258 T. S. Hunt on Lithology. 
ene or hornblende, the feldspar sometimes predominant; while 
in other cases the pyroxene or hornblende makes up the princi- 
pal part of the rock. The presence of these lattef minerals gen- 
erally gives to the fine grained rocks of this class a dar 
a hardness somewhat inferior to the more siliceous class, and a 
density which may vary from 2-7 to more than 38-0. il 
however be found that the line between the two classes cannot 
always be distinctly drawn, inasmuch as rocks containing ortho- 
clase and quartz often include triclinic feldspars such as albite 
and oligoclase, and by an admixture of hornblende offer a tran- 
sition to rocks of the second class. On the other hand, quartz is 
sometimes found with triclinic feldspars and hornblende in the 
rocks of the second class. Besides these two feldspathic classes, 
there is a third small but interesting group, in which an alummn- 
ous silicate of high specific gravity, such as garnet, epidote, or 
zoisite replaces the feldspar wholly or in part. These minerals 
ing basic silicates rich in alumina, the relations of this group 
are naturally with those of the second class, although varieties of 
these species are found in rocks which belong to the first class. 
The silico-aluminous crystalline rocks may thus be convenient 
ly divided into three families. The first of these includes those 
rocks in which the aluminous mineral is orthoclase, (orthose) 
from which they may be conveniently designated by the name 
of the orthosile family. The second includes those in which the 
aluminous element is an anorthic or triclinic feldspar, and may 
be designated as the anorthosite family ; chemically related to this 
are those rocks holding as one of their elements nepheline, leu 
cite, or scapolite. ‘The third family includes those rocks which 
contain an aluminous silicate of high density, as epidote, zoisile, 
garnet, andalusite, or kyanite, in place of a feldspathide. Iolite 
or dichroite, which enters into the composition of some orthosite 
rocks, appears from its atomic volume to be related to the feld- 
spars, and should take its place along side of anorthite and s¢a- 
polite as a magnesian feldspathide, while beryl in like manner 
appears to be a glucinic feldspathide. 
It is worthy of notice that some feldspars having the erystal- 
lization and density of orthoclase, nevertheless contain large ae 
portions of soda. The loxoclase of Breithaupt appears from thé 
analysis of Smith and Brush to be a true soda-orthoclase ; ye 
Journal, [2], xvi, 43,) while the sanidine or glassy feldspar 
many trachytes contains potash and soda in nearly equal prop 
tions. The name of potash-albite has been given to some woe 
— of this composition, but the trachytic rocks hereafter pe 
ribed contain feldspars, which without being glassy, 29" 
the composition of sanidine, together with a cleavage mene 
coeenty which show them to belong to orthoclase eK jee 
to albite. The anorthic feldspars offer in their composi 
