MEMBERS OF THE PYROCRYSTALLINE ROCKS, 
73 
near its junction with limestone it has become porous, preserv¬ 
ing at the same time its vitrified character, and its disposition 
to break with a conchoidal fracture. Its luster is resinous 
rather than vitreous. One of the most interesting localities is 
at Theresa on Indian river. The junction of these rocks may 
be observed in a gorge below the falls of the river. Many 
interesting points are exposed in the vicinity of this place, 
either in the iron mines or the various ravines connected with 
the creeks in the vicinity. 
Fig. 5. 
1 1 Gneiss, 2 2 Primary Limestone, 3 3 Potsdam Sandstone, changed at its 
junction with the limestone into vitrified quartz. 
The common kind of sienite is composed of feldspar, quartz, 
and hornblende, arranged as the minerals are in a true granite. 
The quartz is usually a .light smoke gray, the feldspar is also 
usually gray; but in some cases the latter is red. These min¬ 
erals are mixed uniformly together, and the individuals are 
small. If mica is present, it does not materially alter the 
appearance or character of the rock: ususally both are black 
and in small particles. The whole compound will be fine¬ 
grained, and in this condition constitutes a good building stone. 
§ 62. Porphyritic sienite. The feldspar in this variety 
appears in large individuals, imbedded in a finer ground. 
Either of the preceding varieties may pass into this by a change 
in the size of the particles of feldspar. 
Sienite occurs in Orange, Essex, and Clinton counties in 
New York, and in several counties in the eastern part of 
Massachusetts, particularly in the vicinity of Boston. So also 
sienite is a common rock in Pennsylvania, Virginia, and North 
and South Carolinas. In Virginia it occurs in Nelson and 
