LITHOLOGY. 187 
regarded as an eruptive mass. The rocks are alike in both places, and 
one description will apply to both. They are either gray or very light 
red. The ground mass is usually very fine in its texture, but it is often 
porous and rough, and often very much resembles that of some modern 
trachytes. Were it not that the feldspar is opaque orthoclase instead 
of clear sanidin, one would immediately think of trachyte upon examin- 
ing these rocks. At some points on Mt. Pleasant the rocks are cracked 
up into angular blocks and fragments, as are many modern eruptive 
rocks; and in other places large crystalline grains of hornblende are 
prominent, and cause other kinds of trachytes to be recalled. Trachyte 
is a newly erupted rock, which, in addition to its glassy feldspar, con- 
tains noncrystalline substances,—nephelin, leucite, and the like; and 
though in special varieties any or all of its distinctive features or char- 
acteristic minerals may be wanting, the rocks as a class are very well 
distinguished, and no one would wish to confound such old rocks as 
these that we are considering with them. Still, I think these orthoclase 
porphyries from Albany and Mt. Pleasant are very interesting, since, by 
their very close macroscopic resemblance to certain trachytes, they show 
the close relationship which exists between these rocks and their younger 
kindred. Under the microscope no peculiarities of note are developed. 
Hornblende is found to be a common constituent; the feldspar is usually 
troubled by impurities and decomposition, but in some specimens it is 
quite fresh, though never glassy. On the summit of the mountain the 
ground mass is reduced to a minimum in quantity, and the rock ap- 
proaches sienite in appearance and composition. 
Quartz-Free Orthoclase Porphyry. This rock has the same structure 
as the other porphyries, but possesses quartz neither among the macro- 
scopic crystals nor in the ground mass. A rock of limited distribution 
anywhere, it is of no practical importance in New Hampshire, for it oc- 
curs as a mere exceptional variety in one or two places among the quartz 
porphyries. For example: a specimen of red porphyry from Waterville 
and another from Albany are remarkable because no quartz can be de- 
tected in them, while at the same time a triclinic feldspar, conspicuous 
in polarized light by its bands of color, appears as an ingredient, and by 
its presence suggests the more basic nature of the rock. 
In these varieties of porphyry in which large crystals of quartz do not 
