42 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
the Chamber sburg turnpike, and one-eighth mile west of Musser's store. 
Pied mori tit e occurs here in cavities associated with sclieelitc.' 
Epidote is also abundantly present in the porphyries, passing by 
gradations of rose colored epidote to the deep carmine of the piecl- 
montite. Both of these silicates are undoubtedly secondary products. 
Innumerable black globulitesof manganese oxide and black and red 
iron oxide crowd the groundmass. In their arrangement they show 
rluxiou structure, or form meshes inclosing the quartz areas which 
constitute a part of the micropoikilitic structure. This arrangement is 
sometimes so marked as to be conspicuous in the hand specimen, giving 
the rock a mottled appearance. Crystals of zircon are not infrequently 
present. 
A study of the quartz porphyries of the South Mountain shows us 
that they are present there in characteristic development, and offer no 
marked variations from the normal type described from other localities. 
For this reason a brief space only has been devoted to them. 
APORHYOLITES. 
DISTRIBUTION. 
There remains to be described a large and important portion of the 
acid rocks of the Monterey district. These will be termed aporhyolites, 
for reasons already suggested and to be more fully discussed in the 
sequel. 
The localities colored red upon the geological map of the Monterey 
district (PI. Ill) are aporhyolitic areas associated, in the cases already 
mentioned, with quartz-porphyries. The largest area begins just south 
of the Bigham copper mine, and, widening toward the north, extends 
far beyond the Monterey district. This area furnishes most of the 
spherulitic structures. A small detached area south of the Clermont 
House furnishes slaty and spherulitic aporhyolites. These aporhyolites 
and those of the four detached areas near the Maryland State line, 
separated from each other by a distance of a mile to a mile aud a half, 
show some marked points of resemblance, to which attention will be 
called later. 
The areas covering the foothills southeast of Jacks Mountain show 
an altered aporhyolite. There are several small areas of red aporhyolite 
on the Gladhill road leading to the Bigham property. At the forking 
of this road with the Fountaindale turnpike occurs the aporhyolite slate 
mentioned on page 29. 
MACROSCOPICAI. DESCRIPTION. 
The aporhyolites have about the same range in color as the porphy- 
ries, varying from light bluish-gray, or buff, to many shades of red 
and purple. While usually compact and always fine-grained, they are 
1 G. U. Williams Piedmontite and scheelite from the ancient rhyolite of South Mountain, Pennsyl- 
vania. Am. Jour. Sci., Vol. XLVI, 3d series, July, 1893, pp. 50-57. 
