57 
HALL 66. 
LITHOLOGY. 
This section includes two collections: one of rock specimens 
systematically arranged, and one of polished and ornamental 
stones. The former includes about i,8oo specimens of uniform 
size, 4x3x1 inches, representing the different kinds of rocks. The 
latter is made up of about 200 specimens of polished slabs of dif- 
ferent sizes, and is intended to illustrate the different ornamental 
stones, chiefly marbles and granites. It may, however, be consid- 
ered a part of the systematic collection, the polished slabs having 
been placed as near as possible to the corresponding specimens 
in the former, so that the two may be studied as one if desired. 
They will be so described here. As in other sections of this 
department, the numbering of the cases indicates the order of the 
arrangement. The series begins at the left of the eastern entrance 
to Hall 65. 
The rock specimens are classified under three heads: Erupt- 
ive, Sedimentary, 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 
considered to be the primary rocks of the earth’s crust, so far as 
it is known. From these, aqueous agencies form the Sedimentary 
rocks by erosion and deposit, or by chemical precipitation. The 
latter in turn maybe changed to Metamorphic rocks by dynamical 
and chemical agencies, which, however, do not usually destroy 
the lines of stratification. 
Cases If 4 , and part of 5 . — 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, being placed at the beginning of 
the series, then those having lower percentages, down to the 
lowest, or basic rocks. 
Under the divisions representing different percentages of 
silica are placed, first, the coarse grained, or holocrystalline rocks, 
then those of finer grain or having a porphyritic structure (porphy- 
