46 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
There is some epidotizatioii of the groundmass, resulting in the pro- 
duction of finely granular epidote concentrated at certain centers of 
development. 
The mica by which these lavas are characterized has a fibrous char- 
acter, and is the andesitic type described by Koscnbusch. 1 It is in 
ldiomorphic clusters of reddish-yellow fibers. The absorption is char- 
acteristically strong and the pleochroism marked. Parallel to the 
cleavage the fibers show a deep reddish brown color, and at right 
angles a light yellowish-red or straw-yellow color. The marked resem- 
blance of the lava from these localities suggests a continuity in the 
lava liow — a continuity probably maintained underneath the greenstone. 
If this is the case these areas are exposed by erosion of the overly- 
ing basic lava, if it ever completely overlay the aporhyolite. That the 
continuity was maintained above the greenstone is a scarcely tenable 
hypothesis, because of the extended erosion which such a superposition 
would necessitate. 
If any other ferromagnesian constituents were present in the apor- 
hyolites — and there are indications that there were — they have been 
totally decomposed and removed. 
Harker 2 found it to be true of the pre Cambrian acid volcanics of 
Wales, that clusters of biotite Hakes were preserved in a compara- 
tively fresh condition, while only slight trace of augite or hornblende 
remained. 
In the South Mountain aporhyolites these minerals must have been 
rare originally, and are now completely replaced by epidote. Occa- 
sionally the outline of a perfect crystal section parallel to 010 is pre- 
served, while the substance of the crystal is entirely replaced by epi- 
dote individuals. In the absence of basal sections there is no clue to 
the specific character of the original crystal. 
GROUNDMASS. 
The groundmass is always a quartz-feldspar aggregate, of varying 
structure and grain (about - 8 \ of a millimeter in diameter), more often 
finely than coarsely microgranitic. The structures of the groundmass 
are of the highest interest and importance in their disclosure of the 
original character of the rocks which possess them. 
The fluidal, micropoikilitic, spherulitic, axiolitic, lithophysal, rhyo- 
litic, micropegmatitic, perlitic,taxitic, amygdaloidal, and trichitic struc- 
tures are characteristically developed and merit detailed description. 
Fluidal structure. — The fluidal structure, Avhich is so familiar to all 
students of rhyohtic lavas, is a conspicuous feature of the aporhyolites, 
both macroscopically, as has been described, and microscopically. 
Globulites of magnetite and hematite and indefinable opaque crystal- 
lites follow sinuous lines of flow, twisting around the pheuocrysts and 
1 Rosenbusch, Massive Gesteine, 2d ed., Vol. II, p. C58. 
2 Bala Volcauic Series of Rocks, p. 18. 
