CRYSTALLINE ROCKS OF OAK HILL AREA L571 
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THE GABBROS 
Occurrence.—Among the igneous rocks of the Oak Hill area the 
gabbros are smallest in amount. They occur on Quarry Hill and 
seem to grade over, on the one hand, into the basic pyroxenite- 
peridotites and, on the other, into hypersthene gabbro or norite. 
Among themselves the gabbros are very variable in mineral compo- 
sition. ‘They range from a typical olivine gabbro to a hypersthene 
gabbro. There is no evidence of intrusion. On the other hand, 
the field relations together with the microscopic evidence demonstrate 
a transition into serpentine. A tunnel into the base of Quarry Hill 
on the north side penetrates the norite for about seventy-five feet. 
An open cut on top of the hill immediately above this tunnel shows 
exposures of the same norite. In the field a distinct gneissic structure 
is noticeable, which in this phase of the rock is scarcely evident in the 
hand specimen. Outcrops of the other gabbro are irregularly scattered 
over Quarry Hill, and none are found over the other parts of the area 
studied. 
Petrographic characters.—The gabbro and norite are granitic in 
texture and of dark-gray color. The typical gabbro is usually medium 
grained, while the norite is slightly finer-grained. In the hand speci- 
men the plagioclase, olivine and diallage of the gabbro are plainly 
visible to the naked eye. The plagioclases with their shiny sur- 
faces and the diallage crystals with their greenish shiny faces appear 
to be unusually fresh. In the norite the plagioclase and the hyper- 
sthene are likewise visible to the naked eye. Here the plagioclase 
appears slightly more weathered than that of the gabbro. The 
hypersthene occurs in numerous brown crystals evenly scattered 
throughout the mass. 
