XIII.] ORIGIN OF CRYSTALLINE ROCKS. 285 



alkalies, lime, magnesia, and iron, and thus will have the com- 

 position of the more basic rocks. [We find, in fact, in the 

 sediments of various geological periods, not only beds of clay 

 and marl corresponding to the second class, but strata made 

 up in great part of mechanically disintegrated though chemi- 

 cally unchanged orthoclase, with quartz, the debris of granitic 

 rocks, constituting what 'is called arkose. Beds of this kind, 

 as will be seen in the following paper on the Geology of the 

 Alps, have even been mistaken for granite and gneiss, and 

 similar recomposed rocks occur in the mesozoic sandstones of 

 New England and ISTew Jersey. Such processes of disintegra- 

 tion and decay have probably been going on from very re- 

 mote times, and the crystalline rearrangement of the resulting 

 rocks may be supposed to give rise to true crystalline schists, 

 or their aqueo-igneous fusion to eruptive rocks. (Ante, pages 

 14 and 23.)] 



A little consideration will, however, show that this process 

 is inadequate to explain the production of many of the vari- 

 eties of crystalline silicated rocks. Such are serpentine, steatite, 

 chrysolite, hornblende, diallage, chlorite, pinite, labradorite, 

 and orthoclase, all of which mineral species form rock-masses 

 by themselves, frequently almost without admixture. No 

 geological student will now question that all of these rocks 

 occur as members of stratified formations. Moreover, the man- 

 lier in which serpentines are found interstratified with steatite, 

 chlorite, argillite, diorite, hornblende, and feldspar rocks, arid 

 these, in their turn, with quartzites and orthoclase rocks, is 

 such as to forbid the notion that all of these various materials 

 have been deposited, with their present composition, as me- 

 chanical sediments from the ruins of pre-existing rocks of plu- 

 tonic origin. 



There are two hypotheses which have been proposed to ex- 

 plain the origin of these various silicated rocks, and especially 

 of the less abundant, and, as it were, exceptional species just 

 mentioned. The first of these supposes that the minerals of 

 which they are composed have resulted from an alteration of 

 previously existing minerals of plutonic rocks, often very unlike 



