rising up to the higher ones by means of numerous large 

 "steps" with or without rock basins on the "treads." 



Furthermore the forms just described are those which 

 are characteristic of stream action, 1 and in the valleys 

 under consideration the only mountain streams known to 

 exist which were large enough to have occupied such 

 huge channels in their entirety were glaciers. 



Before proceeding farther with the description of the 

 Yosemite and associated valleys it will be advisable to 

 discuss the principal methods by which streams corrade 

 their channel bases and sides. Having done so it will lie 

 easy to make an application of such principles to the case 

 of the valleys under consideration. 



Some Principles of Glacial Mechanics.— A mass of any 

 material, whether fluid or solid, is composed of various 

 textural units and these same units may vary considerably 

 according to the physical conditions obtaining at the time. 

 Consider, tor example, the granite bathylith out of which 

 the Yosemite and bpper Merced \allcys have been exca- 

 vated. The textural units of such a mass may consist of : 



(1) The huge blocks of material bounded by the strong fault 



and joint planes occurring at frequent intervals. 



(2) The parallelepipeds of rock, ranging in weight from a 



few hundredweights to 100 tons, determined by the 

 frequent master joints of the region. 



(3) The individual grains and crystals of the rock. 



(4) The molecules of the granite itself. 



It is evident that under certain conditions several sets 

 of textural units may exist simultaneously. For instance, 

 the great structural planes, the master joints, the- individual 

 grains and crystals, and the very molecules themselves 

 may all form surfaces between which relative motion or 



"Corrasion by Gravity Streams," p. 209 and reference. 



