28 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIGURE 19. Photomicrograph of chert of the Franciscan group con- 

 taining numerous remains of Radlolaria. 



tions cut normal to the bedding, thin seams of 

 chlorite deformed after the fashion of stylolites are 

 visible. Most thin sections of the chert contain at 

 least a few veins of clear sutured quartz, and some 

 of these are veined and partly replaced by later 

 calcite. 



Associate*! shales 



The slialy rocks associated with the well-bedded 

 chert are distinctive owing to their content of iron 

 and the resulting color, which is red or less com- 

 monly green, in contrast to the tan. pray, or black of 

 typical siltstone and shale of the Franciscan group. 

 They are faintly laminated. Although some are ac- 

 tually shale, others are so coarse grained as to feel 

 gritty and should l>e classed as siltstone. They con- 

 tain chlorite and very fine goethite or hematite dust 

 imbedded in an unidentified line-grained matrix which 

 makes up most of the rock. 



Chemical features 



No analyses of cither the cherts or the shales that 

 accompany them in the New Almadeu district have 

 been made. Complete and partial analyses of the 

 typical chert and shale of the Franciscan group made 

 by Davis (1!MS, p. 2(58-^)!)). together with analyses 

 of a gray \\acke and black shale of the Franciscan 

 group and a composite of "S shales for comparison, 

 are shown in table 7. Comparison of the three cou- 

 plets of chert and associated shale indicates that a 

 mere addition of silica to the shale might yield the 

 corresponding chert, but a very large quantity of 

 silica would l>e required. The shales associated with 

 the cherts differ chiefly from the average shale, 

 column 10. in having a higher content of silica, and 

 ferric iron, and a lower content of aluminum, cal- 

 cium, and potassium. They compare more closely 



TABLE 7. Analyses of cherts and shales of the Franciscan group 



NOTE. Description of sample and Innillty as follows: 



1. Br.-imlsh-red chert fro-n Huglpy Canyon. Mount IMahlo. W II. Melville, 



-m ilvst. Krom MeMlle, I8UI, p. 411. 



2. Brownish-red shale associated with I. \V II Melville, analyst. From Melville, 



I Ml. p. 411. 



3. Siliceous red chert from Red Rock Island. K. F. Davis, analyst. From Dux-Is, 



1918, p. 208. 



4. Portlnl analysis of red shale accompanying 3. E. F. Davis, analyst. From 



Duvls, 1918, p. 2B9. 



A. Red In n from I'olnt Richmond. E. F. Davis, analyst. From Davis, 1918, 

 i' M 



8. Partial analysis of red shale accompanying 3. E. K. Davis, analyst. 



7. Altered graywuckc ..i the Frim m-un urnup from tu-:i.hv:itors of Hagley ' 



MOUTH ' Recast 



by the writer to exclude .!() percent COj, oal iCOj, and remainder 



(Mil-Ill 



8. Average of 2, 4, an<l '>; ' '' total 100 IK i 



. Black siltstone of the Franciscan Kroup, New .Mniinlfii district; minor oxides 



omitted, sec table 2. 



10. ComiKwite of 78 shales from Clarke, IOIA, p. Mfl. N'oncomparable constituents 

 (COi, SOi, I'lOi, BaO, TlOi) omitted. 



