('().-\ll'Ai;iS()X WITH TIIK AIv'CM.KAN. ]39 



that more tlian one well known geolog-ist has believed that he recognizeil 

 as Archaean, areas now known to be Neocomian. The metamorphisni of the 

 Coast Ranges may fairly be considered, at least in part, as regional, since 

 most of the rocks of the Knoxville gronp are considerably altered and there 

 are areas of many hundred square miles now exposed in which no patches 

 of unchanged or very slightly modified rocks are known to exist. Nothing 

 like the uniformity often prevalent in Archaean areas, however, is to be 

 found in the Coast Ranges. On the other hand, though the general appear- 

 ance of many of the Knoxville rocks differs widely from that of correspond- 

 ing Archaean masses, there is much similarity in detail. Recrystallization is 

 very prevalent in the California metamorphics and a crystalline development 

 is characteristic of the Archaean. Muscovite rocks are frequent in the Coast 

 Ranges, while biotite, though rare, is certainly one of the authigenetic min- 

 erals in the California metamorphic area. Plagioclase, augite, and horn- 

 blende are also abundantly developed. Mineral combinations similar to 

 those of diabase, gabbro, and diorite are common both in the Coast Ranges 

 and in many Archaean areas. Gneissoid rocks carrying albite and orthoclase, 

 though not predominant in the Coast Ranges, are found there, and the mixt- 

 ure of zoisite and plagioclase, called saussurite, is frequent in both series, as 

 are also the accessory minerals ilmenite, titanite, rutile, apatite, and chromic 

 iron Finally, serpentine is even more common in the Coast Ranges than in 

 most Archa!an areas. Of the more important features of the Archasan series 

 none appears to be entirely absent among the metasomaticallyrecrystallized 

 rocks of the quicksilver belt which have thus far l)een investigated, but the 

 quantitative relations of the various minerals and rocksinthe two series are 

 widely different. A slight difference in the chemical composition of the sed- 

 iments of the Coast Ranges, or of the solutions by the help of which their 

 recrystallization has been effected, or of the pressure under which the reac- 

 tions took place would have considerably changed the cpiantitative relations 

 of the minerals formed. A greater depth from the surface would manifestly 

 also have promoted uniformity. Whatever, then, is the real origin of the 

 Archaean series, it appears certain tliat rocks indistinguishable from them 

 might have been produced under conditions not greatly dissimilar to those 

 which prevailed in the Coast Ranges at the close of the Neocomian. 



