bascom.1 ACID VOLCANIC BRECCIA. 63 
thus set free was carried off by percolating* water, a low silica percent- 
age and a correspondingly high alumina percentage would result, whde 
the alkalies would remain about the same. The increase in iron may be 
due to infiltration. 
ACID VOLCANIC BRECCIA. 
DISTRIBUTION. 
The presence of acid pyroclastics in the Monterey district has already 
been mentioned. Although a conspicuous feature of a portion of South 
Mountain, notably of the Buchanan Valley north of the Chambersburg 
turnpike, where they cover about 2 square miles, they play an insig- 
nificant role among the rocks of the Monterey district. Their charac- 
ter, however, is unmistakable. They may be classified as tuffs, flow 
breccia, and true breccia or tuffaceous breccia. 
This is a dark-purple banded rock, the clastic character of which is 
hardly evident in the hand specimen. Microscopic examination dis- 
closes its tuffaceous nature. Minute angular fragments, not exceeding 
a millimeter in length, are thickly distributed through a crystalline 
groundmass. 
The fragments are usually spherulitic, with the replacement of the 
spherulitic crystallization, as in the massive aporhyolites described on 
page 52, by a fine mosaic, so that in ordinary light the spherulites are 
traceable only from their outline. In the same way, under crossed nicols 
a uniform granular crystallization obscures the fragmental character 
of the rock — a character which, in ordinary light, is sharply brought 
out by an outlining pigment of red iron oxide. Fragments of quartz 
and feldspar are among the inclusions. The groundmass, which is of 
the same chemical and mineralogical constitution as the included frag- 
ments, doubtless represents an ash recrystallized. 
FLOW BRECCIAS. 
These occur at widely separated localities — northwest of Old Maria 
furnace, near the source of Toms Creek, on the Gladhills road, and on 
the brow of the hill east of the Clermont House. These breccias are 
composed of fragments of considerable size, which plainly were caught 
in a viscous acid magma, as is evidenced by their linear arrangement 
in How lines and by the way in which different fragments fit together, 
forming what was once a larger fragment. On the weathered surface 
of the rock its brecciated character is rendered very manifest in the 
varying tints of pink, red, purple, and blue (PI. XII). The fragments 
range in size from the submacroscopic to those that are 2J inches in 
diameter. Their spherulitic character is discernible by the naked eye. 
Under the microscope, in ordinary light, the fragments frequently show 
either perlitic parting, a spherulitic character, or a regular arrangement 
