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



under this, the deposit always takes place ; and its extent is nearly 

 in the inverse ratio to the slope. 



The exact turning-point of the two systems still remains to be 

 fixed ; but my own impression is, that we are not far wrong in 

 taking 6 inches as the limit at which the deposit begins to take 

 place ; in many instances, however, 5 or even 4 inches may be taken 

 as the starting-point*. 



4. Secular Elevation of Deltas. — The only other point to which I 

 will venture to call attention is what is called secular elevation, 

 which I shall endeavour to define. 



There was a time, before the formation of the Deltas of the Nile and 

 the Ganges, when the sea or tide extended to Memphis in the one case, 

 and to Kajmahal in the other. If at that time the slope of these rivers 

 had been measured from any fixed point, such as the cataracts in 

 Egypt in the one case, and the rock of Chunar in the other, it would 

 have been found that the slope of both of them was very much 

 greater than it now is, it having been diminished in the exact ratio 

 in which the ground at the apex of the deltas has been raised — about 

 80 feet at Rajmahal, and 60 feet at Memphis. Little or no silt was 

 consequently deposited in the up-country in early times, but every- 

 thing was swept to the sea, and the extension of their deltas has 

 consequently been in an immensely more rapid ratio than at present. 

 But, besides this, for every mile that the delta extended seaward, 

 the slopes, as shown on the diagram (fig. 2), would tend to become 

 parallel in a geometric, and not in an arithmetical, ratio. And 

 when the elbows at Memphis and Rajmahal were completely oblite- 

 rated, the secular increase must have been infinitesimally small. 



Taking first only the mathematical view of the subject — if we 

 assume a point such as Patna, say 200 miles above Rajmahal, and 

 that the delta is now extended 200 miles below that point, assuming 

 also that Patna is now 200 feet above the level of the sea, and Raj- 

 mahal 100 feet, all which figures are sufficiently correct for illustra- 

 tion, it is evident that, if we divide the time since the sea was at 

 Rajmahal into four equal epochs, the level at Rajmahal during 

 the first period of the extension of the delta fifty miles seaward 

 would have risen 40 or 42 feet, during the next 27, during the third 



mile, it is not a depositing river. The only part of its course where the slope is 

 less, judging from the lateral extent of its oscillation, is between Kyrpore and 

 Sehwan. Between these places it may deposit to some extent, but elsewhere its 

 course is steep and straight. 



* No argument on this subject can be depended upon if derived from obser- 

 vations on the coiirse of the Nile, the slope of whose bed from the cataracts to 

 the sea appears to be about 6 inches per mile, for the reason that, between the 

 bank of the river and the hajar, or desert, the natives have at different times 

 constructed numerous causeways, as may be seen in the atlas prepared by the 

 French savants in the beginning of this century, and explained by Sir Gardner 

 Wilkinson (Journ. Royal Geogr. Soc. vol. ix. p. 432 et seq.). These act as so 

 many dams or silt-traps during the inundation or depositing season. But for 

 these dykes, it is by no means clear that there would have been any deposit in 

 recent times on the plain of Thebes, instead of 7 feet in 1700 years, which Sir 

 Gardner calculates ; and the variation in the thickness of the deposits in different, 

 places seems to be almost wholly due to these artificial obstructions.. 



