178 MINERALOGY AND LITHOLOGY. 
Porphyritic felsites are sub-divided into three species. Quartz por- 
phyry consists of a felsitic mass of quartz and orthoclase, in which ma- 
croscopic crystals or grains of both these ingredients are developed. 
Orthoclase porphyry consists of the same ground mass in which ortho- 
clase alone is porphyritically developed. Quartz-free-orthoclase porphyry 
contains quartz neither porphyritically developed, nor in the ground mass. 
The last division, though represented, is of no practical importance in 
New Hampshire. 
Quartz Porphyry. Almost all the porphyries which occur in the state 
belong to this division. The ground mass is a felsite in which the pro- 
portion between the quartz and feldspar is variable, and in which mag- 
netite, augite, hornblende, biotite, chlorite, hematite, apatite, and some 
other minerals may be present as accessories. Of the porphyritic min- 
erals, the feldspar has usually a crystalline outline, and sometimes the 
quartz also. The greatest diversity in the appearance of the rocks pre- 
sents itself. Some are black, some are gray, and some are red, and they 
may have a ground mass of any one of these three colors, and porphyritic 
crystals developed in it of any other of the colors; and so the variations 
-become very numerous. This rock forms small dykes; and, on the other 
hand, immense mountains like Mt. Kearsarge are largely composed of it. 
Our porphyries are commonly massive rocks with no signs of structure, 
except in those cases where they possess a schistose nature that has 
been induced in them by external agencies. Their geological relation- 
ships, though sometimes evident, are more often obscure. 
In the north-east part of Waterville a beautiful d/ack porphyry is abun- 
dant, which, on account of the fine opalescence of its clear grains of feld- 
spar, has been supposed to be a dolerite or labradorite rock, but the thin 
sections show that only orthoclase feldspar is present. The ground mass 
is very feldspathic, and contains some augite, chlorite, magnetite, apatite, 
and scales of hematite. The feldspar is clear, but is filled with minute 
fissures and microlites, reflections from which cause the irridescence. 
The rock is a beautiful one, and it would be much admired if cut and 
polished. It was analyzed by Mr. Pease, in the Sheffield laboratory, with 
the following result : 
Silica, . 5 7 . 7 . . ‘ . : : 4 63.63 
Alumina, . . 3 ‘ 5 . 3 q ‘ 3 A ‘ 17.42 
