bascom.] SLATES. 33 
red iron oxide. The quartz crystals are frequently cracked, testifying 
to more intense dynamic action. The extreme induration at both these 
contacts is confined to a selvage of the sandstone. 
In a cut on the Gettysburg- Railroad, southwest of the Old Maria Fur- 
nace, the sedimentary rock is locally of a different character from that 
which has been described. It resembles the second specimen described 
from the contact in the tunnel more closely than any other of the elas- 
tics. Shearing has been accompanied by the development of sericite 
and chlorite, producing' a soft, green, slaty rock. The chloritic areas 
have usually clearly denned boundaries, and possibly represent former 
hornblende grains. Zircon is present. The major part of the rock con- 
sists of quartz grains, with undulatory extinction. 
Another local alteration of the clastic rock is to be found on the hill, 
tops southeast of Jacks Mountain. Here it appears as a yellowish 
schistose rock. The thin section is characterized by the development of 
a large amount of sericite and granular epidote. The sericitic scales 
show a parallel arrangement, which is the product of shearing. The 
color of the rock is due largely to an iron hydroxide. 
Among the sedimentary rocks within the Monterey district there 
rarely occurs, associated with the sandstone, an argillaceous slate. 
This is exposed just north of Jacks Mountain Station. It is the only 
representative in the Monterey district of the interbedded slates occur- 
ring west of that district. It is silky, pearl-gray, crinkled, and cleaves 
readily into slabs. 
The thin section shows that the rock has been somewhat recrystal- 
lized, though its clastic origin is still apparent in the angular shape of 
its quartz fragments. Quartz and sericite are the principal constitu- 
ents of the rock. The other constituents are magnetite, hematite, and 
a gray cloudy aggregate which, under the highest power, obscurely 
suggests leucoxene and granular epidote. 
CHEMICAL ANALYSIS. 
The analysis of the quartzite given below shows proportions which 
would be possible only in a derivative rock where the exact chemical 
composition is a matter of accident and subject to no fixed laws. 
The silica percentage is disproportionately high for any but a clastic 
rock. The alkalies indicate the presence of a considerable amount of 
either orthoclase feldspar or sericite, probably the latter. The alumina 
and lime percentages denote the presence of epidote. 
Bull. 136 3 
