39 
Collection of basalt columns from the Giant’s Causeway, 
Ireland, and the Rhine Valley. The grouping of these is in- 
tended to illustrate the stair-like arrangement which usually 
characterizes cliffs of basalt. The columns are entirely of na- 
tural formation, being produced by the shrinkage of the basalt 
in cooling. 
HALL 66. 
LITHOLOGY. 
The collections shown in this Hall aim to illustrate the 
different varieties of rocks as they are known to petrographers 
and also to exhibit the characteristics of these rocks and their 
order of succession 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 1 inch. 
The specimens are classified under the heads of Eruptive, 
Aqueous and Metamorphic rocks. 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 Aque- 
ous 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 usual- 
ly 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 the percentage of silicOr 
The highest in silica, or acidic rocks, are placed at the begin- 
ning 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 cor- 
responding series begins with the syenites, and ends with the 
nepheline rocks. 
