LITHOLOGY. 209 
carbonic acid were generated as suggested, the conditions of heat and 
pressure were such as to bring about only a partial reaction; and the con- 
ditions under which granitic rocks have been recrystallized, as indicated 
by other circumstances, are just about such as would account for the 
associations observed in this rock. 
Another sienite, which in most respects is exactly like this, occupies a 
dyke in Sandwich. Calcite is absent, and with this absence the quantity 
of carbonic acid decreases. In many cavities in the quartz, however, 
liquid carbonic acid is found, but in the larger number only water and 
the customary crystals of salt. These two rocks are our only ones in 
which I have found this interesting enclosure of liquefied gas, but they 
are remarkable examples. 
On Mill mountain, in Stark, a dark-colored sienite occurs. Macroscop- 
ically, it appears as if wholly composed of orthoclase ; but in thin sec- 
tions considerable hornblende is found in small fragments, which are 
quite thickly distributed: if the rock were light-colored, as are the 
ones thus far described, the hornblende would be prominent enough. 
The feldspar is of that clear grayish waxy variety (adularia) which is so 
prominent in a group of granites which have been described, and to 
those granites this sienite is related. It contains also plagioclase, apa- 
tite, sphene, and epidote, besides a little microscopic quartz. 
A very different kind of sienite is abundant about the town of Albany. 
This is a very fine-grained feldspathic rock which is found in small 
dykes. It is red in color, and full of minute black specks. When thin 
sections are cut, these specks are found to be of a very deep-colored 
hornblende with some little crystals of magnetite, while the mass of the 
_ Tock is composed of small but well defined crystals of orthoclase, in the 
angles between which a little quartz is found. These rocks have com- 
monly been called felsites ; but in no respect do they differ from ordi- 
nary sienite, save in the fineness of their texture. On Sabba-Day brook 
there are favorable localities to see these rocks in place. 
GNEISS. 
Gneiss is in composition exactly like granite, being composed essen- 
tially of quartz and orthoclase as constant constituents, and muscovite. 
‘s 
biotite, hornblende, chlorite, etc., as essential but variabl 
€ consti 
VOL. IV. 27 stituents, 
