Mr. J. Prestwich on the Drift Deposits. 61 



that of the present rivers, is so vast, that in no possible way, 

 except by the sea, could they now be filled with water. Sec- 

 tions are given of the valleys of the Waveney, Ouse, Somme, and 

 Seine, showing a disproportion between the rivers at their highest 

 floods and the old valleys, on the average, about 1 : 500 ; and it is 

 shown, with respect to the great flood of the Seine in 1658, when the 

 waters at Paris rose to a height of 29 feet, that it would require a 

 flood of at least one hundred times that magnitude to fill (with the 

 water even in a state of rest) the valley of the Seine to the level of 

 the high-level gravels of Gentilly and Charonne. 



That the isolated beds of high-level gravels must at one time have 

 been connected in length and breadth is evident from the circum- 

 stance of these detached parts having certain characters in common, 

 and from the fact that if the deep valleys which they overhang, and 

 the transverse valleys which they pass over, had then existed, they 

 would have presented insuperable barriers to the deposition of the 

 gravels at levels so much higher. 



That the transport of this drift could have been caused by the 

 bursting of lakes, by the sudden melting of the glaciers and snow of 

 mountain-chains, or by the transient passage of a body of water over 

 the land is not possible, because the spread of the debris would have 

 been more general, would have held its course more irrespective of 

 the existing watersheds, and would have shown an amount of wear 

 in proportion to the distance travelled ; whereas in each basin the 

 debris is local, however low the watershed. None of the slate and 

 oolitic debris of the Oise valley traverses into the valley of the 

 Somme, notwithstanding the watershed between them is only six 

 miles broad and eighty feet high. 



There are two ways in which the author conceives the spread of 

 the debris in the various directions and distinct areas could have 

 been efi'ected ; the one by the rise of the land from beneath the sea, 

 and the other by the action of rivers on a larger scale than the 

 present ones. As the later tertiary deposits show the existence of 

 seas or of lakes over the districts in question, it follows as a necessary 

 consequence that when the land rose from beneath them, a mass of 

 debris, in quantity and length of transport proportionate to the 

 greater or lesser rate of elevation, must have been spread over the 

 bottom of the channels along which the water flowed ofl'. Nearly 

 associated with the high-level gravels there are remnants of another 

 drift which may have had this older and independent origin. This 

 mode of formation could not, however, be applied to the valley gravels, 

 as they contain freshwater shells such as live in rivers, with land 

 shells and mammalian remains, proving the existence of a dry land. 



The author concludes that the high-level gravels are the result 

 of river-action which took place at a period before the excavation of 

 the present river- valleys. With regard to the mode of formation of 

 these gravels, he remarks on the materials being often transported 

 a considerable distance, — the frequent presence of large blocks or 

 boulders of the harder rocks, — the presence of a certain proportion 

 of angular debris, — and the commonly confused bedding and con- 



