumstances, containing much soda, the reverse of which 
is observable in similar more modem formations ; and ancient as are 
those old Laurentian rocks, they were doubtless derived from the 
ruins of other rocks in which the proportion of soda was still greater. 
And it is from the detritus of the felspar-constituents of those 
old primitive rocks all over the globe that, from the paleozoic era to 
our own, the alkaline waters have been derived by winch the rock- 
constituting silicates have been carried down to the sea in the form 
of carbonate of soda to be transfonned by the chloride of calcium 
(lime) parting with, its chlorine into the chloride of sodium, or 
common salt, while the carbonic acid was liberated from the soda to 
combine with the calcium, whence the formation of cai'bonate of lime 
in the same water, the mechanical deposition and segregation by or- 
ganic agency of which have given rise to the great masses of lime- 
stones we find intercalated at various epochs with the other strata 
of the earth's crust. 
Out of this investigation we derive another conclusion on another 
point, namely, that the decomposition of rocks is much less rapid 
7WIV than in primeval times, because the soluble soda-sihcates are less 
abundant, and because the " quantity of carbonic acid, so powerful an 
agent in these changes, has been diminished by the formation of 
various limestones and of coal. 
