Various explanations have been given of the origin and deposition of the bowlder-clay formation, as well as of the erratic block-drift, for the two can scarcely be separated in the question of cause and effect. The bowlders, for example, are sometimes in the mass of clay itself, sometimes they are lying loose on the surface, in many instances they are spread over areas where no clay exists; but in most cases maintaining their unmistakable character of being water-worn, rounded, and covered with striæ. Both classes of phenomena, therefore, are supposed to be referable to the same period of time, as they probably have originated in the same series of causes. One theory advanced in explanation of both, is the agency of powerful currents that swept over Britain and the adjacent continents, generally in a north and north-westerly direction, bearing along with them soil, gravel, and the larger debris of rocks; and as obstructions occurred, or the violence of the currents subsided, the heterogenous materials were deposited in the various countries and at the different elevations in which they are found. The direction of the currents, often from different centers, is indicated clearly by the position and lithology of the mountains from which the blocks have been transported, no less than by the fact that the greatest accumulations of drift and bowlders are to be observed at the south-eastern extremities of such gorges and valleys as were open to the diluvial action. But the hypothesis fails in giving a satisfactory account of the transport of the larger blocks, often of sixty to a hundred tons weight, over a course of many hundreds of miles, plunging through hollows, and now stranded on mountain slopes several thousand feet above. The theory of icebergs, as the transporting agency, meets this difficulty; and accordingly, in one form or other, such a cause or agent, of widely-prevailing influence, is almost universally adopted into the creed of geologists. This theory implies, that those portions of Europe now covered with the bowlder-clay formation were submerged after the deposition and consolidation of the tertiary strata—that this submergence was the result of a change in the earth’s axis or some extraordinary alteration in its planetary relations—that a great arctic glacial continent subsided and disappeared beneath the waters—and that vast floating masses of ice, inclosing rock loosened from the sinking land, penetrated southward, grazing and polishing the harder substances that lay in their course, or carrying along with them the more yielding and transportable materials. Admit all or even a limited number of such assumptions, and we know from what is occurring in recent times, that the cause is quite adequate to the production of the effect. Sir James Ross, in his late humane exploratory expedition, encountered in the polar regions icebergs from a hundred to three hundred feet in height, and from a quarter to half a mile in length.