XIII. ] ORIGIN OF CRYSTALLINE ROCKS. 283 



II. THE ORIGIN OP CRYSTALLINE EOCKS. 



We now approach the second part of our subject, namely, 

 the genesis of the crystalline schists whose history we have 

 just discussed. The origin of the mineral silicates which make 

 up a great portion of the crystalline rocks of the earth's sur- 

 face is a question of much geological interest, which has been 

 to a great degree overlooked. The gneisses, mica-schists, and 

 argillites of various geological periods do not differ very greatly 

 in chemical constitution from modern mechanical sediments, 

 and are now, by the greater number of geologists, regarded as 

 resulting from a molecular rearrangement of similar sediments 

 formed in earlier times by the disintegration of previously ex- 

 isting rocks, not very unlike them in composition ; the oldest 

 known formations being still composed of crystalline stratified 

 deposits presumed to be of sedimentary origin. Before these 

 the imagination conceives yet earlier rocks, until we reach the 

 surface of unstratified material which the globe may be supposed 

 to have presented before water had begun its work. It is not, 

 however, my present plan to consider, this far-off beginning of 

 sedimentary rocks, which I have elsewhere discussed. (Ante, 

 page 63.) 



Apart from the rocks just referred to, whose composition may 

 be said to be essentially quartz and aluminous silicates, chiefly 

 in the forms of feldspars and micas, there is another class of 

 crystalline silicated rocks, which, though far less important in 

 bulk than the last, is of great and varied interest to the litholo- 

 gist, the mineralogist, the geologist, and the chemist. The rocks 

 of this second class may be defined as consisting in great part 

 of the silicates of the protoxide bases, lime, magnesia, and fer- 

 rous oxide, either alone, or in combination with silicates of 

 alumina and alkalies. They include the following as their 

 chief constituent mineral species : pyroxene, hornblende, chrys- 

 olite, serpentine, talc, chlorite, epidote, garnet, and triclinic 

 feldspars, such as labradorite. The great types of this second 

 class are not less well defined than the first, and consist of py- 

 roxenic and hornblendic rocks, passing into diorites, diabases, 



