48 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136 
intergrowth, in most of the occurrences described these have been the 
component minerals. In the South Mountain rocks the feldspathic 
material is usually so abundant as not to admit of the determination 
of the mineral character of the host. In such cases a clue to the 
nature of the cementing material is found in its optical continuity with 
the porphyritic quartz. These phenocrysts are severally included 
within a single micropoikilitic area, with which they are always simi- 
larly oriented (PL XVH, b). The cementing material acts as a sort of 
secondary enlargement of the quartz phenocrysts. The feldspar phen- 
ocrysts, on the other hand, have no effect upon the orientation of the 
cement. 
In the basic rocks, which are coarser-grained, the character of the 
host can be directly tested and proved to be quartz. The mottled 
appearance, previously alluded to, is usually emphasized by the 
arrangement of globulites, longulites, and trichites of iron oxide. This 
is such as both to accord with the flow structure and to outline the 
quartz areas, which in these cases have a somewhat oval shape. 
When shearing has led to the production of sericite, this mineral is 
formed around the micropoikilitic areas, rarely traversing a single 
area, when it seems to be cementing material filling a crack (PL XVII, 
b). These areas are persistent, and slowly disappear in the develop- 
ment of a slate. 
While in some cases this structure is undoubtedly of primary char- 
acter, as Professor Iddings considers it to be in many porphyrites, in a 
large class of rocks its secondary origin seems equally certain. Irving, 
who gives one of the best as well as the earliest (1881) descriptions of 
this structure, considered it of a secondary character. His statements 
as to its nature and origin are so applicable to the South Mountain 
aporhyolites that they are quoted here in full: 1 
Although wholly absent from some sections, a very highly characteristic feature of 
the sections of many of these rocks, and more particularly of the fclsites without 
porphyritic quartz, is a network quartz which can only be regarded as of secondary 
origin. I find no mention of such a feature in any of the descriptions of the felsites 
of other regions which I have examined. Only occasionally is this net- 
work quartz coarse enough to be readily seen with a low power in the ordinary 
light. Usually both a high power and the use of the polarized light are required 
for its detection, when it appears in its most characteristic development as a deli- 
cate aborescent tracery or frost-work saturating the ground mass in all directions. 
In the polarized light all of the quartz network within each of these numberless 
irregularly round areas, whose existence would not be suspected in the ordinary 
light, is found to be similarly oriented. 
From these more pronounced developments the secondary quartz is found through 
many degrees of lessening amount and less plainly marked character until it disap- 
pears altogether. It is plainly of the same nature as the secondary quartz of the 
already described orthoclase-gabbro, diabase-porphyrite, and quartzless porphyry, 
and of the augite-syenite described below. It never, however, reaches in the rocks 
now under description the coarseness nor presents the graphic forms with which it 
appears in the augite-syenites, its characteristic development here being the deli- 
^oc. cit., pp. 99-100. 
