38 [March 27, 



March 27, 1862. 



Major-General SABINE, President, in the Chair. 

 The following communication was read : 



" Theoretical Considerations on the Conditions under which the 

 Drift Deposits containing the Remains of Extinct Mam- 

 malia and Flint-implements were accumulated; and OD 

 their Geological Age/' By JOSEPH PRESTWICH, Esq., 

 F.R.S., F.G.S, Received March 20, 1862. 

 (Abstract.) 



In his former paper on the subject of the Flint-implements*, the 

 author postponed the consideration of the theoretical questions, to 

 allow time for a fuller investigation of the physical phenomena. The 

 points then sought to be established were, the artificial make of 

 the specimens, their position in undisturbed ground, and their 

 contemporaneity with the extinct animals. The points for present 

 consideration relate to the structural and physical phenomena, and 

 to various theoretical questions. 



In the present paper the author proceeds to show that the flint- 

 implements are found along the line of existing river-plains at heights 

 varying from 20 to 100 feet above the rivers, and that the beds of 

 sand and gravel in which they are imbedded can be divided into 

 two more or less distinct series, one continuous along the bottom of 

 the valleys and rising but little above the river-level, and to which he 

 proposes to apply the term " Low-level Gravels," and the other in 

 detached masses on the heights flanking the valleys, and at 50 to 

 200 feet above the rivers, and which he designates as the "High- 

 level Gravels f." Both gravels consist of debris derived from 

 rocks in the valleys through which the present rivers or their tribu- 

 taries flow, and they both occasionally contain organic remains ; 

 both are, in fact, related to former plains and present valleys. 



* Read before the Royal Society 26th May, 1859 ; Phil. Trans. 1860, p. 277. 



t At the reading of this paper, the author used the terms " Terrace Gravels " 

 and " Valley Gravels ;" but he thinks it better to revert, with limitations, to terms 

 which he suggested some years since, but has not hitherto defined. 





