55 
HALL 66. 
LITHOLOGY. 
The collection shown in this Hall is one which aims to 
illustrate the different varieties of rocks as they are known to 
petrographers. 
The characteristics of these rocks as they appear in different 
localities and their succession in certain regions is also exhibited. 
About 2,000 specimens are shown, most of them being of the 
uniform size adopted by petrographers — 3X 4 x 1 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, the 
lowest, or 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 
