OLDHAM: GEOLOGY OF MANIPUE, AND NAGA HILLS. 19 



complete removal of the talus by the streams flowing over it, which, as 

 may be seen on the map, are much larger than those to the south. The 

 numerous minor irregularities of the surface better accord with this 

 theory than with the alternative one of its being an old lake gradually 

 silted up. 



40. But though the Tiki would probably have cut its bed down, as 

 Probable past exten- ** actually has done, if the supply of water had 



sion of the Tiki valley. decreased together with that of the debris, yet I 

 hope to be able to show that it is not improbable that the stream may 

 not only not have decreased, but may actually be greater now than at the 

 time the high level terrace was formed. At the watershed between the 

 Tiki and the Ngordai the road crosses over a low gap, hardly rising 

 perceptibly, and passes over a fan brought down from the west by the 

 stream marked on the map (opposite) as forming the head- waters of the 

 Tiki, part of the drainage from which, if not part of the actual waters of 

 the stream, flows into the Ngordai and part into the Tiki. This can only 

 have originated by one of the streams having extended its valley at the 

 expense of the other, nor is there much difficulty in determining which 

 it is, for not only does the fan bear the appearance of having been 

 formed by a stream originally flowing away to the north but the level of 

 the Ngordai at four miles from this place is 400 feet higher than that of 

 the Tiki at a similar distance from the same point, thus giving a more 

 rapid fall and hence a more rapid erosion towards the south. 



41. This has an important bearing on the much vexed question of 



Bearing on theory of the . conversion of transverse into longitudinal 



conversion of lateral in- drainage, f or we have here one valley extending it- 

 to longitudinal drainage. < _ ° 



self at the expense of and in opposition to another ; 



and were the action to continue till the Tiki valley reached the junction 



of the Ngordai and the Barak, and consequently effaced the existence of 



the former, no inconceivable result, for the Barak is here 500 feet higher 



than the valley of Manipur, the waters of the Barak would be directed to 



flow along the Tiki valley to the south, and so transverse would be con° 



verted into longitudinal drainage. 



( 235 ) 



