214 MINERALOGY AND LITHOLOGY. 
monly twinned, and usually, though not always, lie with their long direc- 
tion in the plane with the mica. Very often these crystals are ill formed, 
and lenticular in shape; and the black mica surrounding such crystals 
gives to them the appearance of eyes, whence the German name “augen 
gneiss.” Such gneisses occur at Newbury and Antrim. This formation 
reaches it northern limit at Bethlehem. At this point the gneiss is much 
lighter, on account of the greater abundance of muscovite. At New 
London some of the mica, by decomposition, has acquired a brilliant 
golden yellow color. Inexperienced persons have often had their curi- 
osity excited by such mica in certain of our gneisses. This porphyritic 
gneiss is a very important and widely distributed rock. 
The members of the group of rocks to which Prof. Hitchcock has 
given the name of Bethlehem gneiss belong mostly in this place. They 
are in general characterized by a faint green tint that is due to the pres- 
ence of some chlorite, which is a decomposition product, and which allies 
them to the protogene gneiss. At Hanover this rock contains epidote, 
and crystals of tourmaline are found by an examination of thin sections. 
It also contains pyrrhotite, which does not appear to be an objection 
to its employment as building material,—for the base of the fine Culver 
Hall, one of the Dartmouth College buildings, is made of it, and none of 
the stains are found which some expected would shortly appear. A 
specimen of this gneiss from Bethlehem is almost white, and only faintly 
and irregularly streaked with micaceous layers. 
Most numerous appearances are presented by gneiss, which are de- 
pendent on the color, structure, and arrangement of its constituents. A 
kind from Carroll is what is called wavy gneiss, because the layers of 
mica are undulating. Such and numerous other structures are induced 
by the same agencies that crumple up strata. A kind from Marlow con- 
tains andalusite, and also many red garnets, which dot the black surface, 
making a very pretty rock. A gneiss from Ossipee contains numerous 
and quite perfect rhombic muscovite crystals. And so we might con- 
tinue ;—but the causes of the great number of variations to which gneiss 
is subject are self-evident, and need no elucidation. 
Hornblende Biotite Gneiss. In some gneiss, hornblende is visible to 
the unaided eye, as, for example, in one from Frankenstein cliff; but 
more often it is found, by the aid of the microscope, as a constituent of 
