Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/81

 commenced 240,000 years ago, and lasted 160,000 years), and was stored up in the polar regions in excess of the quantity now existing there or what existed previously.

This excess must have been an enormous quantity ; and if equal surfaces of the globe were covered with ice and with water, then every foot of the average thickness of ice stored up in the polar regions above the former surface-level would cause an immediate fall of the sea-level also of one foot.

The author's supposition of a fall in the sea-level of 600 feet does not appear to him excessive, if the Glacial period was so important as we have reason to suppose it to have been.

Discussion.

The President called attention to the fact that in the neighbourhood of coral reefs the dead corals extend to such a vast depth that, supposing them all to have been formed near the surface, and that surface only lowered by abstraction of water to the Poles, the accumulation of ice must have been so great as to become incredible.

Sir Charles Lyell had already suggested to Mr. Croll that, assuming the accumulation of ice at the Pole depressing the centre of gravity of the earth, the submergence that would have resulted had the quantity of water in the sea remained the same would, to some extent, be counteracted by the reduction in volume consequent on the formation of the ice. With regard to the delta of the Mississippi, the data on which he argued had considerably altered since first he wrote on the subject, inasmuch as recent calculations had doubled the estimated volume of water flowing into the sea, and thus it was capable of producing the same effect in half the previously calculated time. The progress of the delta at any spot was of necessity variable, as the position of the mouth changed. The American engineers had allowed only 40 feet as the depth of the fluviatile deposits, whereas from boring Sir Charles had concluded it to be at least 500 or 600 feet. There was now reason to suppose that it was much more, possibly as much as 1500 feet. This being the case, notwithstanding the amount of work done by the river being doubled, his calculation as to the time required for the formation of the delta might not after all be so excessive.

Mr. Prestwich suggested that Mr. Croll's theory involved probably a transfer of ice from one Pole to the other, and not only a diminution of volume of the sea. The raised beaches round the coast of Britain varied considerably, and were not on one uniform horizon, as they must have been had they resulted from a lowering of the sea. The elevation of the old sea-beds during the Glacial period could not be accounted for by any supposition of the mere alteration in the volume of the sea.

Mr. Evans pointed out that, the Cyrena being a freshwater shell, its position at a certain level was not connected directly with the height of the sea. He doubted the curve of the rivers being in all cases parabolic.