Vol. I, No. 9.] The Geology of the Gangetic Plain. 235 
[N.S.] 
depths, the pressure of the superincumbent earth renders imper- 
vious to water everything but the coarser and cleaner kinds of sand. 
Extensive and continuous deposits of such sand would only 
be found along the beds of the larger rivers which may be pre- 
sumed to have existed though they have been long since deeply 
buried by alluvial deposits. 
If a deep borehole struck such an old river-bed, it is concei- 
vable that the necessary head to bring the water to the surface 
might be supplied through a deposit of sand in the former river- 
bed continuous up to a point in its uppper course where it attained 
the necessary elevation. 
The observed facts at Lucknow do not, however, support this 
explanation, as there were several rises in the level of the water in 
the borehole, and it seems unlikely that one borehole should have 
struck the beds of several such former rivers one above the other. 
The second possible explanation is that the surface deposits 
were laid down in running water and the deeper deposits in still 
water. Anyone acquainted with the country can see in the cres- 
cent-shaped swamps and the alternations of clay, light soil and 
sand on the surface of the country, the result of the same agency 
that is reproducing similar features in the alluvium of the great 
rivers, namely deposition by running water; and the inference is 
inevitable, that the surface of the country was formed by the 
deposit of the material derived from the mountains in a labrynth 
of vast morasses amd deltaic rivers. 
This agency, however, would not explain the enormous area of 
clay and sand revealed to us by the Lucknow borehole; and it 
seems necessary to assume the existence of an enormous sheet of 
water with currents sufficient to transport sand great distances 
to account for the phenomena. 
In Oldham’s Geology of India, p. 144, the occurrence of a 
species of fresh-water porpoise, common only to the Ganges and 
Indus and their tributaries, is cited as showing that the Ganges 
and Indus flowed at one time into the sea through a common delta. 
The sea is shown to have extended up the Indus valley within 
a geologically recent period ; and it seems possible that it may have 
extended east much further into the gangetic plain than is usually 
supposed, 
The absence of any indications of marine origin in the upper 
strata might well be due to their having been deposited in fresh 
water after the communication with the sea by the Indus valley 
had been cut off. 
At any rate, whether the water was fresh or salt, the con- 
tinnity of the deeper strata over great distances seems to strengthen 
the theory that the lower strata were deposited in a great sheet of 
still water. 
Such a great sheet of water, originally salt but gradually be- 
coming fresh as the communication with the sea became gradually 
more and more obstructed, would satisfactorily account for the 
change of the salt-water porpoise into an animal inhabiting fresh 
water. 
