598 A. J. JUKES-BROWNE ON THE RELATIVE AGES OF 



the base-level to which the stream would cut down at its mouth, S, 

 would be much lower than the bottom of the Blackwater valley at 

 C ; and just as the Blackwater may have intercepted and diverted 

 certain transverse streams to the westward, so might the stream S 

 have intercepted the waters of the main river at C. In this process 

 the detritive action of rain in lowering the retreating watershed (W) 

 would be as much concerned as the action of the stream in eroding 

 its channel and transporting detritus ; and the whole process would 

 form part of the gradual development of the longitudinal valley in 

 the manner explained by Jukes. Eventually the watershed W might 

 be so lowered that the difference of level between the Blackwater at 

 C and the upper tributaries of the stream S might be very slight ; 

 and during a time of flood the waters of the main river might force 

 a way into the channels of these tributaries. When once this diversion 

 was effected, the river would not be likely to regain its former 

 course through the narrow gorge at V, which would be ultimately 

 converted into a dry pass or gap through the ridge ; and the channel 

 of the Blackwater would be confined to the longitudinal valley north 

 of the Drum ridge, and would appear as a river running from west 

 to east and emptying itself into the sea at Dungarvan Bay. 



The final result, therefore, of the whole process might have been 

 this, that the course of all the transverse streams which originally 

 crossed the Drum ridge from north to south, might have been di- 

 verted into the longitudinal valley, and so have been converted into 

 one continuous river flowing from beginning to end in this longi- 

 tudinal valley from west to east. At the same time the volume of 

 water in the original valley of the Blackwater south of the Drum 

 ridge would be very greatly diminished, owing to the great con- 

 traction of its drainage-area by the diversion of its upper tributaries. 

 I have no doubt that a very good reason can be found why this 

 last diversion did not happen in the case of the Blackwater ; but 

 what local conditions prevented from happening in that case may 

 have happened in other cases, and I have only used the instance of 

 the Blackwater in order to make my meaning clear. 



I will now, therefore, state in general terms the special thesis 

 which I hope to establish in the following pages. Wherever a 

 succession of escarpments and intervening longitudinal valleys has 

 been developed out of a surface of marine denudation, and a river 

 crosses any one of the longitudinal valleys which happens to stretch 

 to the sea-coast, then this transverse river is liable to interception 

 and diversion by the backward extension of a stream flowing down 

 the longitudinal valley into the sea or into a tidal estuary. 



In considering the possibility of such an occurrence it must be 

 remembered that most escarpments have an inclination in the 

 direction of their strike, and that their base-line often has a decided 

 slope from a given point inland towards their termination at the 

 sea ; in such cases the intervening valleys have a general slope in 

 the same direction. It is only under such conditions that the inter- 

 cepting stream could come into existence, and that it could extend 

 itself up the longitudinal valley far enough to intercept the inland. 



