Studies in the Sierra 



307 



reflect the sunbeams like glass, and though they have been sub- 

 jected to the corroding influences of the storms of countless 

 thousands of years, to frosts, rains, dews, yet are they in many 

 places unblurred, undimmed, as if finished but yesterday. The 

 attention of the mountaineer is seldom arrested by moraines 

 however conspicuously regular and artificial in form, or by. 

 canons however deep, or rocks however noble, but he stoops 

 and rubs his hand admiringly on these shining surfaces, and 

 tries hard to account for their mysterious smoothness. He has 

 beheld the summit snows descending in booming avalanches, 

 but he concludes that these cannot be the work of snow, be- 

 cause he finds it far beyond the reach of avalanches; neither 

 can water be the agent, he says, for he finds it on the tops of 

 the loftiest domes. Only the winds seem capable of following 

 and flowing in the paths indicated by the scratches and grooves, 

 and some observers have actually ascribed the phenomenon to 

 this cause. Even horses and dogs gaze wonderingly at the 

 strange brightness of the ground, and smell it, and place their 

 feet upon it cautiously ; only the wild mountain sheep seems to 

 move wholly at ease upon these glistening pavements. 



This polish is produced by glaciers slipping with enormous 

 pressure over hard, close-grained slates or granite. The fine 

 striations, so small as to be scarcely visible, are evidently 

 caused by grains of sand imbedded in the bottom of the ice; 

 the scratches and smaller grooves, by stones with sharp grav- 

 ing edges. Scratches are therefore most abundant and rough- 

 est in the region of metamorphic slates, which break up by the 

 force of the overflowing currents into blocks with hard cutting 

 angles, and gradually disappear where these graving tools have 

 been pushed so far as to have had their edges worn off. 



The most extensive areas of polished surfaces are found in 

 the upper half of the middle region, where the granite is most 

 solid in structure and contains the greatest quantity of silex. 

 They are always brighter, and extend farther down from the 

 axis of the range, on the north sides of canons that trend in a 

 westerly direction than on the south sides ; because, when wet- 

 ted by corroding rains and snows, they are sooner dried, the 

 north sides receiving sunshine, while the south walls are most- 

 ly in shadow and remain longer wet, and of course their gla- 



