40 
Second, under the divisions representing different percent- 
ages of silica, a vertical arrangement is adopted by which the 
coarse-grained dr holocrystalline- rocks are placed first, then those 
of finer 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, etc., which are amorphous, last. 
CASE 1. — First row, granite and its varieties, such as 
granitite, graphic-granite, etc. These are rocks having quartz, 
potash-feldspar, and one or more minerals of the mica, amphi- 
bole, or pyroxene groups as essential constituents. 
Second row, granite-porphyry, quartz- porphyry, vitrophyre, 
felsophyre, etc. Like the proceeding 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. 
The order of the series now passes on to Case 5, described 
on page 47. 
It is interrupted in the numbering of the cases in order to 
give place to some collections illustrating rock regions and to 
the syenite-nephelinite series. 
CASE 2. — Collection of rocks of the copper and iron-bear- 
ing regions about Lake Superior. 
Rocks of Manhattan Island. These were obtained chiefly 
from excavations made in and about New York City. They are 
crystalline, metamorphic rocks, and illustrate the great varia- 
tions possible in kinds of rock in a small region. 
UPPER PART OF CASE 2. — Lavas of well-known vol- 
canoes, including Vesuvius, Mauna Loa, and the extinct vol- 
canoes of Central France. 
CASE 3. — Lavas and other volcanic products of the vol- 
canoes of the Valley of Mexico. 
Rocks of the Green Mountain Range, as seen in 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 distinguished by geologists 
in the region. 
