24 



GEOLOGY AND QUICKSILVER DEPOSITS, XLW ALMAUKX DISTRICT, CALIFORNIA 



the New Almaden district with various parts of the 

 better developed section at the Perrnanente quarry 

 in the Santa Cruz quadrangle. He concluded (p. 74) 

 that ". . . the zones in the limestone can be cor- 

 related over considerable distances by means of in- 

 soluble residues * * *" and "the correlation means that 

 the limestone in different areas must have been laid 

 down simultaneously, under similar environmental 

 conditions, if not as a part of a continuous bed." 



Chemical feature* 



The limestone of the Calera type, exclusive of the 

 interlayered lenses of chert, consists largely of cal- 

 cium carbonate with only a very small quantity of 

 magnesium, iron, aluminum, and phosphate. No anal- 

 yses of this type of limestone from the New Almaden 

 district are available, but three analyses of the similar 

 limestone quarried near Permanente Creek, a few 

 miles northwest of the district, are shown in table 5. 

 The bulk analyses shown in table 6 are of the same 

 limestone; they were kindly provided by the Perma- 



TABLE 5. Analyses of Calera-lype limestone from near Permanenle 

 Creek, Santa Cruz quadrangle, California 



NOTE. Description of sample and locality as follows: 



1. Limestone from I'ernianente Canyon. W. L. Lawton, analyst. From I/awson, 



iu, i>. .',. 

 1. "Black Mountain" liniwtone from Permanente. 8. A.TIbhetts. analyst. From 



HuKurnln and Costello, lic.'l, p. 188. Recast to sbow oxides rather than 



carbonate*. 

 1. Same as 1. 



nente Cement Co. These latter analyses include the 

 siliceous chert lenses, but extrapolation to a silica-free 

 rock shows perhaps even letter than any single analy- 

 sis the low magnesium content of the calcite, for such 

 an extrapolation yields a theoretical rock containing 

 &5.C percent Ca() and but 0.1 percent MgO. 



An analysis by A. ( '. Vlisidis, U.S. Geological Sur- 

 vey, of the unusual oolitic limestone of the Franciscan 

 group from the north bank of Longwall Canyon in the 

 southeastern part of the New Almaden district fol- 

 lows. 



SiO, 

 A1 2 3 

 Fe 2 3 

 MgO 



0.91 

 .61 

 .30 



.77 



CaO 55.11 



TIO, .10 



P 2 5 31 



MnO .01 



CO 2 42.24 



Total - 10036 



CaCO 3 96.0 



This analysis shows the similarity in chemical com- 

 position between the Calera-type limestone and the 

 oolitic limestone. Minor differences are due to the 

 fact that the oolitic rock contains fragments of dark 

 volcanic rock, whereas the impurities in the other 

 limestone are chiefly quartz and clay. As the oolitic 

 limestone contains virtually no chert lenses, it offers 

 good commercial possibilities, even though the ex- 

 posed Ixxlies are somewhat smaller than some of the 

 bodies of Calera-type limestone in the district. 



Fossils 



A microfauna and a few megafossils were obtained 

 from the limestone of the Franciscan group in the 

 New Almaden district. The microfossils. which were 

 all Foraminifera, occur in abundance in most of the 

 Calera-type limestone, but generally they cannot l>e 

 freed from this rook for study. To Senor Jose Pant in. 



TABLE 6. Bulk analyses of Calera-type limestone from quarries of the Permanente Cement Co. near Permanente Creek, Santa Cruz 



quadrangle, California 



, and 10) From I .of an. 1047, p. 316. 



I Obtained from Permanente Cement Co. by the writers. 



