156 The American Geologist. September, 1001. 
along- the serpentine border soiitli of Rock Springs ; about 
Sylmar and the Mount Hope church ; near the point where the 
west branch of Stone run crosses the state Hue, and along the 
railroad near the section-house below Conowingo. At the two 
last named localities the rock is exposed in outcrops and is as- 
sociated with peridotyte and serpentine. Near Conowingo it 
forms dikes in the noryte. These pyroxenytes are coarse, 
granular aggregates composed of rounded grains of diallage 
and hypersthene. In a hand specimen it is easy to distinguish 
these two minerals which make up the bulk of the rock. The 
diallage is dark green, almost black, while the hypersthene is 
commonly a reddish brown. The constituents often reach a 
length of from one-quarter to one-half an inch (7-13 milli- 
meters) and sometimes more. They vary in color from the 
dark green, almost black rocks rich in diallage to dark purple 
or reddish brown ones composed almost entirely of hypersthene. 
The hypersthene resembles that already described as occur- 
ring in the norytes and gabbros, though in the pyroxenytes 
the tabular inclusions were not observed. The mineral is 
readily distinguished under the microscope by its brilliant 
trichroism. 
The diallage is green in transmitted light and shows no 
pleochroism. The columnar structure so characteristic of 
bronzite and enstatite is often present in great perfection, the 
mineral being composed of thin columns or prisms grown 
closely together, which give the diallage a coarsely fibrous ap- 
pearance. The prismatic cleavage is w^ell developed but even 
more noticeable is the very distinct parting parallel to «. P oT 
(100). Small cleavage pieces parallel to the orthopinacoid 
g-ave in converged polarized light an optic axis which had an 
eccentric position in the field. The presence of this inter- 
ference figiu"e, together with the inclined extinction (40°) 
shows the mineral to be diallage and not bronzite or enstatite. 
Basal sections also sliow an optic axis surrounded by several 
bright rings. 
The only other constituents besides those already given 
are an occasional grain of magnetite and sometimes a little 
feldspar. As the latter increases in amount the rock grad- 
uates into a noryte. A common intermediate type is a 
pyroxene rock very rich in hvpersthene and in which the 
plagioclase forms about one-tenth of the entire mass. 
