iv

atmosphere. Their quantitative proportion differs however from
that in which they exist in the air, and is characterized by a more
predominant quantity of Oxygen and Carbonic Acid.

The Constituents of the Earth—Earthy masses—Rocks with
crystalline texture.

The solid crust of our globe is composed either of earthy masses,
more or less loose and friable, or of hard and compact rocks with
crystalline texture. The former are called aqueous locks; are the
most extensive, at least in the horizontal direction; and were pre-
cipated from their solution or suspension in water, in the form of
plainly visible parallel layers, called strata, They may be classed
as follows:

Calcareous Rocks and Salts.	Broken Masses.
Limestone,		Sand-stone,
Dolomite,		Clay,
Anhydrite,		Calcareous Conglomerations,
Gypsum,		Breccia,
Rock Salt,		Tufa,
	Organic Rocks.
	Coal.

The latter are called volcanic rocks, are of igneous origin, pro-
duced by the action of subterranean heat, and form massive blocks,
especially extending in the vertical direction and without any
regularity in the arrangement of their particles. They very proba-
bly existed once in a fused state, forming a liquid and homogeneous
mass, from which, on cooling, such constituents were separated as
had the greatest tendency to crystallize, whereby the entire mass
assumed a peculiar crystalline texture. The various crystalline
rocks can best be classed as follows:

Silicates,	containing	Silicates, containing Lime
Alumina and Alkalies.	and Protozyd of Iron.
Granite,	Felsparphophyrite,	Hyperit,	Melaphyrite,
Gneiss,	Phonolite,	Gabbro,	Dolerite,
Granulite,	Trachyte,	Syenite,	Basalt,
Mica-Slate,	Obsidian,	Diorite,	Leucitopyrite.

Action of the Atmosphere on Earth.—Mechanical Influences
—Chemical Influences.

Geology teaches us that there was a time when no plants were
developed upon our globe; when neither soils nor any friable
masses of earth existed; at that time the entire crust of our globe
consisted solely of those hard and compact rocks of a crystalline