on American Geology. 85 



a progressive change in the composition of sediments, and shown 

 how the gradual removal of alkalies from aluminous rocks has led 

 to the formation of argillites, chloritic and epidotic rocks, at the 

 some time removing carbonic acid from the atmosphere, while the 

 resulting carbonate of soda by decomposing the calcareous and mag- 

 nesian salts of the ocean, furnished the carbonates for the formation 

 of limestones and dolomites, at the same time generating sea salt.* 



Closely connected with these chemical questions is that of the 

 commencement of life on the earth. The recognition beneath the 

 Silurian and Huronian rocks of 40,000 feet of sediments analogous 

 to those of more recent times, carries far back into the past the 

 evidence oftheexistenceofphysical and chemical conditions, similar 

 to those of more recent periods. But these highly altered strata 

 exclude, for the most part, organic forms, and it is only by applying 

 to their study the same chemical principles which we now find in 

 operation that we are led to suppose the existence of organic life 

 during the Laurentian period. The great processes of deoxydation 

 in nature are dependent upon organization ; plants by solar 

 force convert water and carbonic acid into hydrocarbonaceous 

 substances, from whence bitumens, coal, anthracite and plum- 

 bago, and it is the action of organic matter which reduces 

 sulphates, giving rise to metallic sulphurets and sulphur. In 

 like manner it is by the action of dissolved organic matters 

 that oxyd of iron is partially reduced and dissolved from great 

 masses of sediments, to be subsequently accumulated in beds of 

 iron ore. We see in the Laurentian series beds and veins of me- 

 tallic sulphurets, precisely as in more recent formations, and 

 the extensive beds of iron ore hundreds of feet thick which abound 

 in that ancient system, correspond not only to great volumes of 

 strata deprived of that metal, but as we may suppose, to organic 

 matters, which but for the then greater diffusion of iron oxyd in con- 

 ditions favourable for their oxydation, might have formed deposits 

 of mineral carbon far more extensive than those beds of plumbago 

 which we actually meet with in the Laurentian strata. 



All these conditions lead us then to conclude to the existence 

 of an abundant vegetation during the Laurentian period, nor are 

 there wanting evidences of animal life in these oldest strata. Sir 

 "William Logan has described forms occuring in the Laurentian 



* Am. Journal of Science (2) xxv. 102, 445. xxx. 133 ; Quar. Journal 

 Geol. Soc. XT. 488, and Can. Naturalist, December 1859. 



