330 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [^P r - 1, 



Journals ' (vol. ii. p. 256). He sailed for some days among these 

 Jheels, and found by his barometric levellings that they were very 

 slightly raised above the Bay of Bengal. Their bottom generally 

 consisted of accumulations of decaying vegetable matter — incipient 

 peat — through which he could not reach a bottom by thrusting in the 

 boat-poles. 



It is not necessary here to insist on this depression being the effect 

 of the upheaval of the Madoopore Jungle, or to inquire whether it 

 pre-existed ; but I think there can be very little doubt that the dis- 

 turbance caused by the upheaval was what turned the Brahmapootra 

 towards the east into these jheels. 



A mere inspection of the map is sufficient to establish the proba- 

 bility that this change must have taken place not very long ago ; and 

 though the length of the course of the Brahmapootra is only half 

 that of the Ganges (for it is not clear that it has any connexion with 

 the Sampo of Tibet), still the two rivers are quite equal in volume, 

 inasmuch as both enter the deltaic plains of Bengal with a ten- 

 mile oscillation before parting with any of their waters through their 

 distributaries below Rajmahal on the one river, or at Rangamutty 

 on the other. 



Although equal in volume, the Brahmapootra brings down an 

 immensely greater quantity of silt than the Ganges — probably one- 

 half more ; as Buchanan Hamilton phrases it, " It is the dirtiest 

 river I ever saw." This arises, principally, from the fact that the 

 slope of the valley of Assam, from Sudya to Goahuttee, appears to 

 be more than 6 inches per mile. The consequence is, that neither 

 the principal branch of the Brahmapootra nor any of the tributaries 

 deposit their silt, but all is swept onwards ; and, owing also to the 

 greater quantity of rain that falls there than falls to the westward, 

 the denudation of the land is much more rapid. This condition of 

 matters will not be changed till the section across the valley opposite 

 Goalparah has been considerably raised beyond its present level, 

 or, in other words, till the land between Goahuttee and Goalparah 

 attains an elevation corresponding to that attained in the valley of 

 the Ganges about Rajmahal. At present the elevation of the river 

 at Goahuttee, with 350 miles to run, is apparently lower than the 

 Ganges at Rajmahal, within 250 miles of the ocean*. 



Until this extra elevation takes place, the physical condition of 

 the valley of Assam will so closely resemble that in which the 

 valley of the Ganges probably was when the sea was at or near 

 Rajmahal, that few problems connected with this subject would be 

 more interesting than to compare the condition of the two valleys, 

 in so far as materials exist for the purpose. 



At that early time the River Ganges must have flowed with greatly 

 increased velocity nearly in the middle of its valley ; but as its slope 

 and consequent velocity decreased, it was pushed southward against 



* The level of the river at Goahuttee is assumed from a long series of baro- 

 metric observations, checked by those of the brothers Schlagintweit (' India,' 

 vol. ii. p. 103) ; that at Rajmahal, from the levels of the East Indian Railway, 

 checked by those of the great Trigonometrical Survey. 



