Little Studies in the Yosemite Valley. 7 



each fragment, they also embody a time record of the 

 journey. Some of them represent a lapse of many 

 years, the more impressive when it is reflected that 

 during all that time no being, human or other, hap- 

 pened by in this solitude to interfere with the orderly 

 continuance of the process. 



The above explantion, however, accounts only for 

 the movement of the debris. It does not yet make 

 clear the production of the stripes themselves. That 

 a heavy boulder might grind off the lichens from the 

 bed it passes over seems quite natural, but that a bit 

 of rock weighing an ounce or two should clear a path 

 does not seem at all self-evident. The weight of the 

 fragment, if it is a factor in the process, apparently 

 plays but a minor role. 



On picking up one of these traveling fragments, one 

 finds it invariably imbedded in a small pad of loose 

 rock grains that have collected under it. Now lichens 

 cannot thrive under the thinnest veneer of sand or soil, 

 as may be observed in a thousand places throug'hout 

 the Sierra. Slanting rocks uncovered by the grading 

 of a wagon road, for instance, show plainly by the 

 boundary of their lichen growth where the surface of 

 the ground used to be. Shallow basins in a rock floor 

 or on large boulders that tend to accumulate sand, 

 pine needles, and other litter, similarly remain white 

 and bare of lichens. It is safe to say, therefore, that 

 it is the sand pad under the fragments rather than the 

 fragments themselves that clears the lichens from the 

 stripes. And here, again, is substantiation of the view 

 expressed regarding the slowness of the process. For, 

 were the movement at all rapid, the lichens in any one 

 spot would not remain covered for a sufficient length 

 of time to utterly die and loose their hold. As a mat- 

 ter of fact there are places where they were not en- 

 tirely stamped out and the stripes appear dim or in- 

 terrupted. The debris must have advanced here with 

 more than usual rapidity, owing to some local acceler- 



