164 MINERALOGY AND LITHOLOGY. 
Plagioclase Diorite. (Porphyritic Diorite.) There is another kind of 
diorite, which has been found in boulders scattered all over New Hamp- 
shire, and which is in place in the Dixville Notch and at other points in 
the northern part of the state. This diorite is characterized by the por- 
phyritic development of all its ingredients; but large grains, which are 
sometimes an inch and a half in diameter, of a clear white, glassy feld- 
spar, are particularly conspicuous. This feldspar is in condition for 
accurate determination, and proves to be a variety of plagioclase; and 
hence this diorite can be more definitely classified than the others thus 
far described. The feldspar is very striking in appearance. At the first 
glance it looks like quartz, for, in some directions, its fracture is vitreous, 
but on examining further, in other directions, bright cleavage faces are 
identified. In composition, it is near andesite, and its analysis has been 
given under that head (see p. 96). In polarized light few bands are 
found, for its separate laminz are quite broad. Its optical properties 
are those of oligoclase, and, according to Des Cloizeaux, andesite is 
identical with oligoclase. I call the diorite, plagioclase diorite, because 
a variation no greater than what is very liable to occur would make this 
feldspar labradorite or oligoclase; and the members of the class of plagio- 
clase diorites are subject to variations which embrace this sub-species. 
The hornblende of this rock was also analyzed. It was found to be 
quite aluminous. The magnetite is also porphyritically developed; and 
even the apatite is macroscopically visible in long, slender, clear needles, 
which pierce indiscriminately through all the other ingredients. The 
ground mass of this rock is an aggregate of the same constituent. The 
strong contrasts between the bright black and clear white crystals, and 
the dark, compact ground mass, make this one of the most striking rocks 
that occur in our state. 
When microscopically examined, this andesite is found to be fresh 
and clear, and in polarized light shows no effect of decomposition. In 
this respect it is almost isolated among our basic feldspars, and furnishes 
a good illustration of the greater power of a glassy mineral to resist 
decay; but in a specimen of this rock from Dorchester, in which the crys- 
tals are very large, all the minerals are much altered, and epidote becomes 
a prominent ingredient of the rock. 
Another diorite at Dixville is worthy of mention. This rock cuts the 
