﻿J. LeConte — Elevation of the Sierra IVevada. 169 



level of the lower parts of the stream, which therefore also re- 

 news its cutting. Thus, as already stated, in a steady crust, 

 even the lower part of a stream never completely reaches its 

 base level, for this is ever being lowered by the lowering of the 

 beds of the upper parts. The only complete limit to erosion — 

 the only final and absolute base-level is, of course, the sea level. 

 But although a river never completely reaches its base-level 

 until it reaches the sea level ; yet in a steady crust, it so nearly 

 reaches it in its lower parts that for our purposes we may re- 

 gard this level as being often practically reached. 



Suppose then a country to be raised and then to remain 

 steady. In such case as already explained the streams will cut 

 until in their lower parts they reach or nearly reach their base- 

 level. Now they cut no longer or only at extremely slow rate. 

 As soon as the down-cutting ceases, the stream begins to sweep 

 from side to side under-cutting its banks and widening its 

 channel. But meanwhile the valley slopes on either side and 

 the separating divides are being worn down lower and lower 

 by rain-erosion or weathering. Thus in a country which has 

 remained steady for a long time the topography consists of wide 

 trough-like river-beds separated by low rounded divides. On 

 the contrary, in a country rising or recently risen we have deep, 

 narrow canons and high, sharp divides, or else high table-lands, 

 between. Therefore deep, narrow canons are evidence of recent 

 and rapid rising ; the deeper the canon the more rapid has been 

 the rising. But since the widening is meanwhile also progress- 

 ing, the narrower the canons, the more recent has been the 

 rising. 



We have seen that the more rapid the rising, the more rapid 

 the cutting ; also that after a while if the crust remains steady, 

 the cutting ceases and the divides are lowered. It follows 

 therefore that the depth of river canons cannot be taken as a 

 measure, even a rough measure, of time ; (1) because the rate 

 of cutting is so greatly dependent on the difference between 

 the actual river-bed and the base-level, and (2), because when the 

 deepening ceases the lowering of the divides still continues and 

 the canons are thenceforward lessening in depth. Therefore 

 so far from deep canons being evidence of geologically long 

 work, the very reverse is true, viz : wide, shallow river-beds are 

 characteristic of old topography. 



Again: we have seen that if a country rises and then re- 

 mains steady the rivers will cut to base-level and stop cutting — 

 the channels will stop deepening and will only widen. If now 

 another rise commences the rivers will cut again, making an 

 inner and narrower gorge. Thus successive risings may be re- 

 corded in successive benches more or less marked. 



