Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/80

 channel through the sand-hills to the sea. This is now the mouth of the Vistula, that passing Dantzic having been turned into a canal*.

I do not intend here to discuss the question of subsidences and elevations which have affected the surface of the earth so largely, and have no doubt occurred in some localities during the period under consideration. I would, however, remark that in Wealden, Eocene, or Miocene deltas there is no instance of any large fluviatile deposit having been elevated or depressed evenly over a large area ; while all over the world a perfectly even movement of subsidence is supposed to have taken place just at the mouths of large rivers in the Quaternary, or most recent, period, in order to account for modern freshwater delta-deposits containing shells living in the adjoining seas being now found hundreds of feet below the sea-level.

It would be the safer plan, in considering the remarkable gravel- and crag- deposits which characterize so distinctly the Quaternary period, to infer the size of rivers, amount of rainfall, and elevation of tides from the deposits themselves. Further acquaintance with meteorological phenomena may find a fitting explanation of the difficulties we meet with in explaining the position of the gravel at such heights above our present streams and freshwater clay, and sands at such depths below the sea-level.

If the hypothesis we have been considering is a true one, that the sea-level fluctuated 600 feet in the Glacial period, falling gradually and then rising again to its former level, we ought to find the best evidence in the Pacific Ocean among the vast littoral accumulations of the coral-zoophytes.

The same theory of gradual subsidence, as was proposed for explaining the delta-deposits, has been applied to explain the formation of the remarkable coral-islands over a tract of ocean 5000 miles long. The sea-bottom by this theory is supposed to subside so regularly and slowly that the coral-zoophytes build up their reefs and coral- banks at an equal rate with the fall of the sea-bottom.

Mr. Croll's hypothesis of an immense mass of ice at the poles sufficient to make the polar diameter equal to the equatorial (Phil. Mag. p. 302) is well known. He has (p. 305) assumed that the quantity of liquid water would be unchanged, as the ice in the southern hemisphere would be transferred to the northern hemisphere.

The author's hypothesis is different.

He thinks the geological evidence of a Glacial period indicates an immense collection of ice at the north pole, accompanied by a low temperature in the tropics, and probably a very low temperature at the same time at the south pole.

This would involve the supposition of a fall in the ocean-level in proportion to the quantity of ice collected at the poles.

It is of course doubtful what quantity of water was abstracted from the ocean in this Glacial period (which Mr. Croll considers

of the Thames, Seine, and Somme may have assisted in the production of some of the remarkable gravel-beds in these rivers.
 * Pfeiffer, ' On the Vistula,' Dantzic, 1849. The gorging of ice at the mouth