1868.] tylob, formation of deltas. 7 



Discussion. 

 Sir R. I. MuRCHisoN differed from the author on one point only. 

 He regarded the upper member of these saliferous rocks, which is 

 overlain by 1500 feet of Bunter Sandstone, as completely separated 

 from that formation, and as forming the upper pai*t of the Permian 

 group. 



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., E.G.S., &c. 



(Abstract.) 

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

 delta- deposits of the Po, Mississippi, and Ganges, by means of the 

 descriptions of the strata obtained from borings in their deltas for 

 water. 



The surfaces of these deltas and the alluvial plains above them 

 are compared together with reference to their inclination and height 

 above the sea-level ; and it is found 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 devia- 

 tion being 30 feet in some of the largest deltas. 



The delta-deposits are found to be coarser and more sandy near 

 the bottom, indicating that the rivers were more rapid during the 

 earlier portion of their existence. 



Messrs. Wheatley and Abbott's descriptions of the delta of the 

 Mississippi are compared with those of earlier writers ; and a de- 

 scription is given, from their work, of the late extensions of the 

 delta into the Gulf of Mexico. 



The formation of delta-deposits is explained by the hypothesis of 

 a change in the level of the sea, instead of in the level of the land 

 and sea-bottom. 



The littoral deposits around Great Britain are investigated by the 

 author, to ascertain if his hypothesis, of a fall in the sea-level of 

 600 feet during the Glacial Period, is tenable. 



Some evidence of the extent of the Glacial Period is given ; and 

 the ice-cap hypothesis advocated by Mr. Croll is alluded to as a 

 probable cause of a great reduction in the level of the sea through 

 abstraction of water from the sea, and its deposition at the poles in 

 the form of ice. 



The positions of the fossiliferous strata of the Quaternary Period 

 are discussed with relation to Mr. Godwin- Austen's former sugges- 

 tion of a great river where the German Ocean now is, formed by 

 the junction of the Rhine, Thames, and Humber. The probable 

 age of the Straits of Dover is also alluded to. 



Prof. Forbes's examination of the fauna and flora of the British 

 Isles, with a view to the determination of the sources of Alpine 

 plants, induced him to believe that the British plants and animals 

 migrated from Scandinavia, Germany, and France at different 

 periods, some before and some after the Glacial Period; and the 



