328 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 1, 



30 or 40 feet below the level of the country. Since then the river 

 has passed on, and a new village now stands on the spot where my 

 bungalow stood, but 40 feet above its ruins ; and any one who 

 chooses to dig on the spot may find my " reliquiae " there, and form 

 what theory he likes as to their antiquity or my age. If we add to 

 this local disturbance the varying degree of elevation just pointed out 

 in the secular increase, it must be seen that the problem is of a much 

 more complex nature than has hitherto been assumed. 



5. Mode in which Deltas are elevated. — Independently of the changes 

 wrought by the varying quantity of water in the different branches 

 of the deltaic rivers, and the consequent necessity for enlarging or 

 contracting the extent of their oscillations, there is another class of 

 changes superinduced by the accidents to which so complicated a 

 system must always be liable. If one of the tributaries, for instance, 

 which before fell into the hollow side of a curve presses on the 

 convex side, if a sand-bank is formed anywhere, or if any natural 

 or artificial obstruction forces the river to change its bed in any part, 

 the whole system is so rigid that the alteration is felt in every direc- 

 tion, both above and below, as far as the alluvial plain extends. 



One consequence of any such alteration in the course of the main 

 stream is, that the initial or terminal oscillation of any tributary or 

 distributary is continually altering its position, and the oscillations 

 cut their way through the whole plain * of the river, both in an 

 upward and downward direction. 



Such a river as the Ganges between Patna and Rajmahal must 

 have eaten through its plain several times since it has occupied its pre- 

 sent position. But when the delta is so raised as to reduce its slope 

 from 6 to, say, 3 inches per mile, it is not difficult to foresee a time 

 when the river will so raise its bed as to be obliged to seek a new 

 " plain " further north, and may again resume the position it once 

 occupied in the centre of its valley, but from which it has been forced 

 against the southern hills, by the greater "vis viva" which the 

 Himalayan streams derive from the rapid slope of their beds and 

 their preponderating body of water. 



None of the rivers of the delta have in historical times, so far as 

 we know, worked their way twice through the same plains. The first 

 operation so raises their plains that they generally find it easier to 

 seek a new course in the lower lands on either hand ; and the stream 

 in their old beds consequently becoming sluggish, they gradually silt 

 up and become, after a while, altogether obliterated, except here and 

 there where a reach has been cut oft' in the process, and remains, like 

 a fossil, to mark the previous existence in that spot of a river of a 

 given oscillation, which may still be measured with perfect certainty 

 in the fragment that is left. It is by this continual shifting of the 

 plains of rivers that the whole delta is gradually raised to a higher 

 level. 



* By "plain" in this sense is meant the district occupied by the river be- 

 tween the extreme outward edge of its oscillations on either side. With a river 

 1000 feet wide, its plain may extend from one to two miles in width, and others 

 in like proportion, varying, of course, according to the slope of the country. 



