84 VOLCANIC ROCKS OF SOUTH MOUNTAIN. Ibull.13G. 
Other structures which furnish additional and convincing proof of an 
igneous origin need only be mentioned. A detailed description of their 
appearance in the aporhyolites and porphyrites has been given in the 
previous chapters. Such structures are the spherulitic, axiolitic, litho- 
physal, perlitic, rhyolitic, tiuidal, and amygdaloidal. 
Miner alogical. — Metamorphosed sedimentary rocks are always accom- 
panied by certain characteristic minerals. The absence of such minerals 
in these South Mountain rocks is conspicuous. Epidote and sericite 
are the only prominent alteration minerals. Of these, the former is the 
product of weathering rather than a true metamorphic mineral; the 
latter is more or less limited to shear zones, where its development is 
directly related to the dynamic force acting upon massive porphyries 
and aporhyolites. The absence of all evidence of contact action indi- 
cates their effusive character. 
Chemical. — The close conformity of the composition of these rocks in 
the one case with that of the rhyolites and in the other with that of the 
diabases and melaphyres from all parts of the world, as tabulated by 
Roth, indicates an igneous origin. Their uniform composition is a con- 
trast to the composition of a series of clastic rocks, where the chemical 
proportions are largely a matter of accident. A similar test has been 
applied by Eosenbusch l to the determination of the origin of Archean 
gneisses. The association of these types of acid and basic lava accord 
with the laws of petrographical consanguinity. 
ORIGINAL ROCK TYPES. 
ACID IGNEOUS ROCKS. 
The acid-lava flows in South Mountain are regarded by the writer as 
quite comparable, at the time of their consolidation, to similar flows in 
post-Tertiary time, such, for instance, as those which have been recently 
studied in the Yellowstone National Park. Certain portions of the flow, 
as in the case of the Obsidian Cliff, were completely vitreous save for 
spherulitic and lithophysal crystallization. In other localities the lava 
was lithoidal, and in the central portion of thick flows holocrystalline. 
In this way three types of acid volcanics would be developed — 
rhyolites, lithoidal rhyolites, and quartz-porphyries. Every gradation 
between these types would accompany these. 
Thus, while there are certain areas in the South Mountain, notably 
the JBighain Copper Mine and Eaceoon Creek localities, which exhibit 
typical ancient rhyolites, other regions display genuine quartz-por- 
phyries. While in the latter rocks, which constitute a not inconsider- 
able portion of the acid flows, the groundmass may have been, and prob- 
ably was, originally noncrystalline, as in some modern lavas, m the case 
of the former rocks it is supposed that the groundmass was, at the time 
of consolidation, wholly or partly glassy. 
•Zur Auffassung der chemischen Natur dea Grundgebirges: Tscherniaks mineral. Mittheil., Vol. 
XII, 1891, pp. 49-61. 
