58 
finer grain or those having a porphyritic structure and, last, the 
amorphous rocks. Thus among rocks having from 8o to 65 per 
cent, of silica, the granites, being coarse-grained, are placed first 
in the vertical order, the granite porphyries second, and rhyolite, 
nevaditc, obsidian, etc., which are amorphous, fast. 
Case 1 . —First granite audits varieties, such ?iSg?‘anititey 
graphic-granite^ etc. These are rocks having quartz, potash 
feldspar, and one or more minerals of the mica, amphibole, or 
pyroxene groups as essential constituents. 
Second row, gra 7 iite-porphyryy quartz-porphyry ^ vitrophyre, 
felsophyre, etc. Like the preceding in composition, but more or 
less porphyritically developed. 
Third row, rhyolitey nevaditey pumiccy obsidiaUy 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 59. 
It is interrupted in the numbering of the cases in order to give 
place to some collections illustrating rock regions and to the sye- 
nite-nephelinite series. 
Case 2 . — Rocks of the Saarbriicken, Germany, coal fields. 
These show, besides the coal, the upper and under clays, iron 
ores, conglomerates, etc., which usually characterize a coal-bear- 
ing region. Rocks of Manhattan Island. These were obtained 
chiefly 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. 
Upper part of Case 2 .— Lavas of the best known volca- 
noes, such as Vesuvius, Mauna Loa and the extinct volcanoes in 
Central France. 
Case 3 .— Lavas and other volcanic products of the volcanoes 
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 dis- 
tinguished by geologists in the region. 
