DIABASE-PORPHYRITE. 719 
of a non-polarizing, cloudy, grayish, or red-stained substance, which repre- 
sents the original magma. This substance, when present in small quan- 
tities, fills the sharper angles between the feldspars. From these smaller 
quantities it increases in amount until it becomes a preponderating ingre- 
dient in the denser and more highly conchoidally fracturing kinds, when 
the feldspars are seen floating about in it in wholly separate particles. The 
red ferrite, which is an important ingredient in all of the browner kinds, 
has evidently come from the alteration both of the original magma and the 
augite. 
Among the porphyritic ingredients of these rocks the feldspars are 
much the most prominent. Commonly they are red, though occasionally 
white or colorless. The size usually is one-eighth to one-sixteenth inch in 
length or less. Orthoclase occurs among these feldspars, but they are more 
commonly oligoclase and rarely Jabradorite. Augite occurs as a porphy- 
ritic ingredient, but much more rarely than the feldspars. It is commonly 
much altered to chlorite. As adventitious ingredients may be mentioned 
epidote, quartz, calcite in pseud-amygdules and true amygdules, and apa- 
tite in the usual crystals. 
These diabase-porphyrites frequently assume an amygdaloidal char- 
acter in the upper portions of the flows, when they are commonly extraor- 
dinarily vesicular, very often with the vesicles elongated in a common 
direction. Frequently these extraordinarily vesicular amygdaloids have 
mingled with them, and often filling the vesicles, a red, shaly matter. 
The rocks here included vary considerably in silica content, ranging 
from 48 to 60 per cent. It is possible that some of the more basic, blackish 
kinds may represent the half-glassy forms of the olivinitic diabases, but 
this has not been proved by analysis or recognition of olivine as an ingre- 
dient. On the other hand, there is evidently in some kinds, especially 
in some of the redder varieties nearly free from augite, a greater amount 
than usual of orthoclase material, and with this often is a little secondary 
quartz. These kinds make up much of the so-called quartzless por- 
phyries, and are plainly the half-glassy form of the more acid orthoclase- 
gabbros. These kinds have about 55 to 60 per cent. of silica, and stand 
