GEOLOGY OF THE SAN JOSE DISTRICT L^91 



The extinction angle is large, nearly 45°. Zonal structures are 

 developed. The pyroxenes are often surrounded by prisms of 

 monoclinic amphibole. 



The hornblende makes up by far the largest portion of the 

 rock. The individuals of the mineral are in fibrous blades 

 poorly defined by crystal faces in the prism zone. They attain 

 a maximum extension of .4 mm. and are eight or nine times as 

 long as they are broad. Their color is dull green. By pleochro- 

 ism changes to dull brown are produced. The cores of the 

 hornblende crystals are at times different in composition from 

 the outer rims. Twins are rare. Extinction 20°. 



Some plagioclase, evidently of secondary origin, occurs as 

 minute patches in the orthoclase ground. Biotite foils . i mm. 

 or less in diameter are now and again associated with the clus- 

 ters of hornblende crystals. Titanite is disseminated through 

 the rock in microscopic lozenge-shaped individuals. The larger 

 magnetite grains may be surrounded by rims of this mineral. 



5 . Liinlmrgite 



The rock to be described under this name is found in a dike 

 on the western flanks of the San Carlos Mountains five miles 

 south from San Jose. The hand specimen shows it to be a 

 lusterless black aphanitic rock. 



Microscopic Characters. — Under the microscope a well-de- 

 veloped porphyritic texture with a fine-grained holocrystalline 

 ground-mass is at once apparent. The phenocrysts are augite 

 and olivine. Hornblende, augite and plagioclase make up the 

 ground-mass. 



The augite phenocrysts as a rule are sharply bounded indi- 

 viduals which attain a length of .6 mm. or less. Their color 

 is violet gray and they are slightly pleochroic. A zonal struc- 

 ture with many concentric shells may usually be noted, and 

 with this the hour-glass structure is developed. Inclusions of 

 magnetite grains are arranged zonally. At times there is 

 evidence of magmatic resorption, and around the main crystal 

 a rim of secondary augite is developed. Again the center is 

 corroded while the outer zone is clear. Twins may often be 



