FRANCISCAN GROUP 



23 



lens to have organic shapes. Much of this limestone 

 shows no recrystallization even in thin section, except 

 that the replaced Foraminifera may consist of sutured 

 calcite grains somewhat coarser than the rest of the 

 rock; however, locally it is recrystallized. Coarsely 

 crystalline calcite fills fractures, and in recemented 

 breccias it amounts to nearly half of the rock. Pyrite, 

 generally altered to limonite, is fairly common, and 

 the more impure varieties contain altered tachylite 

 and a small amount of quartz and plagioclase. 



The second variety of limestone, which is strati- 

 graphically a little higher in the section, crops out 

 on the south slope of Los Capitancillos Ridge as 

 isolated bodies, of which the most prominent is about 

 2,000 feet southwest of the apex of Mine Hill. This 

 limestone differs from that found lower in the section 

 in that it is everywhere fairly coarsely crystalline, 

 contains numerous lenses and pellets of glauconite, 

 and lacks chert and Foraminifera. 



An unusual oolitic limestone, unlike any known to 

 have been reported heretofore from the Franciscan 

 group, 4 forms large conspicuous outcrops in Longwall 

 Canyon about 1 mile west of its junction with Llagas 

 Canyon. These outcrops are massive, and where the 

 limestone is purest they show no bedding. Solution 

 has commonly roughened their upper surfaces, and 

 where they are jointed, the cracks have been enlarged 

 by solution. This oolitic limestone is dull battleship 

 gray, and the purest consists almost entirely of oolites 

 about 2 mm in diameter, embedded in a matrix of 

 smaller oolites averaging about 0.1 mm in diameter 

 (figs. 14, 15). The substances forming the nuclei of 

 the larger concentrically layered oolites are shell 

 fragments, bryozoan(?) fragments, carbonatized ve- 

 sicular mafic glass, and aggregates of smaller oolites. 

 No Foraminifera were found in the rock. The more 

 common impure facies, which grade into tuffs and 

 shales, contain shale fragments, considerable glauco- 

 nite, and sparse grains of quartz and feldspar; they 

 also contain some interesting but generally fragmen- 

 tary fossils of gastropods and echinoids. The oolitic 

 limestone could not be traced continuously to typical 

 Calera-type limestone beds and cannot be definitely 

 correlated with either of the two limestone units, but 

 because it crops out along their projected strike, the 

 writers believe that the oolitic limestone is merely an 

 unusual variety of the Calera-type limestone, de- 

 posited at the same time and under nearly the same 

 conditions. 



FIGURE 14. Oolitic limestone of the Franciscan group from Longwall 

 Canyon in the southeastern part of the New Almaden district. An 

 analysis of this limestone is given on page 24. 



Insoluble residues 



A study of the insoluble residues of the limestone 

 from several parts of the district has been made by 

 Pantin 5 to determine whether the isolated outcrops 

 in the New Almaden district could be correlated by 

 this means with the thick section of Calera limestone 

 exposed in the quarry of the Permanente Cement Co. 

 several miles to the north. He found the insoluble 

 residues to consist of allogenic gray silt, very fine 

 grained sand, clay, and chert, and authigenic glau- 

 conite, pseudocubic crystals of quartz, barite, chert, 

 lignite, limonite pseudomorphs after pyrite, and limo- 

 nite replacements of microfossils. In different parts of 

 the section these insoluble minerals were present in dif- 

 ferent proportions, and their aggregate quantity ranged 

 from 2 to 15 percent. On the basis of their distribution 

 Pantin was able to correlate the sampled outcrops of 



FIGURE 15. Photomicrograpn of oolitic limestone of the Franciscan 

 group. Note the groups of small oolites that form the cores for 

 some of the larger oolites. 



* This oolitic limestone may possibly be the same rock described in 

 California State Mining Bur., 12th Kept, of the State Mineralogist, 

 p. 394, 1894. 



5 Pantfn, Jos6 Henrique, 1946, Insoluble residues of the Calera lime- 

 stone in Santa Clara County, California : Stanford Univ., unpublished 

 master's of science thesis, November. 



