GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 37 



called the whole the Fordham gneiss. This he correlates with the 

 Grenville of Canada and the iVdirondacks. 



THE HORTONTOWN BASIC ERUPTIVE AND 

 ASSOCIATED METAMORPHIC ROCKS 



General relations. In the orchard by the house and near the 

 barn on the farm of Albert Lawrence at Hortontown, are several 

 outcrops of a massive, compact, greenish rock. One or two ledges 

 are of moderate size, but most of the outcrops are small and incon- 

 spicuous. This rock is traceable only a short way to the north or 

 south by actual outcrops, but in the fields and stone walls south of 

 the orchard there are numerous boulders of this rock. The actual 

 ledges disappear beneath the hill to the southwest of the orchard. 

 At the summit of this hill, in a west by southwest direction from 

 the house, and about 200 or 300 yards away, are numerous ledges 

 of a rusty, blackish rock, which may be followed to the southwest 

 for a short distance and then are lost. Just to the west of these 

 outcrops, on both sides of the road and in the road itself, are numer- 

 ous outcrops of quartzite with southeast dip and a strike east of 

 north. A conspicuous ledge of this quartzite borders the west side 

 of the road. West of this is a gully about 50 or 75 feet in width 

 which at the west is bounded by a perpendicular cliff of the 

 gneisses. The relationships just described are indicated on the 

 accompanying sketch map (see figure 11). 



It was not possible to determine the configuration of the mass 

 to which the greenish rock belongs. The east-west distance be- 

 tween outcrops was estimated at 50 feet, but there is reason for 

 thinking that the rock has a greater extent. 



Petrography and general description. The greenish rock is very 

 tough. It shows variations from a greenish black rock, streaked 

 with lighter green, through a mottled variety to a lighter, greener 

 rock with a tendency to fibrous structure. 



The rock may be cut with a knife. Some varieties, when pol- 

 ished, give a rich, dark, glossy finish. When powdered and tested 

 by the magnet it reveals large quantities of magnetite to which the 

 darker hues are due. Weathered surfaces show freckles of black 

 and greenish yellow, caused by the bleaching of the microscopic 

 crystals among the magnetite grains. The thin section in transmitted 

 light shows innumerable dustings and irregular grains of magne- 

 tite, while the rest of the section is yellowish white. With crossed 



