248 



THE GEOLOGIST, 



entry in Nature's accounts satisfies us of the correctness of our con- 

 clusions ; and we must always find the double- entry if we wish to 

 verify our speculations — they are of no reliance without. 



ISTow 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 magnesia, leaving behind the silica, alumina, and potash ; and as 

 the clavs become more and more sandy and permeable, the action of 

 the filtering waters will be greater, 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 modern 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 o^y^Ll, the alkaline waters have been derived by which the rock- 

 constituting silicates have been carried down to the sea in the form 

 of carbonate of soda to be transformed by the chloride of calcium 

 (lime) parting with its chlorine into the chloride of sodium, or 

 common salt, while the carbonic acid was Kberated from the soda to 

 combine with the calcium, whence the formation of carbonate 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 hme- 

 stones we find intercalated at various epochs mtli 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 

 oiow than in primeval times, because the soluble soda-silicates 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. 



