44 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
deposited along these planes of former spherulitic crystallization. The 
silica occupies the center of the deposit and gives rise to ridged surfaces. 
These layers of sphernlites do not always lie in a single plane. Cross 
sections of the layers show irregular and minute sinuosities, which were 
doubtless caused by movement in the lava during consolidation. 
The bands consist always, when not composed of piedmontite, of a 
central dark line (impurities) with a lighter band (opaque quartz) on 
either side. These in turn are bordered by dark lines (iron oxide) 
(PI. X). This parallel banding, conspicuous even at a considerable dis- 
tance, though recognizable as a true igneous structure by one familiar 
with the acid lavas of the Yellowstone National Park or of the Lipari 
Islands, counterfeits a sedimentary structure so closely that it is not 
surprising that the rocks should be described as "bedded orthofel- 
sites." 
Still other specimens show spherulites with a tendency to an arrange- 
ment in rows and chains which are in turn collected in bands some 
2 inches in width traversing the rock and alternating with bands 
approximately free from spherulites. This arrangement differs from the 
layer spherulites in the fact that the outline of individual spherulites is 
quite distinct and the grouping irregular. 
Some fine examples of the lithophysal structure were found in the 
Raccoon Creek region (PI. XI). Nothing that could be definitely recog- 
nized as lithophysa3 was observed in the Monterey district. 
In the areas southeast of Jacks Mountain the aporhyolites have 
been more or less silicified or epidotized, or both, and are sometimes 
very difficult to distinguish, in the hand specimen, from the neighbor- 
ing sandstone. A good example of this rock is seen in the exposures 
on the turnpike about one-fourth of a mile above the Emmitsburg toll- 
gate, where silicification has produced a close resemblance to a quartz- 
ite. The fresh surface is white. Feldspar phenocrysts are sparsely 
distributed and scarcely discernable. Quartz blebs are more numer- 
ous. Pyrite is finely disseminated, and the exposed surfaces of the 
rock are tinged with yellow and red iron oxide. The rock cleaves in 
two directions oblique to each other. In this respect it resembles 
many of the felsites. 
The microscopic evidence of igneous character is supported by the 
field evidence, which shows a continuity between this altered rock and 
that which would be at once recognized as a felsite. 
MICROSCOPICAL DESCRIPTION. 
PHENOCRYSTS. 
Feldspar and quartz. — The feldspar phenocrysts are very like the 
feldspars of the quartz-porphyries, and little needs to be added to the 
description of them already given. They are fresh, contain inclusions 
of a once glassy magma, show perthitic intergrowth, and are twinned in 
accordance with the Carlsbad (albite) and Manebacher (pericline) laws. 
