PENNSYLVANIA ANORTHOSITES 
43 
Under the microscope the texture is subhedral, and inequigran- 
ular. The primary constituents are andesine-labradorite and diop- 
side, with accessory apatite, magnetite, biotite, and rarely quartz. 
The secondary constituents are actinolite, sericite, epidote, zoisite, 
chlorite, and leucoxene. Garnets are present but are compara¬ 
tively rare. Much of the diopside is altered to actinolite, chlorite, 
and epidote. Some of the pyroxenes show beautiful reaction 
rims of epidote. The proportions of the constituents are feldspar 
90 to 75 per cent, mafic minerals 10 to 25 per cent. 
To sum up, the anorthosite is a medium- to coarse-grained feld- 
pathic rock composed almost entirely of interlocking crystals of 
labradorite. The crystals vary in size from a few millimeters to 
two or three centimeters across their greater diameter. Granula¬ 
tion of the feldspars, which is such a common feature of the 
Morin and Adirondack anorthosites, is not found in these rocks. 
The Lake Superior anorthosite is reported by Winchell to be 
free from secondary textures. The only indications of strain 
subsequent to consolidation of the Honeybrook anorthosite are 
the universal twinning, and the occasional distortion or bending 
of the plagioclase crystals. There is no perceptible difference be¬ 
tween the grain at the centre of the area and that at the borders. 
The medium- to coarse-grained character persists to the farther¬ 
most limits of the anorthosite mass. From the comparatively 
coarse-grained, light bluish-gray hornblendic anorthosite along the 
periphery which contains andesine-labradorite feldspars, one passes 
rather abruptly to the finer-grained, dark-coloured quartz mon- 
zonite with its indiscriminate assemblage of dark and light con¬ 
stituents containing feldspars much higher in potash and soda, 
and also containing quartz. 
Except along the south, where the Cambric quartzites are fault¬ 
ed against the intrusives, the anorthosite is everywhere surround¬ 
ed by quartz-monzonite. The quartzite will not be described, 
since it has no bearing upon the origin of the anorthosite. Scat¬ 
tered areas of gabbro, and a few small dikes of metapyroxenite 
and metaperidotite are found in the Honeybrook area, but not in 
contact with the anorthosite. 
Megascopically the quartz-monzonite is a medium- to fine¬ 
grained, rather dark-gray rock composed of quartz, feldspar, 
34 American Geologist, Vol. XXVI, p. 216, 1900. 
