BASCOM.] 
SERICITE-SCHISTS AND SLATES. 65 
mass. In the next stage the groundmass shows a decided, tendency 
to a parallel arrangement, and sericite is more abundantly developed. 
Eventually the phenocrysts are obliterated and there is much sericite 
in the groundmass. It is not, however, developed to the exclusion of 
the feldspar, while the quartz remains unaltered. 
In general, the development of sericite stands in direct relation to the 
shearing, and increases up to an almost complete, if not quite com- 
plete, replacement of the feldspathic constituents of the groundmass. 
Silica remains as a constituent of the groundmass. 
Material obtained from an artesian well at a point 40 feet below the 
surface furnished a similar shear zone, which displayed an even more 
abrupt transition from a porphyry to a sericite-schist. A single micro- 
scopic section included both porphyry and schist in typical develop- 
ment. The former showed an early stage of the alteration which was 
complete in the latter, where only sericite and a very schistose siliceous 
microgranitic groundmass remained. In this case, and in the extreme 
stage of the transition previously described, it would be impossible, 
with the microscope alone, to decide whether the schists were of clastic 
or nonclastic origin. This is one of the instances where field evidence 
is quite essential to the authoritative determination of the origin of the 
rock. 
At the Bechtel shaft there has been thrown out a mottled red and 
white schist which has been produced by the shearing of a massive 
felsite. Here the phenocrysts have been replaced by a quartz mosaic 
and some sericite, which is also largely developed in the groundmass. 
The red mottling is due to a more or less parallel arrangement of red 
iron oxide globulites. 
A light green sericite-schist found on the railroad near Blue Eidge 
Summit station, and closely resembling some schists in situ exposed 
on the Gettysburg Railroad below the Clermont House, shows under 
the microscope phenocrysts of feldspar containing inclusions of a 
former glassy magma, still well preserved and showing twinning stria- 
tums. These phenocrysts occur in a groundmass of quartz, a little 
feldspar presumably, much sericite, epidote,ilmenite, or magnetite, and 
leucoxene. 
The color of the schist is due largely to the epidote. 
At the exposure just now mentioned on the Gettysburg Railroad, 
east of the Clermont House, there occurs a handsome, light, silvery- 
green, crinkled sericite-schist. Several rods to the north of this 
exposure the railroad cuts through quartz-x)orphyry,'but the contact 
between the porphyry and the schist is not exposed. The schist is sim- 
ilar to those already described, whose gradual passage into a massive 
porphyry could be followed in the field, and shows traces of pheno- 
crysts under the microscope, and in the hand specimen on the surface 
at right angles to the cleavage. The cleavage surfaces often display 
exquisitely delicate and manifold dendritic tracery. (PI. XIV.) 
Bull. 136 5 
