50 
PENNSYLVANIA ANORTHOSITES 
The pre-Cambrian igneous complex of the region around 
Honeybrook includes pyroxenites, peridotites, gabbros, quartz- 
gabbros, anorthosites, diorites, quartz-monzonites, and granites, 
all of which represent a series of differentiation products from a 
single great magma. Whether this complex represents batholithic 
or laccolithic intrusion cannot be ascertained from the structure 
of the pre-Cambrian gneisses and schists which once covered the 
region. 
In certain centres of this great intrusive chamber conditions 
were evidently adapted to extreme differentiation. For the set¬ 
tling out of crystals, such as is implied in the formation of mono- 
mineralic rocks, it is necessary to assume that, locally at least, a 
tongue of dioritic, or gabbroic, magma forced a passage through 
consolidated material and spread sufficiently both laterally and 
vertically to form a chamber of considerable size in which heat 
was maintained for a long time. In this centre of differentiation 
the anorthosite and underlying pyroxenite settled with complete 
sorting, and the squeezed-up, residual magma solidified into quart- 
monzonite. Whether or not there was a still more silicic residue 
above the quartz-monzonite cannot now be determined, for much 
of the roof of the chamber was long ago, even before Paleozoic 
time, removed by erosion. 
