94 GEOLOGY. 



amount than it otherwise would ; and this detention at 

 the sides of the channel will cause more water to flow 

 through it, and to flow more rapidly. Then, as the flats 

 fill up with sediment, this effect upon the flow in the 

 channel will continually increase. 



176. Deposits in Lakes. In every lake there is more 

 or less deposit of sediment from the river or rivers that 

 flow into it, and also from all smaller streams that do so, 

 even to the little rill temporarily made by a shower. 

 The tendency, then, is to fill up the lake, and in time the 

 result would be the conversion of the lake into a river. 

 If a lake have a river entering at one end and issuing 

 at the other, the deposit is made in that part of it where 

 the river enters, and the extent to which it reaches de- 

 pends on circumstances. The more rapid the entering 

 river, and the more shallow the lake, the greater will be 

 the space over which the deposit will be made. The wa- 

 ter of the entering river is of course turbid, while that 

 of the issuing river is clear, the sediment being all depos- 

 ited long before the water reaches that end of the lake. 

 In the Lake of Geneva, which is thirty-seven miles long, 

 and varies in breadth from two to eight miles, we have 

 all this exemplified. The Rhone enters it muddy, but it 

 issues beautifully clear at the city of Geneva. The ra- 

 pidity with which the filling up goes on may be judged 

 of by the fact that the town of Port Vellais, which, eight 

 centuries ago, was on the edge of the lake, is now more 

 than a mile and a half distant from it. The great lakes 

 of this country are continually growing smaller. There 

 are facts which show that their shores, in many places, 

 were once far away outside of where they now are. 

 There is, for example, south of Lake Erie, and distant 

 from it from four to eight miles at different points, a 

 ridge made up of sand, gravel, and rounded pebbles, just 

 as the shore of the lake is now. Moreover, when wells 

 are dug, or any excavations made in this ridge, there are 

 found deeply buried in the soil pieces of decayed wood, 



