76 
GRANITIC GNEISS. 
unite with them chemically." After enumerating 
several instances where granite is found protru- 
ding through the superincumbent rocks, Prof. H. 
proceeds to sum up his reasons for believing this 
rock to be of igneous origin ; which, although we 
purposely design in the present work to abstain 
from theoretical geology, are yet so conclusive as 
to merit particular mention. These are, 1. The in- 
clined position of the older stratified rocks : 2. The 
manner in which it is intruded among the stratified 
rocks : 3. The mechanical effects which it appears 
to have exerted upon the stratified rocks in its im- 
mediate vicinity : 4. Its mechanical effects upon 
the surrounding strata : 5. Its crystalline structure, 
and the numerous crystallizations of other substan- 
ces that have taken place in it. 
Gneiss, 
Gneiss is composed of the same minerals as gran- 
ite, namely, quartz, feldspar, and mica^ arranged in 
distinct layers ; indeed, it may be called a stratified 
granite. Granite frequently passes into gneiss by 
an almost imperceptible gradation ; where the quan- 
tity of feldspar decreases, and the crystals or grains 
become smaller, if the mica increases in quantity 
and is arranged in layers, the rock loses the massive 
structure and becomes slaty, we have then a true 
gneiss. But if the reverse of this happen, the quan- 
tity of feldspar increases and the mica diminishes, 
the rock loses the slaty structure and becomes mass- 
ive, and we have granite again. Gneiss rock con- 
tains beds of crystalhne limestone and of hornblende 
rock ; also most of the metallic ores, garnet, epidote, 
&c. The declivities of granite mountains are often 
covered by rocks of gneiss ; where it forms mount- 
ains, they are not steep and broken as those of 
granite, and the summits are for the most part 
rounded. 
