PART IV.

ELEMENTARY GEOLOGY.

I have been often reminded that many readers of my Report on the Geology of Massachu-setts, would find it convenient to have a summary of the essential principles of that science ap-pended to it. The government have so far approved of this suggestion, as to allow me to an-nex the following brief outline. As I prepared it, it was much more extensive; and being agratuitous labor on my part, I had liberty to print the whole in a separate form. This hasbeen done : and if any readers desire a fuller account of geology than what follows, they willfind it in that work; as well as in many others that have been recently published. I have madefrequent reference to the most valuable of these in the following pages.

I have arranged the subject under short heads ; such as Definition, Principle, Description, In-ference, Remark, Proof, &c. whose abbreviations will be readily understood. This is done chief-ly for the sake of brevity and definiteness. Some of the most important principles and infer-ences are given in larger type.

Section I.—a general account of the constitution and structure of

THE EARTH , AND OF THE PRINCIPLES ON WHICH ROCKS ARE CLASSIFIED,

Definition. Geology is the History of the mineral masses that composethe earth, and of the organic remains which they contain.

Def Every part of the globe, which is not animal or vegetable, including water and air, isregarded as Mineral.

Def. The term Rock , in its popular acceptation, embraces only the solid parts of the globe:but in geological language, it includes also the loose materials—the soils, clays, and gravels—that cover the solid parts.

Principle. The form of the earth is that of a sphere, flattened at the poles : technically, anoblate spheroid. The polar diameter is about 26 miles shorter than the equatorial.

Inference. Hence it is inferred that the earth must have been once in a fluid state } since ithas precisely the form which a fluid globe, revolving on its axis with the same velocity as theearth, would assume.

Prin. Taken as a whole, the earth is about five times heavier than water; or 2 1-2 timesheavier than common rocks.

Inference. We hence learn that the density of the earth increases from the surface to thecenter: but it does not follow that the nature of the internal parts is different from its crust.