Studies in the Sierra 



423 



sheet present no unchanged glaciated surfaces from which, 

 measuring down, we may estimate the amount of post-glacial 

 denudation. The streams of these old eroded countries are 

 said by the poets to "go on forever," and the conceptions of 

 some geologists concerning them are scarcely less vague. 



Beginning at the foot of the Sierra glaciers, and following 

 the torrents that rush out from beneath them down the valleys, 

 we find that the rocks over which they flow are weathered 

 gradually, and increasingly, the farther we descend; showing 

 that the streams in coming into existence grew like trees from 

 the foot of the range upward, gradually ramifying higher and 

 wider as the ice-sheet was withdrawn — some of the topmost 

 branchlets being still in process of formation. 



Rivers are usually regarded as irregular branching strips of 

 running water, shaped somewhat like a tree stripped of its 

 leaves. As far as more striking features and effects are con- 

 cerned, the comparison is a good one ; for in tracing rivers to 

 their fountains we observe that as their branches divide and 

 redivide, they speedily become silent and inconspicuous, and 

 apparently channelless; yet it is a mistake to suppose that 

 streams really terminate where they become too small to sing 

 out audibly, or erode distinct channels. When we stoop down 

 and closely examine any portion of a mountain surface during 

 the progress of a rain-storm, we perceive minute water-twigs 

 that continue to bifurcate until like netted veins of leaves the 

 innumerable currentlets disappear in a broad universal sheet. 



It would appear, therefore, that rivers more nearly resemble 

 certain gigantic algae with naked stalks, and branches webbed 

 into a flat thallus. The long unbranched stalks run through the 

 dry foothills ; the webbed branches frequently overspread the 

 whole surface of the snowy and rainy alpine and middle re- 

 gions, as well as every moraine, bog, and neve bank. The 

 gently gliding rddn-thallus fills up small pits as lakelets and 

 carries away minute specks of dust and mica. Larger sand- 

 grains are overflowed without being moved unless the surface 

 be steeply inclined, while the rough grains of quartz, horn- 

 blende, and feldspar, into which granite crumbles, form ob- 

 stacles around which it passes in curves. Where the current- 

 lets concentrate into small rills, these larger chips and crystals 



