18 
Records of the Geological Survey of India. 
[VOL. HI. 
In every large river basin two distinct alluvial deposits will generally be met with, and this, 
though it may seem a somewhat obvious fact to lay much stress on, has nevertheless been 
overlooked in some instances, where its admission was necessary for the true explanation of' 
the geology of the district. One such instance is afforded in Mr. Fergusson’s paper in the 
Quart. Geol. Jour., Vol. XIX, 1863, where the author, from not properly grasping this fact, is 
betrayed into advocating a theory of elevation of the beds supporting the “ Madliopore 
jungle” which he never would have been, had he rightly comprehended that he was dealing 
not with one, but with two groups of alluvial beds unconformable with each other. The 
older of these groups may be either TacStffini. or °* a m ' xe< l an( l alternating cha¬ 
racter, but the newer group is essentially fluvio-lacustrine, and directly produced by the 
existing river, albeit at one time, under surface conditions widely different from those now 
existing; the former of these groups I shall speak of as the older alluvium; the latter, as 
the “newer” or “Gangetie” or “Irawadi” alluvium, as the case may be. One essential 
distinction between these two groups, apart from mere stratigraphical differences, is, that whilst 
no very considerable thickness of the newer group can anywhere have been deposited, 
without a corresponding subsidence below of the area so raised at top, a very large accumula¬ 
tion of the older or estuary beds may have taken place, during an elevation of the area 
covered by them. 
Under one of three conditions, every river discharges its waters into the sea, namely, 
within an area of either subsidence, quiescence, or elevation, and how largely, not only the 
character of the deposits of a large river are influenced by the prevailing conditions at the 
time of their formation, but the physical peculiarities no less of the delta itself, I shall 
endeavour to illustrate by the Irawadi, and the contrast which its delta presents to that of 
its sister stream, the Ganges—as these two rivers, the Ganges and Irawadi, happily furnish 
us with examples of rivers subjected to respectively the first and last named conditions. 
In the Ganges valley the development of the newer or Gangetie alluvium properly 
so called (or as I would propose to restrict the term) is very considerable, and its relation 
and junction with the older deposits usually well defined. In the upper part of the valley 
it is more or less restricted to the immediate neighbourhood of the river and to the narrow 
limits within which the river alters its channel, but it at once spreads out on either bank 
over a vastly broader area than before, so soon as we descend below the confluence, on their 
respective hanks, of the Gandak and Son; the newer deposit assuming, cast of those rivers, 
much of the importance, as far as area goes, which the older group claimed to the west. 
On the north of the Ganges, in the meridian of Purneali, the newer group is thirty miles 
broad, which corroborates a native tradition, that that city once stood on the Ganges.* 
Eastward from Purneah. in the direction of Rajshaieand Pubua, the newer deposits spread 
over a wide tract of low-lying country, the older clay being, however, often but a few feet below 
the surface, and exposed in the beds of tanks or other artificial sections. Where this clay 
arises from beneath the newer group, we often find it (if not usually) presenting a clearly 
defined boundary, giving rise to an elevated tract of country, which offers a complete contrast 
to the low-lying inundated land occupied by the newer alluvium—as an instance of which, 
I may quote the narrow strip of clay country which runs down through the newer group, 
and strikes the Gauges above the station of Kampore Beauliah, near Burgatchee. South 
of the Ganges, all round the Rajmahal hills, the boundary of the two groups is more 
intricate, and in some of the railway embankments a curious contrast is afforded by the 
difference in color of the clays belonging to different groups of which the embankment is 
composed, the earth at one end derived from a patch of old kunker clay being a bright, 
reddish yellow, whilst at the other, it assumes a pitchy hue, from being taken from a bed of 
the newer deposit , dark-colored from the accident of its forming part of the dried up bed of a 
* It, is possible thai this cationite may require to be enlarged, bu* after examining the ground, I con¬ 
cluded that the sandy beds uorth of Purneah pertained to the older rather than the newer group. Though near 
Purneah very flat and low, they rise and undulate considerably as we approach the hills, and include pebbles 
gradually increasing in size a* we go north (or towards the hills). The gradient of the ground, too, after passing 
Purneah, is at once doubled , going north, that of the thirty miles between Purneah and the Ganges being uniform, so 
that I think there arc substantial grounds for holding the view I have given The junction is, it is true, confessedly 
obscure, but this is the result of th*> sandy nature of the surface beds of the older group, which readily commingle 
and fuse, so to speak, with the newer deposits, the important fact of the gradient doubling along this line not being 
cognizable to the eye, nut where tlio hunker clay of the older group is juxtaposed to the sands and silts ot the 
newer alluvium the case is different and little ambiguity results. 
