Studies in the Sierra 



419 



deposits diminishes, just as the necessity for greater support 

 increases, until a shp is brought on. 



Slips of this species are often of great extent, the surface 

 comprising several acres overgrown with trees, perhaps mov- 

 ing slowly and coming to rest with all their load of vegetation 

 uninjured, leaving only a yawning rent to mark their oc- 

 currence. Others break up into a muddy disorderly flood, 

 moving rapidly until the bottom of the wall is reached. Land- 

 slides occur more frequently on the north than on the south 

 sides of ridges, because of the greater abundance of weight- 

 producing and decomposing moisture. One of the commonest 

 effects of land-slips is the damming of streams, giving rise to 

 large accumulations of water, which speedily burst the dams 

 and deluge the valleys beneath, sweeping the finer detritus be- 

 fore them to great distances, and at first carry boulders tons in 

 weight. 



The quantity of denudation accompHshed by the Sierra land- 

 slips of both species is very small. Like rock-falls, they erode 

 the surface they slip upon in a mechanical way, and also bring 

 down material to lower levels, where it may be more advan- 

 tageously exposed to the denuding action of other agents, and 

 open scars whereby rain-torrents are enabled to erode gullies ; 

 but the sum of the areas thus affected bears an exceedingly 

 small proportion to the whole surface of the range. 



The part which snow avalanches play in the degradation of 

 mountains is simpler than that of free-falling or cascading 

 rocks, or either species of land-slip; these snow avalanches 

 being external and distinct agents. Their range, however, is 

 as restricted as that of either of the others, and like them they 

 only carry their detritus a short distance and leave it in heaps 

 at the foot of cliffs and steep inclines. There are three well- 

 marked and distinct species of snow avalanche in the upper 

 half of the Sierra, differing widely in structure, geographical 

 distribution, and in the extent and importance of the geological 

 changes they effect. The simplest and commonest species is 

 formed of fresh mealy snow, and occurs during and a short 

 time after every heavy snow-fall wherever the mountain slopes 

 are inclined at suitable angles. This species is of frequent oc- 

 currence throughout all the steep-flanked mountains of the 



