PENNSYLVANIA ANORTHOSITES 
49 
extent molten. Granulation, which is a common feature of an¬ 
orthosites, is caused by movement of the magma just prior to its 
solidification. 
4. The silicic rock, syenite, which partly overlies and surrounds 
and intrudes the anorthosite, is separated from the latter by the 
gabbroic facies. It therefore, belongs to a distinctly later 
period of intrusion. 
The Honeybrook anorthosite region furnishes the following 
criteria: 
1. The anorthosite is a coarse-grained, extremely pure, mono- 
mineralic rock of the composition of labradorite. 
2. There is a conspicuous absence of anorthosite as dikes and 
outliers. 
3. There is no protoclastic texture. 
4. The border facies, because of its coarse grain, cannot rep¬ 
resent the chilled border of the parent magma. 
5. There is no intrusive relation between the anorthosite and 
the overlying quartz-monzonite. 
To sum up it is evident that the criteria of the Honeybrook 
anorthosite lend support to the Bowen hypothesis. 
The Honeybrook anorthosite forms a relatively flat, slightly 
domed, nearly horizontal mass which owes its origin to the sort¬ 
ing and accumulation of plagioclase crystals of the composition 
of labradorite (Ab^ An^), and was therefore never fluid as such. 
The coarse-grained border facies is very like the anorthosite in 
composition, rarely more silicic. The change from anorthosite to 
the overlying quartz-monzonite is abrupt, but the absence of in¬ 
trusive relations between the two preclude the possibility of there 
being an appreciable difference in time between their periods of 
consolidation. The finer grain of the quartz-monzonite is the 
result of the crystallization of an increasingly chilled residual 
magma. The anorthosite, on the other hand, owes its relatively 
coarse-grain to the fact that in the early stages of crystallization 
when mono-mineralic rocks are produced, there is opportunity 
for the constant enlargement of the growing crystals. A few 
remnants of the quartz-monzonite found on high land in the entire 
of the anorthosite area suggest that erosion has exposed only a 
portion of the anorthosite mass, all of which was once overlain 
by quartz-monzonite. 
