40 
PENNSYLVANIA ANORTHOSITES 
bled and disintegrated, owing to the weathering out of the less 
resistant mineral constituents. The slopes of the quartzite ridge 
to the south of the anorthosite are covered with buff-colored, flat, 
prismoid rock-fragments which are easily cleavable into thin slabs 
with shiny surfaces due according to Bliss and Jonas,^^ to “small 
flakes of glistening sericite developed along the bedding planes.” 
Gabbro is found in scattered areas over the Honeybrook district; 
but it does not anywhere occur in contact with the anorthosite. 
Small, narrow, diabase dikes trending northeast and southwest 
penetrate the anorthosite, but can be traced only for short dis¬ 
tances. Pegmatite dikes intrude the anorthosite near the southern 
.boundary. 
Honeybrook anorthosite is a remarkably uniform rock. The 
type, characteristic of the whole mass, is a medium- to coarse¬ 
grained rock composed exclusively or chiefly of labradorite or a 
closely related variety of feldspar, which is altered to zoisite. It 
is this alteration product which gives the rock its characteristic 
light bluish-gray color. Fresh anorthosite is obtained from one or 
two localities only, and it is in these localities alone that the feld¬ 
spars are clear and glassy, and easily determinable. Even anor¬ 
thosite which is freshly blasted from the ledge is found to be quite 
as thoroughly zoisitised as the weathered, blue boulders so char¬ 
acteristic of the region as a whole. Four quarries, which are 
opened in the anorthosite for pegmatitic or other intrusions, furn¬ 
ished no less zoisitized material. 
The only variation from pure anorthosite is found close to the 
boundaries of the area where the rock is streaked with narrow, 
irregular bands of mafic constituents. The pure anorthosite and 
the border- facies will be described in detail. 
'In hand specimens, the fresh rock is medium to coarse-grained, 
pinkish gray in color. It is composed almost entirely of clear, 
mutually interfering crystals of feldspar showing bright cleavage 
surfaces. The feldspars for the most part are of uniform size, 
not exceeding four or five millimeters in diameter. There are, 
however, a few exceptional laths measuring over two centimeters 
in length. In the more highly altered specimens which contain 
zoisitized or sericitized areas in great amount, the rock becomes 
blue-gray in color, and the feldspars are clouded. Ferro-magne- 
33 U. S. Geol. Surv., Atlas, Coatesville Folio, p. 21, 1918. 
