CHARACTER OF METAMORPUISM. 63 



Alteration of the sandstones. — TliG saiidstoiies liavG beGii chciiiged, under the 

 conditions which have prevailed in the Coast Ranges, by several distinct 

 processes of varying interest and importance. They are, of course, sub- 

 ject to the ordinary decompositions known as weathering. Here the ferro- 

 magnesian silicates are in part converted into chlorite and in part also into 

 a ferruginous cement; the feldspars become carious, while the quartz is 

 nearly or quite unaffected. Much more interesting is the process of meta- 

 somatic recrystallization, which is in some respects the inverse of weather- 

 ing. In rocks which have undergone this process the clastic grains are 

 transformed into ferromagnesian silicates, feldspar, zoisite, apatite, etc. A 

 third process is that of serpentinization. This sometimes occurs in sand- 

 stones in which metasomatic recrystallization has eithei- not taken place at 

 all or only to an insignificant extent. The recrystallized rocks, however, 

 are also subject to serpentinization, and from them the greater part of the 

 serpentine of the Coast Ranges appears to have been produced. A fourth 

 process is silicification, by which shales have been converted into phthanites 

 and sandstones into quartzites. The serpentines and crystalline metamor- 

 phics also yield to a similar process and are converted into a dark, opaline 

 substance known in some of the quicksilver mines as quicksilver rock, but 

 this seems to be a phenomenon attending the process of ore deposition 

 rather than that of regional metamorphism. A further rather unimportant 

 process manifests itself in many localities by the presence of numerous 

 stringers of calcite or gj'psum intersecting the rocks, particularly the sand- 

 stones, in all directions. This process has affected many of the younger 

 rocks, as well as those of the Neocomian. 



It is important to remark that some of these processes are inconsistent 

 with one another. Evidently chloritization of the ferromagnesian silicates 

 cannot go on simultaneously with the formation of ferromagnesian silicates 

 by metasomatic recrystallization, and tliis process is equalh' inconsistent 

 with serpentinization. So, too, since serpentine is subject to conversion 

 to chalcedony, serpentinization and silicification must go on under different 

 sets of conditions. 



Weathering of the sandstones. — Tlio weatheHug of tlic saudstoucs cau be very 

 briefly disposed of The chlorite which forms in this process from tlie 



