58 



University of California Publications. [Geology 



PEE-FBANCISCAN EOCKS. 



Sedimentary. — In the area mapped no sedimentaries of age 

 earlier than the Franciscan were noted. About twelve miles 

 south of Chittenden and back of the town of San Juan in San 

 Benito County, however, some rather extensive deposits of 

 limestone were studied. These remnants of a once probably 

 extensive terrane are older than the granitic rocks of the Gavilan 

 range. The contacts are intrusive and the limestone, or marble, 

 is highly crystalline. Much of it is very pure, as the accom- 

 panying analyses show. 



Analyses of Limestone. 1 



I II 



Moisture , 10 .10 



SiO, and insol. res 2.62 1.00 



CaC0 3 96.23 96.62 



IY,(>, 30 .05 



ALCK 40 trace 



MgO 24 2.19 



Organic Matter .10 



99.89 100.06 



Very little time was spent studying these interesting deposits, 

 but it may be mentioned that the opportunities for study are 

 excellent owing to the fresh contacts exposed by prospect 

 tunnels. There is a great deal of limestone in this region 

 ranging from a dark blue variety to an almost pure white 

 highly crystalline marble carrying 96 per cent calcium carbonate. 

 Small particles of graphite are scattered through the marble. 

 Similar deposits of limestone have been noted in the Santa Cruz 

 quadrangle 2 and elsewhere. Not all of this limestone is as 

 pure as the two samples analysed. These analyses are of samples 

 taken from the deposits to be used in the manufacture of 

 Portland cement. There are extensive deposits in this same 

 region which have not been so much affected by the intrusion 

 of the diorite, and are worthy of careful search for fossil 

 remains. 



1 By permission of the San Juan Portland Cement Company. 

 2U. S. Geol. Surv., folio No. 163, 1909. 



