T. A. Bendrat — Rocks of Cerro de Santa Ana. 271 



From measurements of extinction angles it is inferred that the 

 plagioclase of the groundmass and also of the larger crystals is 

 anorthite. The pyroxene is a variety of augite which exhibits 

 a faint pleochroism. The olivine, which is well distributed 

 throughout the rock, is also occasionally in allotriomorphic 

 forms but for the most part exhibits crystal outlines. It con- 

 tains black inclusions of iron ore, mostly along cleavage cracks, 

 and this occurs also at times in the augite. 



The rock appears to have undergone crushing through 

 dynamic processes, as shown by the crushed and broken con- 

 dition in which many of the feldspar and olivine crystals are 

 found. The augite granules and shreds are also drawn out into 

 more or less wavy trains which maintain a certain degree 

 of parallelism. There are also different systems of veinlets 

 crossing the rock which are made up of angular grains of oli- 

 vine and predominant feldspar. Three systems of these vein- 

 lets were observed and they were frequently found to interfere 

 with one another, exhibiting thrusts and miniature faults. 

 Such faults are found also in the larger feldspar crystals which, 

 moreover, these veinlets penetrate. 



Hornblende-porphyry. — This rock has a phanerocrystalline, 

 evenly granular groundmass of feldspar and amphibole in 

 which are scattered formless crystals of green hornblende with 

 an occasional one of feldspar. Under the microscope it is seen 

 that the larger hornblendes are to some extent altered, their 

 green color fading in places and at their margins passing over 

 into fibrous actinolite. The larger feldspars are mostly of 

 plagioclase, but sometimes microcline. Like the diabase pre- 

 viously described, this rock appears to have suffered from 

 dynamic processes as shown by the bent condition of the cleav- 

 age cracks in the crystals of hornblende and by faults across 

 them. The plagioclase shows similar features ; like the former 

 rock, the mass is penetrated by a series of veinlets of a felds- 

 pathic nature. 



Hornblende-diorite. — Unlike the foregoing types, this rock is 

 macrocrystalline in that its constituents can be determined by 

 the eye. The chief components are hornblende and plagio- 

 clase, the latter varying in composition from labradorite to 

 anorthite. 



The hornblende in section varies in color from light green to 

 brown even in the same section and in some places it is altered 

 to a colorless variety. It contains inclusions of magnetite and of 

 hematite, the latter being regarded as an alteration product. The 

 plagioclases are twinned according to both the Carlsbad and albite 

 laws. Decomposition of the feldspar is shown in places by its dull 

 and cloudy aspect. It sometimes shows inclusions of apatite 

 needles. The fabric of the rock is somewhat ophitic in that 



