DECOMPOSED SERPENTINE. 127 



the material at liand does not afford tlie means of following- in detail the 

 history of the grate structure. 



In the pseudodiabase and pseudodiorite it is naturally the bisilicates 

 which first show traces of the serj)entinization process, and for these min- 

 erals the history is exactly parallel to that of direct chloritization. The 

 ferroniagnesian silicates yield to this process much more easily than the 

 feldspar and the quartz, which behave as in the slightly altered sandstones. 

 This fact leads to the belief that the greater part of the massive serpentine 

 has resulted from pseudodiabase and pseudodiorite, a view supported by 

 structural considerations; for, since both recrystallization and serpentiniza- 

 tion are dependent on a fissure system, serpentinization in slightly altered 

 sandstones appears to mean that during this process the solutions diverged 

 from their old channels or that, where in the first stage solutions permeated 

 to but a slight extent, they penetrated abundantly at the later period. This 

 would probably be less common than a similar distribution of solutions at 

 each of the two periods. 



Decomposition of serpentine. — In noarl}' all tlic Serpentine localities it is evident 

 that this rock is subject to tolerably rapid decomposition under the action 

 of the atmosphere. The subject has been studied in Bohemia bv Mr. A. 

 Schrauf,' with whose results the observations made in the Coast Ranges 

 agree. Where the serpentine is directly exposed to the action of the atmos- 

 phere, it is often bleached and converted to a porous mass, which is nearl}- 

 pure silica, containing very little magnesia or iron. Wliere serpentine has 

 been subjected to solfataric action in the immediate neighborhood of ore 

 bodies, the bases have often been removed and silica has replaced nearly or 

 quite all of the original mass. While this is a common change near ore 

 bodies, such replacements have also occurred to a small extent at long dis- 

 tances from known occurrences of ore, and it may be tliat tliis process has 

 gone on to some extent at different periods. Since by far the gi-eater part 

 of the silicified serpentine bears such a relation to the ore bodies as to lead 

 to the conclusion that tlie process was attendant on that by which the ore 

 was produced, it will be discussed hereafter in that connection. 



'Zeitsclir. fiir Krjs. uud Miu., GrotL, vol. 6, p. 321. 



