Mode of the Filling up of Lakes. 39 



alluvium will contract the dimensions of the lake continually, 

 and will then be traversed by those streams, which will go on 

 depositing their contents until the lake is obliterated, and one or 

 more gentle slopes or flat plains remain above its level. These 

 slopes and plains are what we find in nature. A tract of allu- 

 vium, which is regarded by many as the bottom of an ancient 

 lake, but which more nearly corresponds with its surface, is of- 

 ten seen either inclosed on every side by mountains, or having 

 on two sides the same elevated ridges, which stretch forward and 

 embrace beyond it a lake, or an arm of the sea. That this tract 

 is not a perfect level, is manifest from the circumstance, that, if 

 we compare its height at many successive points with the height 

 of the river flowing through it, we find that they are still at the 

 same distance the one beneath the other ; and, in addition to 

 this clear proof of the gradual declivity of the plain, we observe 

 its successive portions to be divided by those transverse ledges 

 which are among the most characteristic features of fluvial action. 



IV. The case of a Stream which meets a Stream flowing in another 

 direction. 



At the mouths of rivers, where they enter the ocean, the cur- 

 rent of the rising tide comes directly against the current of the 

 river. Hence they destroy one another"'s motion over a trans- 

 verse line, the exact position of which varies continually accord- 

 ing to the respective force of the two opposite currents. The 

 result is, that the water, being brought to a state of compara- 

 tive rest, deposites its solid contents, and thus forms a bar across 

 the mouth of the river. Where rivers enter lakes, the same ef- 

 fect is sometimes produced by the prevaihng winds, which here 

 take the place of tides. The formation of banks by opposing 

 currents on the shores of the sea, such as the ChesiUbank, and 

 that by which St Michaers Mount is joined to Merazion, are in- 

 stances of the same action. 



Every case in which one stream falls into another on the same 

 level is so far similar, that the motion of one, at the moment of 

 junction, destroys in a greater or less degree the motion of the 

 other ; and the water being so far reduced to a quiescent state, 

 a deposition takes place at the angle between the two streams, 

 varying in its form and extent according to their respective force 



