ORIGIN OF CEYSTALLINB ROCKS. 61 



atmosphere holding- carbonic acid, resulted in the more or loss complete removal of the 

 alkali from the feldspars of crenitic rocks, and their conversion into kaolin, while the corres- 

 ponding changes in the basic exoplntonic rocks v^^re still more noteworthy. These rocks, 

 while containing- feldspars, consisted in largo part of silicates of lime and magnesia, pre- 

 sumably pyroxene and chrysolite, which, as we are aware, yield to the action of the atmos- 

 phere the whole of their lime and magnesia. These, in the form of carbonates, passed into 

 solution together with a large proportion of silica, leaving behind the remaining portion, 

 together with nou-oxyd and the kaolin from the feldspars. The carbonates of alkalies, 

 of lime, and of magnesia, resulting from the subaërial decay of the exposed exoplntonic 

 and the crenitic rocks alike, were carried to the sea, there to play an important part. 

 Besides the direct influx of carbonate of lime into the waters of that time, it is evident that 

 both the alkaline and the magnesian bicarbonates would react upon the calcium-chlorid 

 of the primeval sea, with the production of a farther amount of lime-carbonate, and the 

 generation of alkaline and maguesian chlorids. In this way, the sea becoming magne- 

 sian, a new order of things was established. Henceforth, the pectolitic matters brought 

 up from the primary layer would at once react upon the dissolved maguesian salts, and 

 the production of such comiiounds as chondrodite, chrysolite, serpentine, and talc tvould 

 commence. No one who has studied the mode of occurrence of these silicates in the upper 

 part of the Laurentian series, where serpentine not only forms layers, but frequent concre- 

 tions like flints, often around nuclei of white pyroxene, can fail to recognize the process 

 which then came into play, resulting later in the production of abundance of pyroxene, 

 hornblende and enstatite, and apparently reaching its culmination in the vast amount of 

 maguesian silicates found in the deposits of the Huronian age. 



§ 122 The solutions of simple silicates of alkalies, which by heat had deposited their 

 excess of silica in the form of quartz, as in the case of the soluble matter from glass, probably 

 gave rise by their reaction with maguesian solutions to the basic protoxyd-silicates, like 

 chondrodite, chrysolite, serpentine and pyroxene. That we have no anhydrous quadri- 

 silicates corresponding to apophyllite and okenite is apparently due to the fact that such 

 silicates, in contact with water at elevated temperatures, break up into anhydrous bisili- 

 cates and quartz ; as is seen in the artificial association of pyroxene and quartz in the experi- 

 ments of Daubrée, and the frequent occurrence of admixtures of the two in beds among the 

 ancient gneissic rocks. A noticeable fact in the history of the surbasic silicates of mag- 

 nesia and related protoxyd-bases, mentioned above, is their frequent association with non- 

 silicated oxyds. Examples of this familiar to mineralogists are the occurrence of aggregates 

 of chondrodite and magnetite ; of chromite, picotite, ilmenite and corundum with chry- 

 solite and serpentine ; and of frankliuite and zincite with tephroite and willemite. These 

 collocations are probably connected with the solvent power of solutions of alkaline silicates, 

 already insisted upon (§ 89), and probably also with the dissociation of silicate of alumina 

 in heated alkaline solutions, noticed by H. Deville (§ 98). 



The separation, by the alternate action of decaying organic matters and of atmospheric 

 oxygen, of iron-oxyd, which readily passes from a soluble ferrous to an insoluble ferric 

 condition, and conversely, has probably played an important part in the formation of depo- 

 sits of iron-oxyds, which are much more cosmopolitan in their associations than corundum, 

 or the compounds of chromic, titanic, aluminic, manganic and zincic oxyds mentioned 

 above, to which we have assigned a difierent origin. It will remain for the mineralogist 



