30 NEW YORK STATE MUSEUM 



basal sandstone to a quartzite and metamorphosed the overlying 

 limestone and slate. The partly disintegrated upper portions of the 

 gneisses would have been thoroughly indurated into a compact rock 

 and probably partially recrystallized. The less altered gneiss would 

 also have been changed, although not necessarily in such a way 

 as to form entirely new minerals. Chlorite would now appear 

 in a firm rock as a pseudomorph after biotite, or hornblende, and 

 the old iron oxids would have been preserved as magnetite or hema- 

 tite. In places where alteration had not taken place, the practically 

 unchanged gneiss would be preserved. 



It is possible in this manner to account for the peculiar rock types 

 of the Glenham belt and for the occurrence of such features as a 

 coarse granitic " stratum " resting on upturned gneisses and fol- 

 lowed by a somewhat foliated, finer-grained, quartzitic rock as shown 

 in the gorge of Mount Beacon brook; or for the occurrence of such 

 extensive surface developments of rock as the chief varieties of the 

 Glenham belt, which so certainly rest upon and grade into the in- 

 clined gneisses. Conditions would have been very favorable for the 

 interaction of feldspars and ferromagnesians, which now find ex- 

 pression in the abundant and widely distributed epidote that clearly 

 belongs to an ancient period of alteration. 



A relatively large proportion of the ancient altered gneisses has 

 been preserved in the Glenham belt. The section along the Wap- 

 pinger Falls road, with its assemblage of altered and unaltered types, 

 seems intelligible from this explanation. 



At places, as at Vly mountain, and near " Rock Hollow " in Mat- 

 teawan, fragments of the quartzite have been preserved and these 

 apparently grade into the underlying rock with which they are both 

 unconformable and coextensive. 



These principles of subaerial decay have been applied in the 

 foregoing discussion to certain altered gneisses and hybrid rocks 

 occurring in many places among the Fishkill mountains. They serve 

 to account for an evident ancient alteration in these recks and for 

 the occurrence of certain types that are intermediate in character 

 between the quartzite and the underlying gneiss. 



SUMMARY OF THE MICROSCOPIC CHARACTERS OF THE GNEISSES 



A microscopic examination has been made of about twenty-five 

 sections of the gneisses of the Fishkill mountains, selected from 

 types which were believed to show the principal variations in the 

 gneissic series from west to east. A half dozen were also selected 

 from the Glenham belt. 



