Lithology for Mtcseum Purposes. — Gratacap. 283 
The Idea of ^ock, fourt/ily , can be expressed by contrasted 
states of aggregation as dust, clay, sand, gravel, with lava, slate, 
sandstone, conglomerate, for it is a plain violation of sense to 
call the first rocks. They are not. They are material on the 
way to rock or derived from rock. They are what they 
are called, and form beds, sheets, banks, beaches, layers, 
mounds, etc., but from the point of view of lithology they are 
not rock. 
The Idea of 'R.ozk, fifthly, can be enforced by examples of 
the variation of the same rock as quartzose, micaceous, horn- 
blendic, granite, coarse and fine grained granite, grained and 
glassy basalts, phenocrystalline and obsidian liparytes, etc. 
The Idea of Rock, sixthly, can be further enlarged by thin 
sections, viewed through hand glasses, and by colored 'or un- 
colored drawings, subjects being selected which are the widest 
opposites as andesytes and granites, sandstones and quartz 
porphyry. 
These six aspects of rock will bring the visitor i:-.to com- 
plete contact with a many sided conception of rock, and the 
next steps will neither be abrupt nor unexpected in the devel- 
opment of his appreciation of lithology. 
1 1 . Terms. 
It is unnecessary to introduce many definitions, but enough 
to fix by a name the features of rock already seen, and others 
which succeed. The following terms seem adequatelv com- 
prehensive: 
stratified, illustrated by banded limestone, gneiss. 
massive, " " serpentine. 
granular " " Berea sandstone. 
crystalline " " granite, trachyte. 
schistose " *' mica schist. 
concretionary " " oohte. 
pegmatitic " " graphic granite. 
cong lomeratic " " puddingstone. 
breccia ted " " Taconic marble. 
porphyritic " " quartz porphyry, verde antique. 
clastic " " sandstone. 
glassy " " obsidian. 
vesicular " " pumice. 
