1 28 PREHISTORIC E UROPE. 



their excavation proceeded more rapidly in the past than at 

 present. But even after all due allowance has been made on 

 this score we must still concede for the process of excavation a 

 very prolonged time indeed. It is true that the Cretaceous and 

 Tertiary strata through which so many of the valleys in the 

 south of England and the north of France are cut are by no 

 means so durable as the older rocks of Wales, for example, and 

 the north country generally. Nevertheless, it is obvious that 

 the removal of a mass of Chalk and overlying Tertiary beds, 50 

 to 150 feet in thickness, and a few yards to upwards of a mile 

 in breadth, throughout the course of a valley 50 or 60 miles or 

 more in length, must have occupied, even at the most rapid rate 

 of denudation, an immense period. We have to conceive of the 

 rocks being gradually undermined, and their fallen dtbris 

 triturated on the bed of the river into gravel, sand, and mud, 

 and rolled gradually seawards. The mere rounding of the flint 

 pebbles, which form a large portion of the old gravel-beds, must 

 of itself have taken a very long time. However rapid, therefore, 

 we may suppose the former rate of excavation to have been, we 

 cannot escape the conviction that the work effected implies an 

 extremely old date for the commencement of the Pleistocene 

 Period. 



Some idea of the rate at which a valley is excavated might 

 be gathered by carefully estimating the quantity of sediment 

 carried annually by its river into the sea. To get as near the 

 truth as possible it would be necessary to measure first the 

 mean annual discharge of water, and then to ascertain the 

 amount of material held in chemical solution and mechanical 

 suspension, together with that which the water pushed forward 

 on its bed. Unfortunately only a few measurements of this 

 kind have been made, but these, so far as they go, help us to 

 form a more or less adequate conception of the rate at which 

 denudation progresses under present conditions. My brother, 

 Professor A. Geikie, has collected all the available data bearing 

 upon this subject, and comes to the conclusion that those rivers, 

 concerning which he has been able to obtain information, remove 



