DEPARTMENT OF GEOLOGY 
37 
illustrate the stair-like arrangement which usually characterises 
cliffs of basalt. 
Hall 66. 
LITHOLOGY. 
The collections of this hall illustrate the varieties of rocks 
occurring at different localities. About 2000 specimens are 
shown, most of them being of the size 3x4x1 inch. The 
specimens are classified under the heads of eruptive, aqueous, 
and metamorphic rocks. Of these the eruptive rocks are grouped 
according to their percentage of silica and their grain. Thus, 
beginning with the granites, which have from 80 to 65 per cent 
of silica, the order passes 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 ends with the peridotites, having below 45 per cent. A cor- 
responding series begins with the syenites and ends with the 
nepheline rocks. Again under the divisions representing differ- 
ent percentages of silica, the coarse-grained or holocrystalline 
rocks are placed first, then those having a porphyritic structure 
and, lastly, the amorphous rocks. Thus among rocks having 
from 80 to 65 per cent of silica, the granites, being coarse-grained, 
are placed first, the quartz porphyries second, and obsidian, 
etc., last. 
Entering from Hall 65, in the first case at the left of the 
entrance are shown granite and its varieties, such as granitite, 
graphic-granite , etc. These are coarse-grained rocks having 
quartz, potash -feldspar, and one or more minerals of the mica, 
amphibole, or pyroxene groups as essential constituents. Then 
follow granite-porphyry , quartz-porphyry, vitrophyre, felsophyre, 
etc., which are like the preceding in composition, but more or 
less porphyritically developed. Following these are rhyolite, 
nevadite, pumice, obsidian, etc., which are amorphous volcanic 
rocks, having high percentages of silica, usually more than 
70 per cent. Then follow diorite and varieties, holocrystalline 
rocks having plagioclase feldspar and hornblende or black mica 
as essential constituents, andesites and dacites, amorphous or por- 
phyritic rocks composed of soda -lime feldspar, black mica, horn- 
blende, and in the case of dacites quartz, then porphyrites of 
various kinds. 
The next group begins with gabbros and norites, which are 
