22 University of California Publications in Geology [ VoL 11 



In places where little sericiteis present one occasionally notices 

 very minute lath-shaped sections of an unknown mineral with very 

 low birefringence and negative elongation. 



No other minerals have been recognized in the cement, besides 

 quartz, chalcedony, sericite, chlorite, calcite, epidote and the undeter- 

 mined mineral just mentioned. Only a small loss of weight occurs on 

 leaching the rock powder with 10 per cent caustic potash solution, so 

 that there can be little opaline silica present. 



Possibly other constituents are present which are not visible under 

 the microscope. The presence of some easily decomposed mineral is 

 suggested by the rapid disintegration of the cement on weathering. 

 Possibly this latter effect is the result of the oxidation of the small 

 chlorite flakes and of finely divided chlorite upon exposure to the 

 weather. 



In those instances where secondary quartz is present in the cement 

 there is no sign of secondary enlargement of clastic quartz grains. 

 The new silica shows no relation in orientation to the old grains. 



The peculiar mineralogical composition of this sandstone is re- 

 flected in its chemical analyses : 



I. Analysis of a fresh specimen of typical Franciscan sandstone from the quarry 

 of the Oakland Paving Company, Piedmont. James W. Howson, analyst. 

 II. Analysis of a Franciscan sandstone from Sulphur Bank, California, given by 

 George F. Becker, in U. S. Geol. Surv. Monograph No. 13, p. 92. 

 III. Composite analysis of 253 sandstones given by F. W. Clarke in U. S. Geol. 

 Surv. Bull. 616, p. 28. 



TiC 



Fe 2 3 



FeO 



CaO 



BaO 



MgO 



K 2 



Na.O 



ZrO : 



Water below 110° C 



Water above 110° C 



C0 2 



PA 



so 3 



MnO 



Sp. gr 2.720 



I. 



II. 



III. 



68.84 



68.50 



78.66 



0.25 



0.60 



0.25 



14.54 



12.82 



4.78 



0.62 



1.29 



1.08 



2.47 



3.37 



0.30 



2.23 



1.82 



5.52 



0.04 





0.05 



1.94 



2.21 



1.17 



2.68 



1.26 



1.32 



3.88 



6.03 



0.45 



0.05 







0.35 



0.28 



0.31 



1.60 



2.11 



1.33 



0.14 





5.04 



0.15 



0.16 



0.08 



0.15 



0.02 



0.07 



99.93 



100.47 



100.41 



