Third is the presumption of many that they are the wreck of a mantle of rocks of similar kinds which once covered the surface, and by different agencies have been tossed into alignments, heaped into hills, or left undisturbed upon the mountains.
Fourth, Dolomieu supposed that the summits of the Alps, and those of the Jura Mountains, were formerly connected by a regular incline, down which the masses of rock rolled, so that bowlders from the Alps got perched upon the Jura, and that during subsequent convulsions the ground sunk and assumed its present form.
Fifth, Venturi suggested very early the aid of ice, as glaciers and floating ice, to explain their transportation.
Sixth, a view, received by several, regarded the Jura range to have been once a level plain at the feet of the Alps, and that, when it had become strewed with bowlders, torn by frost and torrents from the latter, it was elevated into a line of hills carrying up its old accumulations.
Seventh, M. de Buch, developing the first theory, thought that the erratics were a conscience of the elevation of the Alps posterior to the deposition of the Tertiary,
Eighth, the Noachian deluge was burdened with the responsibility of their dispersion.
Ninth, glacial action, as explained by Prof. Agassiz.
Tenth, ancient alluvial action, identical in nature with that known at present.
Eleventh, action, by its tractile strength, of the receding waters of the ocean, as mountain-chains were successively upheaved above its surface.
Twelfth, elevation of the arctic seas, which caused a flow of water from the polar regions, transporting ice loaded with rocks and gravel southward.
Fourteenth, an explanation of the phenomena in the United States, viz., a drainage of a vast inland sea through the valleys of the St. Lawrence, Hudson, Susquehanna, Ohio, etc., accompanied or followed by a débâcle of ice and drift from the north.
Fifteenth, a change in the axis of rotation of the earth.
Sixteenth, collision with a comet, which extraordinary jolt loosened the rocks, and rattled them from the mountain-peaks to the adjacent plains.
Seventeenth, shrinkage of the earth, increased velocity of rotation, and consequent rush of arctic waters to the equator, carrying bowlders.
In the above enumeration some theories will be observed to have only a local application, and were originated at a time when the dispersion of bowlders was not known to have so universal a character. Of these seventeen hypotheses we believe it would be impossible to insist seriously upon more than two classes: those which attribute the phenomena to the action of water, and those which enlist the agency of