Page:A Treatise on Geology, volume 1.djvu/333

 CHAP. VI. sufficient explanation in the known daily operation of physical laws, aided by displacements of the relative level of land and sea, such as geology has established. Compared with tertiary phenomena, we must allow that the pebbly conglomerates on the flanks of the Alps are really detrital deposits of an earlier era. Dr. Forchhammer has adopted the view of the 'boulder formation' of Denmark being one very long series of detrital deposits, including the whole tertiary series, and extending from the plastic clay group beyond the ordinary diluvial epoch.

Whether this be correct or not, it is certain that we must apply, for solutions of the problem of the distribution of the diluvial blocks, to the same agencies which have been invoked to explain the accumulation of the tertiary molasse of Switzerland, and the conglomerates of the red sandstones of England. All their causes we do not know; but the predominant one is known to be great change of the level of land and sea, and the consequent origin of new and powerful oceanic currents.

It is certain that during the 'diluvial' period a large portion of the northern zone, which is now dry land, was under the sea. Murchison has shown that this was not the case in Siberia, which, like other then dry lands, may be regarded as the 'feeding ground' of the mammalia, whose remains characterize some of the deposits we are now considering.

Those ancient sea beds, now dried, show, besides sands and gravels, the effect of strong and variable currents, great masses of fine clay, strangely enclosing far travelled rock masses of various magnitude, much, little, or not at all worn, and occasionally marine shells, in which, upon the whole, an arctic character is recognised.

However this may be, it appears absolutely certain that none but oceanic currents are adequate to explain the extensive ravages of the solid land which produced,