1868.] TYLOE, EOKMATION OF DELTAS. 11 



commenced 240,000 years ago, and lasted 160,000 years), and was 

 stored up in the polar regions in excess of tlie 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 neighbour- 

 hood 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 in- 

 credible. 



Sir Charles Lyell had already suggested to Mr. Croll that, as- 

 suming 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 Missis- 

 sippi, 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 pre- 

 viously 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. Peestwich suggested that Mr. CroU's theory involved pro- 

 bably 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. 



