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liner grain or those having a porphyritic structure and, last, the 
amorphous rocks. Thus among rocks having from 80 to 65 per 
cent, of silica, the granites, being coarse-grained, are placed first 
in the vertical order, the granite porphyries second, and rhyolite, 
nevadite, obsidian, which are amorphous, last. 
Case 1. — First row, granite and its varieties, such as granitite, 
graphic-granite , etc. These are rocks having quartz, alkaline 
feldspar, and one or more minerals of the mica, amphibole, or 
pyroxene groups as essential constituents. 
Second row, granite-porphyry , quartz-porphyry , vitrophyre, 
felsophyre , etc. Like the preceding in composition, but more or 
less porphyritically developed. 
Third row, rhyolite , nevadite , pumice , obsidian , etc. These 
are amorphous volcanic rocks, having high percentages of silica, 
usually more than 70 per cent. 
Case 2, — Polished granites and marbles. 
Cases 2 and 3.— Series illustrating the rocks of Manhattan 
Island. These have chiefly been obtained by taking specimens 
from excavations made in and about New York City. 
They are crystalline, metamorphic rocks, and illustrate well 
the great variations possible in kinds of rock in a small region. 
Series illustrating the rocks of the Green Mountain Range, 
passing eastward from Pittsfield, Mass. 
These include a variety of schists, limestones and other 
metamorphic rocks ranging in geological time from the Archaean 
into the Devonian. They illustrate the different formations dis- 
tinguished by geologists in that region. 
Case 4 : and part of 5. — Syenite-nephelinite series. 
First Group. — First row. Syenite y minette , etc. Holocry- 
stalline rocks, having orthoclase and biotite as essential constitu- 
ents. 
Second row. Trachytes. Tertiary eruptive rocks, characterized 
by the predominance of an alkaline feldspar, usually sanidine, 
and freedom from quartz. An iron-bearing mineral is also usually 
present. 
Second Group. — First row. Nepheline or elaeolite syenites. 
Rocks comprised of nepheline, orthoclase, and usually a pyroxenic 
mineral and plagioclase feldspar. 
Second row. Phonolites , rocks consisting of an alkaline feld- 
