GEOLOGY OF THE SAN JOSE DISTRICT 287 



The hornblende prisms of the younger generation average 

 .07 mm. in length by .01 mm. in breadth. They show the 

 same colors and pleochroism as do the larger hornblendes. 

 The pieces of feldspar packed in between the hornblendes are 

 very minute, seldom above .02 mm. in length. No twinning 

 could be observed. The feldspar in thin sections is very clear 

 and fresh. Irregular pieces of magnetite, often nearly as large 

 as the augites, are surrounded as the pyroxene is by serpentinic 

 rims. The perfect crystals of apatite .07 mm. across on the 

 basal section are notable for the inclusions which they contain 

 of microscopic hornblendes, zonally arranged. 



(b) Casa Grande Type. — The variety of camptonite here de- 

 scribed is met in a three-foot dike on the road leading to San 

 Carlos just as one leaves San Jose. 



Microscopic Characters. — Seen in the hand specimen the 

 rock is black and very fine-grained. Only a few shining prisms 

 may be made out in the dense aphanitic ground-mass. 



Microscopic Appearance. — Under the microscope the rock 

 shows a holocrystalline porphyritic texture. The phenocrysts 

 are of hornblende and augite, embedded in a ground-mass made 

 up of little feldspars, hornblende, augite, magnetite and apatite. 

 Calcite and analcite are secondary products. 



The augites of the older generation are by far the most abun- 

 dant phenocrysts. Eight-sided sections are developed, with 

 very perfect crystal boundaries, which are defined by the usual 

 faces w(iio), a(\oo) and <^(oio). The pyroxene prisms are 

 usually half as broad as they are long. They take all possible 

 orientations. The color varies considerably. Violet gray pre- 

 vails but light shades of green are often observed. Some crys- 

 tals are nearly colorless. The mineral is pleochroic between 

 gray and shades of green. A zonal structure is usually devel- 

 oped with green or violet cores, and a colorless outer shell. 

 Inclusions of apatite and magnetite are in linear arrangement 

 parallel with the crystallographic faces. An exterior shell of 

 magnetite grains has often been added by magmatic resorption, 

 or where no secondary magnetite has formed the crystal edges 

 are usually frayed out. The extinction angle is nearly 45°. 



