FRANCISCAN GROUP 



43 



FiGtJRE 33. Polished surface on silicifled tuffaceous greenstone showing typical "orbicular jasper" structure 

 resulting from radial growth of chalcedony fibers In the replacing silica. 



indicates a two-stage process of formation in which a 

 brecciated greenstone was partly replaced by ferrugi- 

 nous silica and later it recrystallized to form an or- 

 bicular jasper. Similar rocks occurring a few miles 

 east of the district in the Morgan Hill quadrangle are 

 valued by lapidary enthusiasts and have found their 

 way into thousands of collections in California. With 

 further recrystallization of the chert, the chalcedony 

 is replaced by a mosaic of sutured quartz and the iron 

 oxides generally are expelled to the periphery of the 

 quartz grains. Where the rock is still more meta- 

 morphosed, the iron forms scales of specular hematite 

 as much as half a millimeter in diameter. Although 

 the hematite is generally most abundant in areas of 

 clear quartz or in vugs that also contain a little car- 

 bonate, one thin section showed strings of minute 

 hematite scales which appeared to have replaced cro- 

 cidolite. 



Crocidolite-bearing chert, characterized by its tough- 

 ness and patches of inky blue color, occurs in boulders 

 along zones that also contain other varieties of meta- 

 morphic rocks. Thin sections of these rocks show that 

 the silica is present in the form of sutured quartz ap- 

 preciably coarser grained than the silica of the un- 

 metamorphosed chert. The crocidolite generally is 

 concentrated in veinlets, but in some places it sur- 

 rounds individual quartz grains and in others it has 

 a random distribution (fig. 34). In some metachert, 

 aegirine replaces the central part of clots of crocido- 

 lite and forms the margins of quartz-aegirine veins. 

 Such crocidolite-bearing cherts are fairly common in 

 the California Coast Ranges, and have been studied 



in greater detail by Louderback and Sharwood ( 1908, 

 p. 659) and by Ransome (1894, p. 211-219). 



ORIGIN OF METAMORPHIC ROCKS 



To emphasize the difficulty of explaining the origin 

 of the metamorphic rocks in the Franciscan group, it 

 is only necessary to point to the many and varied 

 agencies to which the metamorphism has been ascribed. 

 These include dynamometamorphism, regional meta- 

 morphism, and additive contact metamorphism, and it 

 has even been suggested that the metamorphic rocks 

 are merely fragments of an older underlying forma- 

 tion dragged up along faults. The most recent con- 



FIGURE 34. Photomicrograph of metachert containing needles of cro- 

 cidolite (dark) in a groundmass of quartz. Plane light. 



