Page:The Geologist, volume 5.djvu/137

Rh order of their creation. (The last section is termed paleontology, from palaios, ancient, and onta, beings.)

It is necessary to mention that geology throws no light on the origin of the world, or on the nature and state of the materials occurring beneath its crust, or on its condition anterior to the setting-in of ordinary physico-geographical agencies—those are hypothetical subjects.

The following are some of the principal geological deductions:—

1. The rocks forming the known portion of the crust of the earth are of two kinds, as regards the origin of their present condition. One consists of masses which have been subject to a high temperature; and the other of materials deposited by the agency of water. They are respectively termed Igneous and Aqueous. Principal Igneous rocks: granites, syenites, porphyries, diorites, felstone, basalt, obsidian, etc. Principal Aqueous rocks: limestones, sandstone, shale, slatestone, salt, gypsum, marl, chalk, etc.

2. The formation of Igneous and Aqueous rocks has been going on from an immeasurably remote epoch, and still continues.

The existence of some kind of Igneous rocks in a refrigerated and consolidated condition, anterior to the formation of Aqueous deposits, can only be hypothetically assumed.

3. Certain Igneous rocks, as granites and the like, extend, en masse, below the surface of the earth to unfathomable depths; and wherever observed in contact with Aqueous rocks, they form the foundation of the latter; others, as basalts and lavas, have been ejected from great depths; and they generally occur overspreading other rocks.

The doctrine of the Igneous origin of granite and allied rocks has been much contested of late; there is little doubt, however, of their having been subject to a high temperature (see 6th deduction).

4. Aqueous deposits, occurring nearly everywhere, and often several thousand feet in thickness, have been derived from previously existing Igneous rocks, or from prior-formed deposits of their own class, through the mechanical and chemical action of atmospheric agents, springs, rivers, lakes, and seas. In this way, huge mountain-masses have been worn down (denuded), and valleys excavated; while their materials have been transported to the mouths of rivers, or disseminated over the bottom of lakes and oceans. The remains of plants, corals, shells, and other organisms (fossils) frequently enter into the composition of Aqueous rocks: some- times beds are entirely made up of such remains.

5. Aqueous rocks, with few exceptions, have been slowly and gradually deposited in more or less horizontal beds (strata);—the order of superposition of the beds being the order of their successive formation; while their relative position denotes their relative period of deposition in the scale of geological time.

6. Subterranean heat and heated vapours, in many cases emanating from proximate igneous sources, have frequently penetrated deep-seated Aqueous rocks, producing in them molecular re-arrangements, and a more or less crystalline structure, or a change of chemical composition; thereby obliterating many of their original characters, and otherwise metamorphosing them. List of Metamorphic rocks: gneiss, mica schist, hornblend schist, quartz stone, statuary marble, etc.

Possibly all known Igneous rocks were originally Aqueous deposits that have been completely melted.

7. The surface of the earth has repeatedly undergone both slow and sudden upheavals and depressions. The former movements have raised wide-spread horizontal beds from the bottom of seas, often several thou-