﻿Mr. A. Tylor on the Formation, of Deltas. 311 



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. Prestwich suggested that Mr. Croll's theory only involved a 

 transfer of ice from one Pole to the other, and not 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 

 would have been had they resulted from a lowering of the sea. 

 The elevation of the old sea-beds during the Glacial period were not 

 accounted for by any supposition of the mere alteration in the 

 volume of the sea. 



Mr. Evajs t s 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. 



Mr. Mallet had already remarked that the beds of rivers, 

 especially near their sources, appeared to assume curves closely 

 allied to a parabola. He considered that the form was due rather 

 to the elevatory forces than to erosion. He doubted, however, 

 whether they were really parabolic curves, or indeed any other ma- 

 thematical curve. 



Mr. Tyloe, replied that he had not found definite evidence as to 

 the extension of corals downwards to such a depth as that men- 

 tioned by the President. With regard to oscillation, he had merely 

 treated of the southern part of England. The opening of the Straits 

 of Dover would account for the existence of beaches above the pre- 

 sent level, as the tides would have previously risen higher. The 

 parabolic curve was that which, by actual comparison, coincided 

 most closely with the longitudinal section of the banks of the rivers 

 Po, Mississippi, and Ganges. 



