178 A YEAR IN SCIENCE 



flowing; under this is a softer stratum. At Niagara 

 Falls there is a solid compact layer of limestone at the 

 top with softer shale underneath. The water falling 

 over the falls wears away the soft shale. As this con- 

 tinues the limestone is left projecting ; finally, this 

 limestone breaks off and falls into the stream below. 

 Thus the falls slowly recede up stream. At Niagara 

 the rate of recession .is about five feet per year. Falls 

 may disappear from stream courses: first, by the 

 recession of the falls to the source of the stream; sec- 

 ond, by bed erosion, entirely removing the hard strata. 

 Then the falls may become rapids and finally disap- 

 pear. Long periods of time are required to accomplish 

 either of these results. 



Seventh, lakes in the course of a stream indicate a 

 period of youth. Streams are the enemies of lakes, 

 and in two ways are Working for their destruction. 

 First, the outlet is being lowered by erosion. The 

 level of the lake falls as erosion of the outlet pro- 

 ceeds. Second, the stream flowing into the lake brings 

 sediment w r hich is deposited in the lake, and thus fills 

 it. Both of these forces at work finally cause the dis- 

 appearance of the lake. 



The Yellowstone, Upper Colorado, Snake, Green, and 

 Grand Rivers are the best examples of young streams 

 in this country. 



Maturity. A mature stream has the following char- 

 acteristics: first, the slope is rather gentle, as erosion 

 has accomplished this; second, the current is not very 



