bascom.] BASIC SLATES. 79 
The "epidotic rock" (I aud II) shows, as would be expected, an 
abnormally high lime percentage. 
The chief variation from the normal type lies in the addition of lime 
aud iron oxide and the abstraction of the alkalies aud magnesia. 
BASIC SLATES. 
DISTRIBUTION AND DESCRIPTION. 
The localities where the basic slates occur in any considerable extent 
are colored a light yellow-green on the map of the Monterey district. 
The slight rounded eminence opposite the Fountaindale post-office is 
composed of a dark-gray crinkled and finely laminated slate. It is 
darker colored than the clastic slate of the region and is surrounded 
by the basic eruptives. The thin section shows very distinct traces of an 
ophitic structure. Iron oxide, chlorite, and sericite are the only con- 
stituents that can be determined. About three-fourths of a mile north 
of Monterey station, on the road leading from the turnpike to the Old 
Furnace road, there is an exposure of a lighter-colored slate. The same 
constituents are found in this slate, with a larger proportion of sericite 
and the addition of leucoxene. The ophitic structure is barely discern- 
ible, and it is with considerable hesitancy that the slate is referred to 
the group of igneous rocks. Half a mile farther northeast, on the Old 
Furnace road, just beyond Gum Spring, occurs a light-gray spotted 
slate of undoubted igneous origin. It is not so finely foliated as the 
slates that have just been described, and much of the original structure 
remains. The constituents are the same as those of the last-mentioned 
slate. Leucoxene is more abundant and the ophitic structure is pro- 
nounced. 
The amygdules, which give the slates a spotted appearance, are com- 
posed of quartz, sericite, and some chlorite. These slates are related 
to the porphyrites of Group I, some typical examples of which occur 
near by. 
At the west end of the Gettysburg tunnel, just above the iron-bear- 
ing band previously described, another spotted slate has been devel- 
oped from the basic igneous rock. In this case the slate is green, and 
the spots are a brilliant dark shade of the same color. Iron oxide, 
chlorite, epidote, and some silica are the constituents. The original 
structure is entirely obliterated. Some of the epidote grains faintly 
suggest olivinitic forms. Chlorite is the prevailing mineral. The spots, 
which as in the other slate are sheared amygdules, are formed of chlo- 
rite only. This slaty zone is only a few inches wide and passes some- 
what abruptly into a slightly schistose porphyrite. 
The first tunnel on the old Tapeworm .Railroad, which was abandoned 
before an excavation was made, exposes a green slate which differs from 
the one just described only in its nonamygdaloidal character and in the 
greater abundance of epidote. The knoll northeast of Blue Eidge 
