FRANCISCAN GROUP 



17 



TABLE 1. Analyses of graywacke from the Franciscan group, 

 with average of 30 graywacke and 40 granodiorite analyses 

 for comparison 



NOTE. Description of sample and locality as follows: 



1. "Neocomian Sandstone" from headwaters of Bagley Creek, Mount Diablo, Calif. 



W. II. Melville, analyst. From Melville, 1891, p. 412. 



2. New analysis. Graywacke (N A-450) from a point 3,450 ft N T . 66H E. of the apex 



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

 Vlisidis, U.S. Geological Survey, analyst. 



3. Sandstone of the Franciscan group from Sulphur Bank, Calif. W. II. Melville, 



analyst. From Becker, 1888, p. 82. 



4. Fresh sandstone of the Franciscan group from quarry of the Oakland Paving Co., 



Piedmont, Calif. James W. Howson, analyst. From Davis, 1918, p. 22. 



5. Fresh sandstone of the Franciscan group from junction of Buckeye Gulch and 



Hospital Canyon, Carbona quadrangle, Stanislaus County, Calif. Analyzed 

 by Herdsman Laboratory, Glasgow; analyst not known. From Taliafcrro, 

 1943b, p. 136. 



6. Average of 30 gray wackes. From Tyrrell, 1933, p. 26. FC2Oa given as 3.4 percent 



assumed to be in error, as that figure makes the total 102.4 percent. 



7. Average of 40 granodiorites calculated to 100 percent by R. A. Daly. From Daly 



and others, 1942, p. 2. 



Except for the analysis given in column 1, which 

 was made from a graywacke containing an abnormal 

 amount of calcite that probably occurred as veins, the 

 analyses exhibit the range in composition no greater 

 than might be expected in these unsorted sediments. 

 As compared with the "average graywacke," the Fran- 

 ciscan rocks contain less aluminum and more magne- 

 sium and sodium, which suggests that they contain 

 more fragments of soda-rich mafic volcanic rocks than 

 does the "average graywacke." 



The similarity of the graywacke to granodiorite in 

 chemical composition has been pointed out by Tali- 

 aferro (1943b, p. 137, 138), and may be checked by 

 comparing columns 2-4 with column 6. The corre- 

 spondence is fairly good except for three notable de- 

 partures: the ratio of FeO to Fe 2 Os is considerably 

 higher in the graywacke than in the granodiorite, the 

 CaO content is much lower, and the ratio of Na 2 O to 

 K 2 O is higher. 



SUTSTONE AND SHALE 



Siltstone and shale make up less than 10 percent of 

 the Franciscan group in the New Almaden district. 

 Two varieties are common. One, a tan to light-gray 

 phyllitic siltstone, is confined to the southwestern part 

 of the area, where it is interbedded with the feld- 

 spathic graywacke in the lowest part of the Franciscan 

 group that is exposed in the district. The other, con- 

 sisting of dark-gray to black siltstone and shale, oc- 



curs throughout the section, but it is most abundant 

 in a belt trending northwestward through the central 

 part of the district, where it is associated with green- 

 stones that belong to a younger part of the group. A 

 less common third variety, containing considerable 

 iron oxide and commonly having a red or green color, 

 is everywhere closely associated with the cherts and is 

 described with them on page 28. 



Megascopic features 



The older light-colored siltstone crops out locally 

 along the first ridge southeast of Los Gatos, in deeply 

 worn trails or on ridge tops nearly devoid of soil or 

 brush. These rocks are not abundant, and they gen- 

 erally occur in thin beds intercalated with highly 

 feldspathic graywacke; but locally, as in the upper 

 drainage area of Limekiln Canyon, they apparently 

 attain a thickness of several hundred feet without be- 

 ing interbedded with coarser sedimentary rocks. The 

 siltstone is generally more contorted than the surround- 

 ing massive arkose. Bedding can be distinguished 

 with certainty only in the coarser layers, for the silt- 

 stone is invariably phyllitic, breaking into flakes along 

 subparallel parting planes that only approximately 

 coincide with its original bedding. The parting sur- 

 faces are shiny, and under the hand lens they show a 

 myriad of very small unoriented light-colored mica 

 flakes. In thin sections these rocks are seen to con- 

 sist mainly of well-sorted angular grains of quartz 

 and feldspar; but they also contain several percent of 

 muscovite, which appears to be partly detrital and 

 partly authigenic. Clay minerals are also present in 

 apparently small but undetermined amounts. 



The younger and darker fine-grained sedimentary 

 rocks are believed to be largely siltstone because of 

 their fissile to irregular fracture, but they include 

 some shale and mudstone with conchoidal fracture. 

 The color of the siltstone ranges from gray to black, 

 but most of it where fresh has a somewhat greenish 

 cast and where weathered is lighter colored. Expo- 

 sures of unaltered siltstone and shale without inter- 

 bedded coarser rocks are rarely found except in arti- 

 ficial cuts or in sharply incised ravines, but along 

 borders of serpentine masses the siltstones have lo- 

 cally been so hardened that they form good outcrops. 

 In many exposures the siltstone occurs only as thin 

 seams interbedded with graywacke, but in some places, 

 as in Rincon Canyon 3,000 feet upstream from its 

 junction with Guadalupe Canyon, sections as much as 

 500 feet thick are exposed that consist largely of silt- 

 stone. In other places thin layers of siltstone are 

 interbedded with greenstone tuff; good examples of 

 these intercalations can be seen in many of the upper 

 workings of the New Almaden mine, but they are 



