1899] THE DEVELOPMENT OF RIVERS 275 



For the purpose it may be said stronger and weaker rocks, also pervious 

 and impervious. 



The stream cuts down its beds so that different rocks outcrop in 

 the stream-bed, and are exposed on its flanks. Weak rocks are denuded 

 faster to make combes, impervious rocks send forth springs. The springs 

 and combes become side-streams and valleys. The side-streams are 

 subsequents. Now consequents cannot get any better off than at first : 

 they get all the drainage and cannot get more. But subsequents become 

 better off the more they work back. The consequents are dip-streams, 

 they are best off when they start. The subsequents are strike-streams 

 — they are worst off at starting, and best off at finish. 



A strike stream has a dip side and an antidip side. It obtains all 

 the drainage of the dip side by the former streams. If it can give the 

 water a quicker fall to a lower level in a shorter distance, 1 it will 

 start streams on the antidip side. Antidip streams are obsequents. 

 Like subsequents they become better off the more they can work back. 

 They are really stronger than subsequents, and can in time obtain 

 practically all the drainage of a given area. 



In the case of an anticline a consequent could grow by developing 

 obsequents beyond the axis. The Salisbury Avon seems to be doing this 

 in connection with the Pewsey anticline, taking the water and destroy- 

 ing the south drainage area of the Kennet. Artesian action too is possible 

 — it may obtain water which falls far to the north of the Kennet. 



In a drainage area which had not suffered previous denudation 

 subsequent streams would be started along anticlinal axes, because 

 there the impervious rock, and hence the water-level of the upper area 

 would first be reached. 



In an area which had suffered previous denudation so that different 

 rocks outcrop, the same would apply where there was a wide expanse of 

 a rock of uniform composition. The head-waters of the Salisbury Avon 

 in the Pewsey valley, and the growth of the original Sevenhampton 

 branch of the Coin in the Charlton Abbots valley, are examples. In 

 part this may explain the preservation of synclinal areas. 



But in such a drainage area, where stronger and weaker rocks out- 

 crop not far apart, the subsequent streams will be developed along the 

 strike of the weaker rocks. The Ock, and the Thame, and the Thames 

 above Oxford, are examples. So, in fact, are rivers which drain along 

 the strike of New Eed Sandstone, Lias, Oxford Clay, and so on. 



But this is a digression. "What I want to point out is this, there 

 are started, first, dip-streams, secondly, strike-streams, and thirdly, 

 antidip-streams. Where would their position be ? Naturally in the 

 lowest ground — the valleys of the beheaded dip-streams or consequents. 



This seems to be an important matter to consider in drainage 

 restoration. If obsequents tend to occupy and work back in valleys of 



1 This is important : it is not necessarily a shorter route to the sea, but an immediate 

 quicker fall. 



