332 MR. A. J. JOKES-BROWNE ON [A-Ug. I9°4* 



When, however, the easterly slope was modified and dominated by 

 the southerly tilt given to the country, as I suppose, in late 

 Pliocene time, then every eastward-flowing stream would impinge 

 with greater force on its southern banks and would cut deeper 

 curves out of the southern side of its valley ; at the same time, 

 the erosive power of every little rivulet which flowed from north to 

 south would be increased. That part of the valley of the Yeo 

 which lies to the south of Ashburton was doubtless initiated by a 

 tiny tributary of the Dart ; and during the gradual detrition of the 

 country, it would naturally encroach upon the watershed which lay 

 between its head and the valley beyond. This process, even without 

 the aid of any earth-movement, is likely to have resulted in the 

 trenching of the dividing ridge ; and as the Yeo would be cutting 

 away the northern side of this ridge, it is likely that a time would 

 come when it only required a flood in the valley of the Yeo to make 

 its waters overflow into that of the little stream to the south. 



The basis of this theory, by which I have tried to explain the 

 peculiar course of the Teign and the origin of the Teignmouth 

 Valley, is the double assumption that the country had first a general 

 inclination to the eastward and was subsequently given a tilt to the 

 southward; but both these assumptions are in accord with geological 

 facts in other parts of England. They agree also with geological and 

 geographical facts in Devonshire : the drainage-system of Dartmoor 

 is likely to be older than that of the surrounding country; and the 

 biggest rivers of Dartmoor rise near its western border, as they 

 would do if the slope of the Eocene and Oligocene land was towards 

 the east. On the other hand, the long courses of the Tamar and 

 the Exe seem explicable on the supposition of a southerly slope. 

 which has enabled them to extend their system of drainage towards 

 the north. In this connection, I think that an examination of the 

 possible relations between the head-waters of the Exe and the Tone 

 might lead to interesting results. 



I suspect that everywhere throughout Devonshire and Western 

 Somersetshire the extension of southward-flowing rivers at the 

 expense of eastward-flowing streams may be invoked to explain 

 the present somewhat-complicated system of drainage. I desire, 

 however, to guard myself against being understood to suggest that 

 either or any of these earth-movements produced a continuous regular 

 slope in one direction. It is quite possible that the general easterly 

 tilt given to the whole region in Oligocene time was interrupted 

 by undulations striking from north to south, and that, while the 

 principal or primary rivers cut across these incipient ridges, local 

 drainage might in some districts be directed into north-and-south 

 lines at an earlier date than that which I have suggested. 



The series of domes and basins which now exist in the South o+ 

 England — I mean such as the basin of Beer and Axmouth, and the 

 dome of the Yale of Marshwood — may have been produced by the 

 intersection of two series of flexures, an earlier series running from 

 north to south, and a later series from west to east : for we know 



