FRANCISCAN GROUP 



37 



FIGURE 27. Photomicrograph of section of altered tachylitic tuff of 

 the Franciscan group. The shards, which were originally mafic 

 glass, are completely altered to a fine-grained aggregate of chlorite. 

 A few small crystals of albite and augite are visible in the shards. 

 Vesicles are filled with nontronito and secondary albite. The matrix 

 IB fine-grained chlorite similar to that which has replaced the origi- 

 nal tachylite. 



analyses 3 and 4 at least the sodium-calcium ratio is 

 considerably less than in normal spilites. Many more 

 analyses will be needed before one can safely state 

 that the greenstones of the Franciscan group are soda- 



rich spilites, or can draw conclusions regarding a 

 process of spilitization that they may have undergone. 

 In the New Almaden area we know that the plagio- 

 clase, which appears in sections to be primary, con- 

 tains an abnormal amount of soda for such mafic 

 rocks; whether or not this reflects a magma rich in 

 sodium, or even sodium-rich rocks, remains an open 

 question. 



METAMORPHIC ROCKS 



Metamorphic rocks having a truly gneissic or schist- 

 ose texture, and consisting largely of minerals formed 

 by metamorphic processes, constitute an exceedingly 

 small, but very interesting, part of the Franciscan 

 group. These metamorphic rocks are so strikingly 

 different and so much more highly metamorphosed 

 than the typical sedimentary and igneous rocks of the 

 Franciscan group, which show only incipient meta- 

 morphism, that they must have been formed by some 

 additional metamorphic processes. Two varieties, 

 hornblende rocks and glaucophane rocks, occur in suf- 

 ficiently large areas to be shown on the map of the 

 district, plate 1. Other varieties described below are 

 of such local occurrence (hat they are not represented 

 on the map, but elsewhere in the California Coast 

 Ranges they are widespread, though not abundant. 

 The distribution and character of the metamorphic 

 rocks in the New Almaden district is believed to be 

 representative of those occurring in the Franciscan 



TABLE 8. Analyses of greenstones of the Franciscan group, with composite analyses of diabase and spilite for comparison 



0.1 NiO omitted. 



NOTE. Description of sample and locality as follows: 



1. Diabasic greenstone (NA-224), from crest of ridge 3.45 miles S. 70 E. of apex of 



Mine Hill, New Almaden district, Santa Clara County, Calif. A. C. Vlisidis, 

 SOftlyst. Plagioclase is Abes. 



2. Tachvlitic tuflaceous greenstone (NA-325), from point of 1,100 ft alt, 3.44 miles 



S. 554 E. of apex of Mine Hill, New Almaden district, Santa Clara County, 

 Calif. A. C. Vlisidis, analyst. Phenocrysts are Ab>s and vesicles locally filled 

 with Abu. 



3. Fourchite from Angel Island, Marin County, Calif. From Ransome, 1894, p. 231. 



4. Hornblende pseudodiabase, from near Mount St. Helena, Calif. W. H. Melville, 



analyst. From Becker, 1888, p. 98. 



5. Pseudodiabase, from near Knoxville, Calif. W. H. Melville, analyst. From 



Becker, 1888, p. 99. 



6. Average of 90 diabases, calculated by Daly and others, 1942, p. 2. 



7. Average spilite according to Sundius, 1930, p. 9. 



8. Average spilite according to A. K. Wells, 1923, p. 69. 



