396 A. Tylor — Formation of Deltas. 



p. 392. These curves vary on the two sides of a vi^atershed with the 

 respective inclination flexures and difficulties of the strata and 

 position with regard to drainage. The curves vary particularly 

 with the projection of harder beds and retreat of softer beds from 

 the theoretical line (Quetelet, p. 413. Lettres, etc., Curves shown, 

 1846). 



The slope of the bottom of the Dover Valley is about 385 feet in 

 7 miles, or 1 in 55. The Somme drains a more extensive area, and 

 is a larger and slower river. It contains along its course, and near 

 the level of the present river, fluviatile deposits with Cyrena flumi- 

 nalis. As this shell is found fossil abundantly in the Thames and 

 Humber rivers flowing into the German Ocean, it seems probable 

 that the Somme river, since it is inhabited by the Cyrena fluminalis, 

 drained into the German Ocean, and not into the English Channel — 

 the Cyrena is found in no other river draining into the English Channel. 

 The Varne shoal, between Dover and Calais, would appear to mark 

 the former watershed of the ^omme and Dover rivers. 



Sir H. T. De la Beche examined the soundings in the Channel, in 

 order to see if the large valleys which opened on to the coast-line 

 were continuous under the sea. He found they were not now to be 

 traced, but I think this is no proof that they did not exist. 



The action of the sea may have filled up the deeper portions of 

 the valleys with deposit, and abraded the old watersheds lying 

 between the systems of valleys formed when the space occupied by 

 the Channel was dry land. The measure of time for partly exca- 

 vating and filling up the Channel valley may be represented by that 

 requisite for the denudation of the Wealden area and the south of 

 England. 



Prior to the Glacial Period we should expect that there was high 

 land between Dover and Calais, and that when the German Ocean 

 and English Channel became dry this land was lowered immensely 

 by fluviatile and pluvial denudation. 



The work of the sea in opening out the Straits of Dover would be 

 much facilitated if the space between Dover and Calais had been 

 previously perforated by two large rivers, and still more by the 

 action of their tributaries and by rainfall, during a period which has 

 been supposed by astronomers to have lasted 160,000 years. All 

 over the south of England there is evidence of subaerial and pluvial 

 gravel reaching nearly to the summit of the hills, and the character 

 of the debris and the size of the now dry valleys and character of 

 the watershed indicate the kind of climate which existed in the 

 south of England from the time of the elevation of the Wealden 

 area nearly to the historical period. The unsymmetrical flexures 

 in the Wealden and Cretaceous Beds show the exact direction of 

 the elevatory force, which was ohlique ^ to the horizon and to the 

 Channel, bending the strata in a double oblique system, by the 

 author's measurement. The uneven surfaces of the Palceozoic rocks - 



1 Mr. Hopkins treated the force as vertical, and did not observe flexures. 

 ^ The most favourable situation for boring for coal is between Fairlight Glen and 

 Crowboro Beacon. 



