﻿310 Geological Society: — 



mite of the Zechstein, from the depth of 1740 feet to 1884 feet, 

 follow the saliferous beds. 



3. "On the Formation of Deltas; and on the Evidence and 

 Cause of great Changes in the Sea-level during the Glacial Period." 

 By Alfred Tylor, Esq., F.L.S., F.G.S., &c. 



The first portion of this paper was devoted to a comparison of the 

 delta-deposits of the Po, Ganges, and Mississippi. The surfaces of 

 these deltas and the alluvial plains above them were compared 

 together ; and it was stated that a parabolic curve drawn through the 

 extremities of each river, and through one point in its course, 

 nearly represents its longitudinal section — the greatest deviation 

 being 30 feet in some of the largest deltas. 



The littoral deposits around Great Britain described by Mr. God- 

 win-Austen were next investigated, to ascertain whether the hypo- 

 thesis of a fall of 600 feet in the sea-level is tenable. The ice-cap 

 at the poles was also alluded to as a probable cause of a great reduc- 

 tion of the sea-level during the Glacial period. 



The upper 600 feet of deposits in the Pacific Ocean, made by coral- 

 zoophytes, were quoted as cases which might be explained as well by 

 oscillations in the sea-level as by the received hypothesis of the sub- 

 sidence of the sea-bottom. 



Prof. E. Forbes' s investigations into the origin of the fauna and 

 flora of the British Isles were next alluded to, and the author con- 

 sidered that the hypothesis of a fall in the sea-level better accords 

 with the facts of migration than Forbes's suggestion of changes of 

 the level of the land and sea-bottom. 



The origin and age of the English Channel was discussed at some 

 length; and the occurrence of the Crag and fossiliferous gravels 

 and raised beaches near the same level, although of different ages, 

 together with the evidence afforded by the dredging up of fresh- 

 water and littoral shells in the North Sea and English Channel, 

 were adduced in support of the theory of the depression of the sea- 

 level. 



The parabolic curve not only represents the curve of deposition ; 

 for the author had measured other sections, and found that the 

 curves of denudation and deposition approximate often to that of 

 the parabola. 



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 



