74 RIVERS. 



1,235 English feet, which would give less than five inches per mile for 

 the mean declivity, but from the point to which the tides of the ocean 

 reach the declivity, is considered not above 0.2 inch per mile. The 

 Ganges from Hurwar, where it issues from the depths of the Himalaya, 

 has a mean declivity of four inches per mile; that of the Volga is about 

 five inches. The general form of the channels of rivers, however, 

 especiallv in a champaign country, indicates that, if not wholly formed, 

 they are greatly modified by the actions of their waters. In fact, their 

 beds are usually proportional to the force of the stream; on the banks 

 of many large rivers we can still trace the different heights, at which the 

 waters have flowed formerly. Play fair mentions the existence of four or 

 five successive terraces on the Rhine, each of which has evidently been 

 formed in succession by the river; and, hence, he concludes that the 

 Rhine once flowed 360 feet above its present level. Similar remarks 

 were made on the upper Rhone, by Saussau. The general effect of the 

 action of the water must be the erosion of the channels in the lapse of 

 ages; and even in the rocky beds of rivers, this action of water is per- 

 ceptible. Thus the waters of the Niagara have apparently worn away 

 the limestone rock of the falls, and found a deep ravine through the stony 

 bed, six miles in length from Queenstown, where the cliffs terminate 

 abruptly to the present site of the falls. The undermining of the cliffs 

 by this stupendous cataract, has caused a recession of the Falls, equal to 

 about 18 feet in thirty years ; but we cannot hence infer the length of time 

 that this cataract has existed, because we have no certainty of the 

 equability of the disintegration, nor of the other causes which may have 

 aided or facilitated the process. The currents of large rivers may 

 frequently be traced by their color to great distances in the ocean. Thus 

 the Ganges, when in flood, discolors the sea in the Gulf of Bengal to the 

 distance of sixty miles, and its mud covers the bottom at eighty miles 

 from the land. The mud of the Orinoco is carried far into the Atlantic, 

 and on examining water drawn from the sea, in latitude 8 degrees 20 

 minutes north, that is in the parallel of the Orinoco, and 200 miles 

 from its embouchure, Traill ascertained that the specific gravity of the 

 water suddenly fell below what it is was to the north and to the south of 

 this parallel. The enormous mass of water discharged by the Maranon, 

 according to Sabine, discolors the ocean, and has even considerable 

 rapidity in a direction inclined to that of the equinoxial current, at the 

 distance of 300 miles from the American coast. Fresh water may be 

 obtained from the surface of this current, at a great distance from the shore. 

 The periodical inundations of rivers depend on great falls of rain, in 



