Vol. I, No. 9.] The Geology of the Gangetic Plain. 233 
[N.S.] 
exists in the Benares District opposite Saidpur. I have not had 
an opportunity of verifying the statement, and have not therefore 
shown it on the map. 
Mr. Oldham’s is the only possible explanation, wiz,, that the 
river Ganges has gradually eroded the land at some bend in its 
course till it has cut into the course of the affluent at a point 
above the former confluence. 
When once a channel had been made into the course of the 
affluent, centrifugal force would drive the water of the Ganges 
through the breach so made, and the new channel would rapidly 
be widened out till it became the main course of the Ganges. 
The island opposite Chausa must have been caused by the 
Ganges usurping the course of the Karmnasa, and that opposite 
Saidpur by the usurpation of the course of the Gumti. 
Similarly, tradition, which Mr. Oldham considered trust- 
worthy, says that, at one time, the water of the Gogra passed 
down the present river Sarju, or Tons as it is sometimes styled, 
which separates the Chazipur and Ballia Districts, This would 
indicate that the Ballia District consists of a similar island caused 
by the river Gogra (or Sarju as it is called in some parts) usurp- 
ing the course of some smaller river that used to flow down the 
present bed of the Gogra. 
Any such usurpation of another river’s course would probab- 
ly completely alter the set of the current at the old confluence, 
and release the river from the bends in the old alluvium which 
had formerly given the river its general direction. 
At the commencement of the paper it was taken as already 
proved that the area at the mouth of the Ganges was an area of 
subsidence. 
There are, however, indications that within the area of the 
United Provinces of Agra and Oudh the southern portion of the 
Gangetic plain has sunk relatively to the northern portion. 
The first piece of evidence is that supplied by the artesian 
well sunk at Lucknow. (vide Oldham’s Geology of India, p. 434.) 
At a depth of 158 feet the water stood at 61 feet below the 
top of the borehole. Ata depth of 975 feet the water had risen 
to 2 feet. At 990 feet it stood at a depth of 5 feet below the 
top of the borehole, and at 1,189 feet the water rose over the top 
of the casing, itself 24 feet above the surface of the ground. 
This shows that the lower strata must be inclined, though it 
does not indicate the direction of the dip, 
As the Himalayas are known to have risen and the Gangetic 
plain to have sunk, the probability is that the dip of the strata is 
from north to south. 
A river flowing over a flat alluviual plain would naturally 
find its way directly down the slope, and there is no reason why 
the watershed should not be equidistant from either bank. 
If, however, after the river had excavated a channel for itself 
with the watershed equidistant from either bank, the whole allu- 
vial plain through which it flowed was slightly tilted at right 
angles to the river’s course, the result would be that the slope into 
