244 OUR NATIONAL PAEKS 



main trunks of the rivers are well developed, and 

 their banks planted with fine forests, while their 

 upper branches, lying- high on the snowy moun- 

 tains, are still buried beneath shrinking residual 

 glaciers ; illustrating every stage of development, 

 from icy darkness to light, and from muddiness 

 to crystal clearness. 



Now that the hard grinding sculpture work of 

 the glacial period is done, the whole bright band 

 of Sierra rivers run clear all the year, except when 

 the snow is melting fast in the warm spring 

 weather, and during extraordinary winter floods 

 and the heavy thunderstorms of summer called 

 cloud-bursts. Even then they are not muddy 

 above the foothill mining region, 'unless the mo- 

 raines have been loosened and the vegetation de- 

 stroyed by sheep ; for the rocks of the upper 

 basins are clean, and the most able streams find 

 but little to carry save the spoils of the forests, 

 — trees, branches, flakes of bark, cones, leaves, 

 pollen dust, etc., — with scales of mica, sand 

 grains, and boulders, which are rolled along the 

 bottom of the steep parts of the main channels. 

 Short sections of a few of the highest tributaries 

 heading in glaciers are of course turbid with 

 finely ground rock mud, but this is dropped in 

 the first lakes they enter. 



On the northern part of the range, mantled 

 with porous fissured volcanic rocks, the fountain 

 waters sink and flow below the surface for con- 



