38 FIELD MUSEUM OF NATURAL HISTORY 
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. They are popu- 
larly known as trap rocks. Their composition is like that of the 
preceding. 
Pyroxene rocks, diallagite, etc., which are basic rocks com- 
posed largely of pyroxene, come next, and the group ends with 
peridotite and varieties, including Iherzolite , picrite 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, as 
follows: Syenite, minette, etc. These 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 text- 
ure. Then follow N epheline or elceolite syenites, in which nephe- 
line accompanies the feldspar; otherwise they are like syenite. 
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, follow next in order. Kersantite, leucite basalt, leucitite, 
neptheline basalt and nephelinite , which are rocks containing leu- 
cite or nepheline in place of feldspar, 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, caU 
car eons tufa, oolitic and pisolitic limestone, onyx, serpentine and its 
varieties, talc or steatite, including verdantiq^ie marble and ophite, 
gypsum, alabaster, etc. Then follow rocks formed as sedimen- 
tary deposits, and fragmental in structure. The principal 
varieties of these are arranged in this order: Sandstone, con- 
glomerate, breccia, quartzite, shale, clay, tufa or tuff, coquina, 
chalk and limestone. 
Then follow metamorphic rocks, These are divided into 
stratified or bedded, and foliated or schistose. The first class 
includes crystalline limestones, marbles and dolomites. They are 
made up chiefly of the mineral calcite, and are formed from the 
remains of molluscs, corals and other animals. These produce 
