156 



THE EARLY HISTORY OF THE 



[vol. lxxv, 



which must have been very considerable in the Himalayan region, 

 probably played a much greater part. 



Lord Clifford pointed out that in the great waterspout flood 

 of 1868 in the Clarence Valley (New Zealand), the water denuded 

 the mountains of whole forests and blocked up the gorge from 80 

 to 100 feet high with stone and timber, creating an inland lake 

 13 miles long and 5 or 6 miles wide in the course of three days. 

 It then burst away the embankment that it had made, cut 

 30 feet deep through the solid rock of the gorge, and carried out 

 enough stone to reclaim 2000 or 3000 acres of a delta at the 

 estuary of the river. When encroaching rivers cut back into the 

 channel of a higher watershed, it was impossible to say how rapidly 

 what looked like irresistible rock might be cut away. 



Prof. E. J. Garwood asked whether traces of hanging valleys 

 had been observed in connexion with the westward-flowing captured 

 tributaries described by the Author. In the case of well-known 

 captures like those of the Albigna and Forno captured by the 

 Maira from the headwaters of the Inn, the tributaries now entered 

 the main stream over precipitous cliffs 1200 and 800 feet high, 

 and similar cases had been met with by the speaker in the Tongri 

 district in Sikhim : for instance, that of the Kang La valley. It 

 was not necessaiy for the tributary to hang at its entrance into 

 the main valley at the present time — erosion may have cut back 

 the mouth of the tributary to accordant grade with the main valley; 

 but it was often possible to trace the original discordance in the 

 form of steps some distance up the tributary valleys. 



Mr. Gr. W. Yottjtg thought the circumstance that the dolphins 

 of both the Granges and the Indus belonged to the same species 

 suggested that the reversal of drainage was due to differential 

 earth-movements rather than to capture of the head- waters of the 

 longitudinal ' Indobrahm ' by an aggressive transverse river. In 

 the latter case the water would be gradually captured, but hardly 

 such big and active animals as dolphins, which required consider- 

 able depth of water; whereas, if earth-movements were the cause, 

 the same fauna would be found in both the original stream and the 

 severed portion. 



Mr. C. E. N. Brouehead wished to know whether the Author 

 of the paper intended to imply that the capture of the 'Indobrahm,' 

 by which it found an exit to the south and became the Granges 

 and Brahmaputra, was of more recent date than the capture of the 

 river flowing north-westwards in the great depression north of 

 the Himalayas by the southward-flowing Indus. The gorge of the 

 Indus described in the paper was a t3 r pically new feature. If the 

 lower course of the Granges and Brahmaputra was also of recent 

 date, and due to the cutting-through of a hard ridge joining the 

 Indian Peninsula to the Assam Hills, one would expect a gorge 

 similar to that of the Indus instead of the broad open valley 

 which, on the map, had all the appearance of mature age. 



Mr. P. D. Oldham said that, on one point, he was not in agree- 

 ment with the Author. He could not regard the gaps through 



