330 University of California Publications in Geology [Vol. 1° 



Si0 2 



60.60% 



60.78% 



60.30% 



AL0 3 



16.61 



16.04 



16.03 



Fe 2 3 



5.03 



4.82 



4.19 



CaO 



7.93 



8.22 



7.19 



MgO 



2.08 



1.99 



2.26 



Loss 



4.79 



4.75 



4.42 





97.08 



96.60 



94.39 



The alkalies are not determined in these analyses. Probably some 

 potash and soda have been leached out in the alteration of the feld- 

 spars. The granodiorite has its best exposure on Chino Hill and is 

 plainly seen underlying the white limestone. The rock is mined by 

 tunnels into it from the side of the hill below the limestone. 



The quartz-monzonite porphyry forms dikes and laccolithic-shaped 

 masses in the limestone of Sky Blue Hill. The rock is very fine- 

 grained, almost felsitic in character, with an ash-gray color. Under 

 the microscope it is essentially a fine granular mixture of quartz, 

 orthoclase and plagioclase with occasional large plates of feldspar and 

 angite. Some pale green and slightly pleoehroic hornblende is present, 

 but most of the dark magnesian mineral is augite. It occurs in 

 granular aggregates 1 a pale bluish-green color and shows no pleo- 

 chroism. Titanite grains are common in the rock. 



This rock forms a wide intrusion into the limestone of Sky Blue 

 Hill, since tunnels run into the hill from the floor of the Commercial 

 Rock quarry penetrate it for several hundred feet. It has been the 

 most active agent in the formation of the metamorphic minerals of 

 this hill. 



The pegmatite occurs as intrusive dikes but it is difficult to trace out 

 their boundaries. They appear to be associated more with the meta- 

 morphic masses of vesuvianite-garnet rock. The pegmatite consists 

 mainly of white orthoclase and green epidote. Orthoclase occurs in 

 large cleavage masses and is usually pure white in color. The epidote 

 penetrates the feldspathic mass in long slender crystals, most of which 

 are altered to a bronze-brown color. Quartz occurs as smoky granular 

 masses but is not prominent. Zircon, tourmaline, axinite, pyroxene, 

 and a few other minerals are occasionally found as accessories and 

 some minerals have later been developed in the pegmatites by hydro- 

 thermal metamorphism. 



The Limestone. — Daily analyses are made of the limestone and that 

 from the Chino quarry has the following average compositions : 



