74 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
dition or are represented by characteristic alteration products, which are 
often paramorplis of the original mineral. The former is only rarely 
the case, while the latter is the rule. 
The feldspar occurs both as porphyritic crystals and as a constituent 
of the groundmass. The crystals of the first generation are from 0.6 to 
0.8 millimeters in length to 0.2 millimeters in breadth. Those of the 
second generation are lath-shaped, 0.4 millimeters in length to 0.4 
millimeters in breadth, and condition the microophitic structure. They 
are both striated, and show undulatory extinction when any of the 
original substance remains. Usually the feldspars are altered to epi- 
dote and quartz, or they have been completely replaced by quartz, while 
their crystal outline is preserved by the iron constituent. 
It is doubtful whether augite is present otherwise than as a constitu- 
ent of the groundmass where it is allotriomorphic. It is universally 
replaced by the more stable amphibole minerals or by epidote or 
chlorite, and some porphyritic crystals of the latter minerals may 
represent augite phenocrysts. The chief alteration product of augite 
in the schistose porphyrites is actinolite. This mineral is not limited 
in its development to the augite outlines, and it thus obscures the 
ophitic structure. Olivine crystals of two generations are readily 
recognized by means of their characteristic form, their irregular frac- 
turing, and sometimes by their alteration products. Usually the olivine 
is altered to epidote. In a few cases (PI. XXVIII, b) the crystals are 
still sufficiently unaltered to respond to the optical tests for olivine. 
There is a large amount of an opaque black oxide in the porphyrites. 
Where this was tested the pow'der was found to be magnetic. This 
fact, the crystal form of some of the oxide, and the analyses of these 
rocks point to the conclusion that much of it is magnetite, though 
undoubtedly very titaniferous. Occasional rhombohedral forms and 
cleavages indicate that ilmenite is also present. Both these minerals 
have given rise to an abundant development of leucoxene (PI. 
XX VII I, b). 
In order of abundance the original constituents rank as follows: 
feldspar, augite, magnetite and ilmenite, olivine. 
Secondary constituents. — The processes of alteration have been greatly 
assisted by the open-textured, vesicular nature of the rocks, and the 
mineral character of the amygdules is indicative of the character of 
the alteration of the rock mass. The vesicles are almost universally 
tilled with one or two or all of three minerals: epidote, quartz, and 
chlorite. Whatever is the amygdaloidal filling is also the prevailing 
alteration mineral. If quartz fills the amygdules the rock mass is 
more or less completely silicified. The ophitic structure, while pre- 
served in outline, is replaced by the nncropoikilitic. Quartz, titaniferous 
magnetite, leucoxene, and some epidote constitute the rock, which is 
distinguished in the hand specimen by its blue-gray color and white 
amygdules. 
One of the most common amygdaloidal fillings is epidote with a 
