318. Geology is an inductive science based upon the observation of facts, and if the theories adduced to account for the phenomena are shown to be untrue or insufficient, the facts still remain unimpeached. Geological theories relate principally to the modes of action of the two great agencies aqueous and igneous. It has before been stated (§ 13,) that geologists are divided in opinion respecting the intensity with which these agents have operated in different periods; and while they agree in attributing the stratified rocks to deposition from water, in particular modes, the phenomena of the Drift have given rise to several theories. Diverse modes of explanation have also been proposed to account for the phenomena of volcanoes, and the elevation of mountains and continents. All correct reasoning in natural science is based upon the uniformity of nature's laws; a conviction of this uniformity is unceasingly impressed upon the mind by experience and observation.

THEORIES OF THE DRIFT.

319. No part of Physical Geology is so unsettled as that which relates to the dispersion of the Drift. It was formerly imputed to the agency of powerful currents of water alone, and by many it was ascribed to Noah's deluge. This was the view entertained by Dr. Buckland when he wrote his Reliquiæ Diluvianæ. But the short period of that flood and the absence of man's remains and works, with other considerations, have led to the universal abandonment of that view. The agency of water alone is now regarded as inadequate to account for the phenomena; they are attributed to the joint action of ice and water. Three theories are advocated by different writers on the subject. 320. The iceberg theory, supposes that the polar regions

of the earth were depressed beneath the ocean before the drift, and that during that period icebergs loaded with gravel and fragments of rocks were transported by currents, inflicting scratches and grooves upon the rocks over which they passed, and melting in lower latitudes deposited the materials they had conveyed.

It is urged in favor of this theory that such an agency is now witnessed in the phenomena of icebergs bearing thousands of tons of earth and rocks, transported by polar currents, as far as the drift extends. The stranding of the icebergs with fragments of rock frozen in them, is thought to produce striæ and furrows upon the bottom of the ocean like those of the drift.

It is objected to this theory that the boulders of icebergs are brought from high latitudes, while those of the drift appear to have been derived from neighboring mountains and ledges; the parallelism and uniformity of the striæ are not adequately accounted for; and the highest portions, as mountain tops, alone should have been furrowed and striated, since icebergs that would float over such heights would not reach to the bottom of the valleys, whereas the scratches are common in the plains and valleys.

321. The elevation theory attributes the phenomena to numerous elevations of the earth from beneath the polar seas, repeated for successive ages, sending enormous waves toward the equatorial regions bearing icebergs, with their boulders and earth, and urging before them the loose materials lying upon the surface; the striæ were produced by fragments of rocks in the bottoms of immense masses of ice which may have been forced up steep declivities.

The arguments in favor of this theory are, that such

upheavals of the bottom of the ocean are known to occur, as in the elevation of Sabrina, Graham's Island, in the Mediterranean sea, and the Aleutian Islands, (§ 60 ;) that waves caused by earthquakes have great power to prostrate and transport heavy bodies; and that, as the surface of the continents, the mountain ridges and valleys, were essentially the same in form and direction then as now, the origin of the action must be sought for out of the country, and the direction of the drift dispersion indicates that the localities elevated were about the poles.

It is objected, that so numerous and extensive elevations as the theory requires, are improbable; that the phenomena of the drift indicate prolonged action, while such vertical movements are transient; and that the parallelism of the striæ on the rocks is not accounted for by moving fields of ice buoyed up by water, and conveyed by currents which, instead of passing up mountain sides and over their summits, would have swept around such obstacles.

322. The glacier theory supposes that the climate, which in the Tertiary period had been so warm as to allow the palms to grow within the temperate zones, became much colder, causing enormous sheets of ice-polar glaciers-to advance far beyond their previous limits, moving along the surface by alternate advance and retreat, rounding, polishing and striating the rocks, and afterward when melted depositing their loads of boulders and detritus, where the drift is now found.In Europe the center of expansion is supposed to have been the Scandinavian mountains, and in North America in the polar regions, from which the glaciers advanced southerly.

The advocates of this theory contend that the phenomena of glaciers as witnessed in the Alps (§ 32,) are perfect

miniature representations of the drift-its striæ, furrows, boulders and moraines; that the elevation of extensive regions in high latitudes, like those of the Cordilleras in Mexico, and the high plains of Central Asia, would produce such a reduction of temperature as to cause immense glaciers, even thousands of feet in thickness. This theory is advocated by Prof. Agassiz.

The principal objection to the glacial theory is that glaciers are at present entirely confined to valleys, and the origin of such an enormous sheet of ice as it contemplates is altogether hypothetical.

323. Neither of these theories is deemed quite satisfactory; the proximate cause of the phenomena is very generally supposed to have been the joint action of ice and currents of water, but their origin and exact modes of operation are not determined.

THEORY OF VOLCANOES.

324. The cause of volcanic phenomena-eruptions, earthquakes, and elevations and depressions of portions of the earth's surface-has been the subject of much discussion. The prevalent theory on this subject supposes the whole earth, with the exception of a crust fifty or one hundred miles thick, to be in a melted state; that eruptions are produced by the access of water through crevices to this heated mass, which causes steam and other elastic bodies to force out through craters and fissures, lavas, scoriæ, sulphur, and other volcanic products; and that the whole globe has formerly been in a state of fusion, the present crust having resulted from the cooling of the surface.

325. In favor of this view it is urged

1. That the temperature of the earth below a certain depth, as tested in mines and Artesian wells, continually increases as we descend (§5,) at an average rate of about, 1o for fifty feet, which would at a depth of a little more than a mile give the temperature of boiling water, and at a depth of about fifty miles, would be adequate to the melting of any known rock.

2. The spheroidal form of the earth is thought to indicate that it has been in a fluid state; and, if So, it must

Sir Isaac Newton

have been through the agency of heat. has shown that a body having the size and density of the earth, revolving on its axis with the rapidity it has, would, if its particles were free to move, assume its oblate spheroidal form.

3. The numerous extensive volcanoes, whose origin is deep seated, which communicate with each other over vast areas, and the masses of whose lavas thrown out at a single cruption sometimes surpass the bulk of the mountains in which their craters are situated, (§ 54,) require an enormous mass of heated matter; if the interior is in a melted state the materials are abundant, and their extrusion may be produced by the pressure of steam and other elastic bodies, or by the contraction of the crust upon the melted mass.

4. The phenomena of hot springs, deep Artesian wells, and the increase of temperature generally, as we descend beneath the surface, are adequately accounted for by this theory.

5. The ultra tropical character of the climate, and its great uniformity during the periods of deposition of the earliest fossiliferous rocks, have been attributed to this origin.