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[Assembly 
or eggs of fish. The name is appHed to a large group of strata, 
characterized by peculiar fossils. Only a part have this peculiar 
structure. 
Orthocera, orihoceratite. An extinct genus of animals belonging to the 
order Molusca. They inhabited long strait tapering shells, divided 
into parts by septa or partitions. Common in the transition or Tren- 
ton Falls hmestone. 
Outcrop. The edges of strata, as they appear at the surface. 
Outliers. The detachment of a mass of rock from the general formation 
to which it belongs ; or it is a term applied to a mass which appears 
as if it was detached from the general formation, as its outcrop is at 
a distance from it. 
Oxid^. The combination of a metal, or any substance with oxygen, 
which is not acid. 
Oxygen. One of the constituent elements of the air and water ; that 
portion of the air which supports life and combustion. 
Pachydermata. An order of animals with thick skins, as the hog, ele- 
phant, rhinoceros, mammoth, &c. 
Palaeontology. The science which treats of fossil remains, both ani- 
mal and vegetable. 
Pelagian. Belonging to a deep sea. 
Petroleum. Mineral pitch ; a liquid approaching the consistence of 
tar, of a bituminous odor, and which oozes out of a rock. 
Phlegrcean fields. The burnt fields ; applied by the Greeks to the 
region around Naples, from the marks of igneous action every where 
visible. 
Pisolite. A stone composed of rounded particles like peas, aggluti- 
nated together. 
Pliocene. An era or period analogous to the present, and more recent 
than the miocene. A large proportion of the animal remains of the 
pliocene strata still exist. 
Porphyry. A rock having a base whose structure is more or less com- 
pact, in which there are disseminated crystals of feldspar. 
Productus. A bivalve fossil shell, concave on one side, and convex on 
the other. 
Plastic clay. One of the beds of the eocene tertiary period. 
Plesiosaurus. A fossil extinct reptile, having a close resemblance to a 
hzard. 
Precipitate. The fine powder which separates from solutions after 
standing, or which are disengaged by the chemical action of bodies 
added to those solutions. 
Pumice. A light spongy lava. 
Puddingstone. Indurated or cemented masses of rounded pebbles, 
sand, &c. 
Pyrites. A mineral composed of sulphur and iron, or sulphur and copper. 
Quartz. The term universally applied to a simple mineral, composed 
of nearly pure silex. Rock crystal is an example. 
Rock. A term applied to all mineral beds or masses. 
Sandstone. A rock composed of grains of sand. 
Saurians. Animals belonging to the tribe of lizards. 
