50 
Records of the Geological Surrey of India. 
[vo'. VIII- 
rocks are seen in the Tapti river below Bhodhan and in the Kim river as far west as the 
neighbourhood of Elao, but throughout most of the intervening area they are covered up and 
concealed by alluvial deposits, and they are nowhere exposed, except in one or two small 
isolated hills, throughout the country south of the river Tapti. By far the greater portion of 
the country consists of an alluvial plain, the surface being covered with a thick coating of 
black soil. Along the sea-coast are low hillocks of blown sand. 
The alluvial deposits furnish nearly all the water obtained in wells, and these deposits 
demand therefore rather fuller notice. They consist of days, sandy clays, and sand, much 
interspersed in places with concretionary nodules of carbonate of lime. Towards the surface 
they pass into black soil. They may contain beds of gravel (rolled pebbles) in places, but 
such appears to be uncommon, so far as my information extends. The different layers of 
sand and clay are probably very irregular in thickness and extent, but sections are rare, 
and very few borings have been taken. In those made for the Tapti bridge at Surat, however, 
as I am informed by the Executive Engineer in charge, a bed of hard clay with calcareous 
nodules, in which it is proposed to lay the foundations of the piers, was found to be very 
much thinner on one side of the river than on the other, the difference, which was not 
precisely determined, amounting to several feet. It is evident that this bed has an irregular 
and possibly a lenticular section, and the same is probably the case with all the strata in the 
alluvial deposits, whilst the more sandy layers in which, owing to their greater permeability, 
water is generally found, may very often thin out and disappear in the distance of a few 
yards. 
I quite concur in Mr. Medlicott’s remarks on the different reasons which may be assigned 
for the occurrence of brackish water in wells. These are, briefly, the presence of salt in 
the strata when originally formed, salt springs, and infiltration from spots in which salt 
is being deposited at the surface of the ground. To these may be added percolation from the 
sea or from estuaries, which, however, is practically identical with the third form. In the 
case of Surat, I believe that the salt was originally deposited in the alluvial strata. 
The plains of Guzerat have every appearance of being estuarine or marine deposits 
formed from the clay and sand brought down by the Tapti, Narbadda, and other rivers. 
The deposits forming in the salt marshes and flats submerged at high tides near the mouth 
of the Tapti, which I had an opportunity of examining during my recent visit, are covered by 
a deposit differing so little from one form of the black soil, that it is impossible to draw a line 
separating the two, the blackish argillaceous dried mud of the estuarine flats and marshes 
being similar, both in colour and texture, to the black soil of the fields a few inches above the 
level of the highest tides, and this soil again differs but slightly, either in colour or texture, 
from the ordinary ‘cotton soil’ of Guzerat. Such differences as exist are, I think, due to 
surface action; to the effect of rain and chemical changes, impregnation with organic 
matter,* and agricultural processes, and I see no reason for doubting that the whole of the 
surface formations in Surat may have been deposited from salt and brackish, water in tidal 
estuaries and salt marshes, precisely similar to those which are now being reclaimed and 
converted into arable land in places on the sea-board of the district. The more sandy beds 
must have been deposited where some current, due either to tidal or stream action, existed; 
the fine argillaceous black soil has probably been formed in back-waters and marshes.!" 
Evidence of recent rise in the land has been found in several places on the western coast 
of India: instances are known at Bombay, in Katthiawad, and in Sind. There is every reason 
* It is probable that great part of Ouzerat has been covered by forest, and the soil thns impregnated with 
decayed organic matter. In this manner the best and richest cotton soil has very probably been formed. 
| IVIy brother, Mr. H. F. Blantord, several years ago pointed out a similar mode of origin of black cotton soil on 
the Madras coast, and I found a similar deposit forming under the same circumstances in Orissa. 
