bac haaiaalll 
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TRANSACTIONS OF THE SECTIONS. 91 
especially ; but on others, as on the Indus and Sutlej, occupied by fine sand and shells, 
forming barren zones along their shores. 
_ The overflow of the river commences within a few days after the commencement 
of the rainy season ; as the rainy season increases, it swells so rapidly, that by careful 
observation, even without instruments, the increase of the river may be traced from 
hour to hour, like the rising of the sea during the tide of flood, until after uninter- 
rupted rain, it completely fills both its ordinary bed and its rainy river-bed, and then 
has a breadth of four to five miles, a magnitude which is even far exceeded by many 
rivers. 
; The small rivers swell in the same way as the large ones, but, as we have remarked 
above, the rainy river-bed being larger, in proportion as the river is smaller, impass- 
able streams are now produced, where a few days before there stood small pools filled 
With water scarcely an inch in depth. 
So long as the river only fills its rainy river-bed, although it may be four or five 
miles broad, this rise has not by any means become an inundation. ‘The mass of water 
is completely enclosed between the two sides of the rainy river-bed. An inundation 
only takes place, when the river, being swollen by unusually continuous rains, or by a 
greater melting of the snow in the mountains, rises above its rainy bed, which is en- 
closed by high banks, and then covered the neighbouring plains like a lake. Unlike 
the rivers of Europe, destructive inundations are rare in most Indian rivers. 
The commencement of the formation of deltas is situated where the periodical tise 
of the river somewhat exceeds the height of the rainy bed. Inundations, but of a 
harmless kind, occur here regularly every year, as, for example, to the south of Dacca 
and Berhampoor in Lower Bengal. 
But in the upper parts of almost any large river district, inundations do not occur 
regularly, but only occasionally, and they both take place suddenly and leave destruc- 
tive effects behind them. One of the greatest inundations took place along the Indus 
in the summer of 1856, in its middle course between Dera Istnael Khan and Mithah- 
kot, where the river attained a breadth of nearly seven miles, its regular rainy bed 
extending two miles in breadth ; and where, in the winter of 1856-57, after the lapse of 
six months, we Saw profitable fishing going on in pools and channels which the Indus 
had left behind in its return, in places where for years only corn had grown. 
That the phzenomena of erosion in the plains should so many times exceed similar 
phzenomena in Europe, may be satisfactorily explained here by the magnitude of the 
mass of water and its periodical accumulation at particular seasons of the year. 
It is more difficult, however, to explain the enormous erosions in the mountains in 
the north of India. In the Himalaya itself the quantity of rain is one of the most 
essential causes, but here, as in Tibet, the narrowness and steepness of the valleys 
combine with this to increase the action extraordinarily. Hence it is that the entire 
eroding force of the masses of water poured down is definitely concentrated in the 
middle df the valleys. Waterfalls and lakes, wherever they occur, are only, possible 
as long as the erosion has not yet reached a certain maximum. In the Himalaya and 
Tibet both are wanting. Lakes have been emptied by the constantly progressive 
erosion, as is evideticed by numerous beds of lakes now dry; in many cases the evapo- 
ration has so greatly increased, after so many watery surfaces ceased to exist, that the 
lakes still remaining begin to grow saline. 
Waterfalls have also formerly existed, but the lateral valleys have now become so 
fiearly equal in level, in their lower parts, with the principal valleys, in which the 
erosion progresses more slowly in proportion in consequence of their smaller inclina- 
tion, that the subsidiary rivers unite with the principal streams with scarcely an 
acceleration of the current. : 
This gradual production of valleys, of which the bed has become some thousands of 
feet deeper, must naturally have had the greatest influence upon the physical con- 
ditions. 
We Should dread extending our remarks too far, if we did more than indicate the 
essétitial consequences of erosion, to which we may perhaps hereafter have the oppor- 
tunity of adverting again; and we here mention in conclusion, only,—elevation of 
temperature ; thermic action of the currents of air ascending the valleys along their 
steep walls ; alternation of the conditions of moisture, and the changes, so intimatel 
connected with these, in the distribution of plants ; and extension of the glaciers, 
ooo 
