34 FIELD MUSEUM OF NATURAL HISTORY 
less porphyritically developed. Following these are rhyolite, neva- 
dite, 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 porphyritic 
rocks composed of soda-lime feldspar, black mica, hornblende, 
and in the case of dacites quartz, then porphyrites of various 
kinds. 
The next group begins with gabbros and norites, which are 
coarse-grained rocks consisting of a basic soda-lime feldspar, 
with a diallage or other pyroxene. Diabases, rocks having plagi- 
oclase feldspar and augite as essential constituents follow, then 
come basalts, dolerites and melaphyres. The latter are usually 
found in the form of dykes and intrusive sheets and are popu- 
larly known as trap rocks. Pyroxene rocks, diallagite, etc., which 
are basic rocks composed largely of pyroxene, come next, and the 
group ends with peridotite and varieties, including Iherzolite, pic^ 
rite and dunite. These are highly basic rocks, composed chiefly 
of olivine, but often having chromite and other iron oxides 
present. 
Rocks of the syenite-nephelinite series are placed next. The 
syenites are holocrystalline rocks, having dominant orthoclase and 
subordinate ferro-magnesian minerals. These are followed by 
trachytes, which are of the same composition as syenite but of 
porphyritic or felsitic texture. Then follow nepheline or elceolite 
syenites, in which nepheline accompanies the feldspar ; phonolites, 
which are like the above but of porphyritic or felsitic texture 
and tephrites and basanites, rocks having nepheline or leucite and 
lime-soda feldspar as essential constituents, usually porphyritic 
in structure, with a more or less amorphous ground mass. Ker- 
santite, leucite basalt, leucitite, neptheline basalt and nephelinite, 
which are rocks containing leucite or nepheline in place of feld- 
spar, and these usually associated with augite, end the series. 
Then follow aqueous rocks. Those formed as chemical pre- 
cipitates are placed first. These include hematite, limonite, cal- 
careous tufa, oolitic and pisolitic limestone, onyx, serpentine and 
its varieties, talc, or steatite, including verde antique marble and 
ophite, gypsum, alabaster, etc. Then follow rocks formed as 
