344 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 1, 



tributary streams would join the assumed Ganges-canal at right 

 angles, as shown in the diagram, fig. 3. 



For the second hypothesis, let me assume that the sea, or canal, 

 extended from somewhere below Rajmahal to the Himalayas, due 

 north and south, and that the dip of the plain was west and east, 

 say from Cawnpore or Fyzabad to this canal or sea, as shown in 

 fig. 4. It is evident that all the streams, on issuing from the 

 hills, would tend to turn eastward, and to run parallel to the Ganges. 

 Neither of these, of course, truly represents the facts of the case. 

 The valley or the basin of the Ganges, like that of almost all rivers, 

 is compounded of these two plains, varying in slope according to 

 circumstances ; and the course of the tributaries is along the diagonal 

 or mean of these two intersecting plains. 



Thus, if we assume that when the sea was at Rajmahal the 

 slopes of the two plains were about equal, as shown in fig. 5, all 

 the tributaries would join the main stream at about an angle of 

 45° ; but the extension of the delta has now raised the land about 

 this place to nearly 80 feet above the sea-level. This has been 

 equal to tilting back the valley of the Ganges to that extent, without 

 materially affecting the slopes of the lateral plains, as they are 

 shorter, and start from much higher fixed points at the foot of these 

 hills. The consequence is, that the angle of 45° is always tending 

 to increase, and must eventually reach 90°, or nearly so, in all cases. 



There are only slight indications in Bengal of the state of affairs 

 represented in fig. 4, but it can be traced in parts. The upper 

 part of the valley of the Ganges, from Allahabad to the mouth of 

 the Gogra, is in the state represented in fig. 5, with a tendency 

 rather towards that shown in fig. 4. The lower part is fast assuming 

 the form represented in fig. 3. But there is still a fourth form which 

 the rivers must all ultimately assume, and which more resembles 

 fig. 4 than any of the other diagrams. It is this : as soon as the 

 slope of the principal stream has been so reduced by the elevation 

 or extension of the delta, or other causes, that it becomes a deposit- 

 ing river, it will then so raise the level of its plain above the sur- 

 rounding country that the tributaries cannot flow directly into it. 



The form they will then take is shown in fig. 6 : having been re- 

 duced to joining the main river at right angles, as in fig. 3, they will 

 be turned at right angles on reaching the edge of its plain, and, 

 flowing parallel to it, join it at some point lower down, where the 

 tributary may have acquired sufficient elevation to force its way 

 into the bed of the great river. 



We have already examples of this in the way the Soorma was 

 deflected by the Brahmapootra, the Damooda by the Hoogly, the 

 Attree by the Poddah, and all the minor South Behar streams by 

 the Ganges. The Coosy too is fast assuming this shape, and even- 

 tually it will become the normal condition of all the tributaries of 

 the Ganges. 



The first river to feel the effect of the tilting backward of the 

 plane of the Ganges, by the elevation of the land at Rajmahal, was 

 the Coosy, as the nearest to the delta. The consequence is, that 



