OEIGIN OP CEYSTALLINE EOCKS. 59 



to the surface, there to be deposited, would be not unlike those which have been removed, 

 by infiltrating waters in various subsequent geological ages, from erupted masses of similar 

 basic rock ; which, we have reason to believe, are but displaced portions of this same primary 

 layer. The mineral species removed from these latter rocks, or segregated in their cavities, 

 are, as is well known, chiefly silica in the form of quartz, silicates of lime and alkalies, and 

 certain double silicates of these bases with alumina, including zeolites and feldspars, besides 

 oxyds of iron and carbonate of lime ; the latter species being due to the intervention of 

 atmospheric carbonic acid. The absence from these minerals of any considerable propor- 

 tion of iron-silicate, and, save in rare and exceptional conditions, of magnesia, is a significant 

 fact in the history of the secretions from basic rocks, the transformation of which, under 

 the action of permeating waters, has resulted in the conversion of the material into quartz 

 and various silicates of alumina, lime, and alkalies, while leaving behind a more basic and 

 insoluble residue abounding in silicated compounds of magnesia and iron-oxyd with 

 alumina. 



§ 118. The peculiarities resulting from this comparative insolubility of magnesian 

 silicates long ago attracted the attention of the writer. The addition, to solutions like sea- 

 water, of bicarbonate of magnesia, which is a product of the sub-aerial decay of basic rocks, 

 would, it was shown, effect a separation of dissolved lime-salts in the form of carbonate, 

 leaving the magnesia in solution as chlorid or as sulphate ; while on the contrary the action 

 of such a natural water with certain silicates, whether solid or in solution, containing lime 

 or alkalies, would effect a removal of the dissolved magnesia. At the same time it was 

 shown that, " by digestion at ordinary temperatures, with an excess of freshly precipitated 

 silicate of lime, chlorid of magnesium is completely decomposed, an insoluble silicate of mag- 

 nesia being formed, while nothing but chlorid of calcium i"emains in solution. It is clear 

 that the greater insolubility of the magnesian silicate, as compared with silicate of lime, 

 determines a reaction the very reverse of that produced by carbonates with solutions of the 

 two earthy bases. In the one case, the lime is separated as carbonate, the magnesia remain- 

 ing in solution, while in the other, by the action of silicate of soda, or of lime, the magnesia 

 is removed and the lime remains. Hence carbonate of lime and silicate of magnesia are 

 found abundantly in nature, while carbonate of magnesia and silicate of lime are produced 

 only under local and exceptional circumstances. It is evident that the production from 

 the waters of the early seas of beds of sepiolite, talc, serpentine, and other rocks in which a 

 magnesian silicate abounds, must, in closed basins, have given rise to waters in which 

 chlorid of calcium would jpredominate." '* 



§ 119. From this reaction it would follow that the magnesian salts, formed when the 

 first acid waters from the atmosphere fell upon the primary stratum, would be removed 

 from solution, either by the direct action of the solid rock, or by that of the pectolitic 

 secretions derived therefrom in the earliest ages. The primeval ocean, if, as we sup- 

 pose, a universal one, would soon be deprived of magnesian salts, and henceforth the 

 early-deposited rocks would be essentially granitic in composition, a nd uon-magnesian, 

 until the introduction of magnesia into its waters from an exterior source. 



The pectolitic silicates, themselves, which, in the cavities of exotic basic rocks, are 

 deposited in crystalline forms, would, if set free in a sea deprived of magnesian salts, be 



'™ Amer. Jour. Sci., 1865, vol. xl., p. 49 ; also Chem. and Geol. Essays, p. 122. 



