57 
HALL 66. 
LITHOLOGY. 
The collections shown in this Hall aim to illustrate the differ- 
ent varieties of rocks as they are known to petrographers, and also 
to exhibit the characteristics of these rocks and their order of suc- 
cession as they appear in different localities. 
About 2,000 specimens are shown, most of them being of the 
uniform size adopted by petrographers — 3 x 4 x i inch. 
The specimens are classified under three heads: Eruptive, 
Aqueous, and Metamorphic. The Eruptive rocks are those 
which have been formed at great depths, and were once in a state 
of igneous fusion. Being most deeply seated they may be con- 
sidered to be the primary rocks of the earth’s crust, so far as it is 
known. From these, aqueous agencies form the Aqueous rocks 
by erosion and deposit, or by chemical precipitation. The latter 
in turn may be changed to Metamorphic rocks by dynamical and 
chemical agencies, which, however, do not usually destroy the 
lines of stratification. 
Cases 1, 4, 5, and. part of 6 . — Eruptive rocks. The 
classification which has been adopted for these is based upon the 
following plan: 
First, an arrangement according to percentages of silica. 
The highest in silica, or acidic rocks, are placed at the beginning 
of the series, then those having lower percentages and, last, those 
lowest in silica, or the so-called basic rocks. 
Thus, beginning with the granites, which have from 80 to 65 
per cent, of silica, we pass among the coarse-grained rocks to the 
diorites, which have between 65 and 55 per cent., then to the 
gabbros and diabases, having usually more than 45 per cent, 
and end with the peridotites, having below 45 per cent. A corre- 
sponding series begins with the syenites, and ends with the 
nepheline rocks. 
Second, under the divisions representing different percentages 
of silica, a vertical arrangement is adopted by which the coarse- 
grained or holocrystalline rocks are placed first, then those of 
