392 A. Tylor — Formation of Deltas. 



IV. — On the Fokmation of Deltas : and on the Evidence and 

 Cause of Great Changes in the Sea-Level during the 

 Glacial Period.^ 



By Alfred Tyloe, F.G.S. 



Part I. 



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 height above 

 the sea and inclination, 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, and indicate 

 more rapid rivers and greater rainfall at the earlier portion of tbeir 

 history. 



Messrs. Humphreys 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 the 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 Ehine, Thames, and Humber. The probable age of 

 the Straits of Dover is also alluded to. 



Professor 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 differ- 

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



1 Being the text in full of a paper read by the author before the Geological Society 

 of London, November 11, 1868, but only published in brief abstract in the Quart. 

 Journ. Geol. Soc, 1869, vol. xxv., p. 7. 



The views embodied in this paper received little favour at the time of their first 

 enunciation ; but having since been adopted by some geological writers, the author is 

 desirous that the fuU text of his paper (as delivered to the Geological Society in 

 1868) should be made known, in order that his claim to priority may be recognized. 



