CHEMISTRY OF THE EARTH. 



195 



and to diorites aud dolerites on the other. These correspoud to the two 

 chesses of igneous rocks designated by Bunsen as the trachytic and 

 pyroxenic types. 



§ 28. There is, however, a third source of silicated rocks, to wliich 

 some alkision has ah*eady been made in speaking of the production of 

 magnesian silicates by direct precipitation, as the result of chemical 

 changes in solutions. In this way have been formed, besides these and 

 rehited protoxide silicates, other silicates, including alumina. This base 

 in certain conditions as yet but imperfectly understood, passes into 

 solution in water, and has given rise to complex silicates, including pro- 

 toxide bases. As I have elsewhere exjiressed it, not only steatite, pyrox- 

 ene, hornblende, and serpentine, but chlorite and, in many cases, garnet 

 and epidote, have had their origin in the crystallization and molecular 

 re-arrangement of natural silicates, generated by chemical processes in 

 aqueous solutions at the earth's surface. To these must be added other 

 silicates, containing alkalies, chiefly potash, such as glanconite, and a 

 hydrous silicate of alumina and potash which has the composition of 

 pinite or agalmatolite aud forms beds in the sedimentary rocks of dif- 

 ferent geological periods. Evidences abonud of the solution of alumina, 

 and of the generation, as chemical precipitates, of various aluminiferous 

 silicates. These, like the similarly-formed protoxide silicates, are in 

 most, if not all cases, highly basic, and moreover, from the mechanical 

 conditions of their production and deposition, are found associated and 

 even intermingled with the finely-divided basic sediments of mechanical 

 origm. The aluminous silicates thus formed, though mineralogically 

 important, are probabh' small in amount when comj)ared with the great 

 mass of argillaceons sediments. 



§ 29. The chemical changes which are wrought in the silicated ro(;ks 

 diuing their mechanical disintegration are, as we have seen, chiefly the 

 elimination of the alkalies, especially the soda, in a soluble form from 

 its ahiminons componnds, and the separation and accnmnlation of the 

 oxide of iron. The decomposition of the silicates of lime and magnesia 

 which takes place is, to a great extent, compensated for by the regene- 

 ration of similar compounds by the reaction already explained, but the 

 mean composition of the argillaceous sediments of any geological epoch 

 will depend not only ui)ou the age of a formation, but upon the number 

 of times which its materials have been broken up, and the length of the 

 periods during which they have been exposed, in an unmetamorphosed 

 condition, to the action of water, carbonic acid, and vegetation, If, how- 

 ever, we may assume that this action, other things being equal, has on 

 the whole been most comj^lete in the newest formations, it is evident 

 that the chemical and mineralogical composition of different systems of 

 rocks must vary with their antiquity, so that we may find in their com- 

 parative stud}' a guide to their respective ages. Silicious deposits, and 

 chemical precipitates, like the carbonates and silicates of lime and mag 

 nesia, may exist, with similar characters, in the geological formations of 

 any age,* not only forming beds apart, but mingled with the less perme 

 able silico-aluminous sediments of mechanical origin. Inasmuch as the 

 <'hemical agencies giving rise to these compounds were then most active, 

 they may be exiiected in greatest abundance in the rocks of the earlier 

 periods. In the case of the more permeable and more highly silicious 

 sediments already noticed, (§ 21,) whose principal elements are silica, 

 alumina, and alkalies, the deposits of different ages will be marked 

 (•hiefly by a progressive diminution in the amount of potash aud in the 

 disappearance of the soda which they contain. In the oldest or least 



* Geology of Canada, Report, 1666, page *230. 



