54 



NEW YORK STATE MUSEUM 



POST-GRENVILLE INTRUSIVES 



GABBRO-AMPHIBOLITE; PIERREPONT OCCURRENCE 



A further consideration of hornblende schists leads to the subject 

 of the post-Grenville or intrusive rocks. At two localities in the 

 hills south and southwest of Pierrepont direct evidence on 

 the origin of the prominent development of hornblende schists of 

 the sigmoidal area was obtained in the guise of what may be looked 

 Upon as intrusive contacts. Without going too far into detail, it 

 may be stated that at a point one and one-tenth miles east of Beach 



Fig. 8 Xenolith of garnet gneiss in amphibolite ; ground plan. The 

 exposure is terminated on the west end by two joint faces, about 60 feet 

 long; on the east the mass tapers away into the amphibolite. (a) Amphi- 

 bolite; (b) garnet gneiss. 



One and one-tenth miles east of Beach Plains Church, one-tenth of a 

 mile south of road. 



Plains Church, a mass of typical black hornblende schist was ob- 

 served in contact with a xenolith of typical garnet gneiss. The ex- 

 posure (see figure 8) is some 60 feet square and terminated 

 abruptly on the west side by a 6- foot vertical bluit. It shows a 

 thick mass of garnet gneiss tapering eastward into the amphibolite, 

 suddenly at first and then more gradually, and exhibiting foliation 

 concordant with its boundaries (see figures 9 and 10). The latter 

 rock surrounds the garnet gneiss on at least three sides; on the 

 fourth, that of the joint faces, the relations are obscure. On the west 

 face the formations are concordant, but on the southwest edge they 

 are far from exhibiting the usual conformity wdiich characterizes the 

 parallel interbanded strips of these rocks. The amphibolite presents 

 a steep contact (about 70^-75° ) against the gneiss, and near this 



