FRANCISCAN GROUP 



25 



a former graduate student at Stanford University 

 who worked on the Calera-type limestone at the 

 writers' suggestion, goes the credit for noticing 

 the locality shown in figure 13 in which some Forami- 

 nifera had been freed from the limestone by weather- 

 ing; subsequent investigation resulted in finding other 

 Formainifera in f liliaceous limy shales interbedded 

 with the limestone. This locality yielded the Forami- 

 nifera which have been described by Cushman and 

 Todd (1948, p. 90-98) and Kiipper (1955, p. 112-118). 

 It lies in a road cut in the SW14 of sec. 24, T. 8 S., 

 R. 1 W., and it can easily be reached by traveling 

 south 0.23 mile along a poor dirt road that branches 

 from Shannon Road at a point 0.7 mile west of 

 Guadalupe Creek. Cushman and Todd believed the 

 fauna to be diagnostic of a Lower Cretaceous age, 

 and Glaessner (1949, p. 1615-1617) upon comparing 

 their illustration with forms found in the Mediter- 

 ranean region suggested that the age be restricted to 

 the upper part of the Lower Cretaceous (Albian- 

 Aptian). Kiipper, who obtained better material than 

 that available to Cushman and Todd, reported, "The 

 evidence thus favors correlating * * with strata 

 classified as Cenomanian in Europe and Africa." 



Rather poorly preserved megafossils were found 

 in the oolitic limestone along the middle part of 

 Longwall Canyon in the southeastern part of the dis- 

 trict. Although small fragments of fossils, and also 

 wood fragments, are fairly common in this rock, few 

 of the fossils are complete, and as the rock is brec- 

 ciated and recrystallized, only weathered surfaces 



yielded useful fossils. A diligent search resulted in 

 the finding of only two specimens of the gastropod 

 Nerinea, and several spines and a fairly complete 

 test of an echinoid, Cidaris (fig. 16). According to 

 R. W. Imlay (written communication, Oct. 29, 1948), 

 the Nerinea must be either Jurassic or Cretaceous, but 

 the Cidaris has not been identified closely enough to 

 indicate its age. In addition to the above fossils 

 from the limestone, fragments of Inoceramus were 

 found in limy concretions in shale of the Franciscan 

 group near the north end of the Calero Dam, but they 

 were too small to be determined specifically. 



CHERT 



Chert constitutes less than 0.5 percent of the rocks 

 of the Franciscan group in the district, but because 

 of its resistance to weathering, it seems much more 

 abundant than it really is. Its areal distribution in 

 the district, and in the geologic column, is not uni- 

 form, for it is much more abundant along the belt of 

 greenstone trending westward through the central 

 part of the district than it is elsewhere. This belt 

 includes all the major quicksilver mines, but, although 

 quicksilver mines have been developed in the chert of 

 the Franciscan group elsewhere in California (Bailey, 

 1946, p. 219-221), only minute amounts of cinnabar 

 have been found in the chert in the New Almaden dis- 

 trict. Manganese ores also have been mined from 

 cherts of the Franciscan group elsewhere in the State, 

 but in this district only a few scattered occurrences 

 of manganiferous chert have been prospected. 





1 inch 



1 inch 



0.5 inch 



B A C 



FIGURE 16. Fossils from the limestone of the Franciscan group in the New Almadcti district. ,-1, Cidaris sp., side view. Natural size. /?, Cidaris sp., spine. 



Natural size. C, Nerinea sp., ground to show internal structure. X2. 



686-671 O 63 



