i6o The American Geologist. September, looi. 
Conovvingo and is also exposed a mile and a half west of Syl- 
mar, near the state line, associated with pyroxenyte. These two 
types are in fact generally closely associated. While the peri- 
dotytes are at present not very abundant there is every reason 
for believing that much of the serpentine has been derived 
from them and hence that at one time the}^ covered much or 
all of the large area now occupied by the latter rock. 
The rocks under discussion are dark brown or almost black, 
and, while they are as a rule coarse-grained, they vary widely 
in structure and appearance. The variety from the Mount Hope 
church locality exhibits well the peculiar mottled or poikilitic 
structure. There are large, brightly reflecting cleavage sur- 
faces of pyroxene dotted over with small grains of olivine, 
largely changed to serpentine, and it is these dull spots which 
give the rock its speckled appearance. Under the microscope 
the pyroxene extinguishes uniformly over large areas. An- 
other variety from the dike below Conowingo has a porphyritic 
structure caused by phenocrysts of pyroxene scattered through 
a groundmass composed of olivine and accessory hypersthene. 
This groiindmass readily changes into serpentine through 
which are scattered the yet unaltered pyroxenes. Again, the rock 
may have a more or less banded appearance, as in the state line 
outcrop. This structure is produced by layers of compact, ser- 
pentinized olivine, alternating with other granular layers com- 
posed of pyroxene and some comparatively fresh olivine. 
The pyroxenic constituents of these peridotytes are hyper- 
sthene and diallage. Associated with these is more or less 
olivine and occasionally a little feldspar. In one specimen the 
latter had changed into zoisite. The pyroxene is readily dis- 
tinguished by the bright cleavage surfaces and reddish-brown 
or dark green colors. The diallage does not differ from that 
already described in the pyroxenytes. In thick sections it has 
a green color and show^s no pleochroism. The columuar struct- 
ure is well developed and many individuals include rounded 
grains of olivine. 
The orthorhombic pyroxene is represented by hypersthene. 
Cleavage pieces of this parallel to the macropinacoid show in 
converged polarized light two axes and a bisectrix in the field. 
The mineral is optically positive, the brachydiagonal axis be- 
ing the acute bisectrix. The positive character of the mineral 
