G6 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
On the highroad from Fountaindale to Fairfield, not far from the 
"Old Copper Shaft," occurs a dark purple-gray spotted slate. The 
light-green spots are sometimes irregular, but more frequently possess 
crystalline outlines and prove under the microscope to be a sericitic 
alteration of feldspar phenocrysts. Much of the feldspathic material 
still remains. The groundmass consists largely of iron oxide, which by 
its prevalence obscures the other constituents — leucoxene and quartz. 
Microscopic evidence is sufficient in this instance to determine the 
origin of the rock. It is plainly a sheared eruptive, and probably a por- 
phyritic aporhyolite, although the irregular outline of some of tbe 
sericitic areas suggests a brecciated aporhyolite. 
The occurrence of these slates is an interesting feature of the geology 
of the South Mountain. In the hand specimen they might readily be 
confused with a porphyroid, that is, a metamorphosed clastic rock, and 
have been so confused by geologists. They did not escape the atten- 
tion of Professor Rogers, who alludes to them as "the fissile talcose 
rock" near the "reddish gray rock, containing specks of reddish feld- 
spar," and includes them among the primal slates whose highly altered 
condition he repeatedly contrasts with the other slightly altered sedi- 
ments (sandstone). If these slates were of clastic origin, a high degree 
of metamorphism was necessary to produce their present crystalline 
condition, and Professor Bogers was quite right in drawing a contrast 
between their extreme metamorphism and the comparatively unaltered 
condition of all the other sediments. The very fact that their develop- 
ment from a sediment calls for such a high degree of metamorphism con- 
fined to limited and isolated zones, and for which no adequate cause can 
be assigned, renders such an origin as iuq^robable as it is unnecessary. 
Such a "selective metamorphism" is not demanded by the facts. 
As a matter of fact, these slates are scarcely more altered than the 
sandstone. Dynamic action in the latter has developed a quartzite. 
Dynamic action in the less resistant porphyry and aporhyolite has 
produced a sericite slate. That the chemical character of the acid 
rock remains essentially unaltered is evinced by analysis I, given on 
page 61. This shows exactly the composition of a rhyolite, and is 
totally unlike that of the sediments of the region. The sedimentary 
argillaceous slates of South Mountain are very little altered, and 
exhibit no tendency toward the development of porphyroids. 
All evidences — field relationship, successive stages shown in the 
hand specimen and under the microscope, chemical character, inherent 
improbability of clastic origin — combine to reveal the igneous character 
of these acid slates. 
SUMMARY. 
The acid igneous rocks of the South Mountain have proved to be 
quartz-porphyries, devitrified rhyolites (aporhyolites) with accompany- 
ing pyroclastics, and sericite- schists. 
