GEOLOGY OF THE SAN JOSE DISTRICT 289 



pletely isotropic. It clouds over when the section is heated to 



redness, and gelatinizes with acid. 



An analysis of the above rock gave the following per- 

 centages : 



SiOj 42.49 The calculated norm of the 



^h^i 17.68 rock is given below. 



FcgOg 5.12 Anorthite 20.02 



FeO 5.90 Leucite 13-95 



MgO 5.28 Nephelite 19.60 



CaO 15.81 Diopside 14.84 



NajO 4.29 Olivine 9.14 



^2^ 2.97 Ackermanite 14-55 



H2O 38 Magnetite 



7.42 



9992 99.52 



It belongs with order 8 of class III and falls in rang 3 and 

 in grad 4, covose. 



(c) Rincon Type. — The variety of the camptonite here de- 

 scribed occurs in narrow dikes at various points to the east and 

 southeast of San Jose. 



Macroscopic Appearance. — In texture this rock is holocrys- 

 talline porphyritic with phenocrysts of plagioclase in a ground- 

 mass consisting of feldspar, augite, hornblende and magnetite. 

 Chlorite, muscovite, calcite and epidote are the common altera- 

 tion products. The plagioclase is in large complex individuals 

 twinned polysynthetically with many fine lamelL^, the crystals 

 being often i mm. on either dimension. They afford favorable 

 cases for the estimation of the basicity when a section cut at a 

 right angle to the albite lamellae may be shown to represent 

 the two halves of a carlsbad twin as well. Such readings indi- 

 cate anorthite, and the extinction angles on many albite lamellae 

 are high enough to give confirmation to this determination. 

 The phenocrysts of plagioclase often show an inner core sur- 

 rounded by a rim which has a different orientation optically, or, 

 more rarely, zonal structure, consisting of many shells, is 

 observed. Colorless mica, in patches i mm. long has re- 

 sulted where the feldspar is weathered, and with it more or 

 less chlorite is associated as having come from the same 

 source. 



