Geol.— Vol. I.] TURNER— ORIGIN OF YOSEMITE VALLEY. 28 1 



that the granite of the Sierra was divided into polyhedrons 

 by sets of partings; two vertical horizontal and others diag- 

 onal to these. Although he does not clearly state it, he evi- 

 dently regards the vertical walls as due to erosion along 

 vertical partings, the benches as due to erosion along hori- 

 zontal partings, and the inclined surf aces in some cases due 

 to erosion along diagonal partings. Muir writes: — 



The Yosemite Half Dome, the noblest rock of the Sierra, perhaps, also fur- 

 nishes admirable illustrations of all kinds of cleavage and of their tftects upon 

 form. Its upper part, for about one thousand eight hundred feet, is almost abso- 

 lutely vertical, while the lower three thousand feet is inclined at an angle with 

 the horizon of about 37°. On the notch a section of the dome may be seen, 

 showing that it is there made up of immense slabs.set on edge, evidently pro- 

 duced by cleavage planes which, cutting the dome perpendicularly, have 

 determined the plane of its face. Along the base of the vertical portion the 

 stumps of slabs are exhibited, which have been successively split off the face 

 while the rock 'was in process of formation by the Lyell, Tenaya, and Hoff- 

 mann glaciers. 



Moreover Muir noted a mode of erosion by ice which 

 has since been brought into greater prominence by Mr. Wil- 

 lard Johnson. Rewrites: — 



Again, we find this mighty ice-tool undermining from beneath what it 

 could not grind away from above, eroding backwards into the basis of peaks 

 and pinnacles. 



This presumably refers to the formation of glacial cirques, 

 which will be referred to further on. 



Mr. Gilbert^ thought that " the great glaciers of the 

 Sierra Nevada occupied an antecedent system of valleys, 

 shown by their form to be the product of stream erosion. 

 The period of ice was, therefore, preceded by a period 

 when there was no ice or little ice, and this antecedent 

 period" was of relatively great duration." 



In an important paper,^ Dr. G. F. Becker describes in 

 detail the sets of joints or fissures to which Muir refers. 

 As my own results are a confirmation of those of Becker, 

 as to the distribution of these joints and their relation to the 

 formation of canyons, I will quote from him at some length. 

 I have not myself seen, however, much evidence of the 



1 Lake Bonneville, Monograph I, U. S. Geol. Surv. 1890, p. 269. 



2 This would be the Sierran Period. H. W. T. 



3 Bull. Geol. Soc. Amer., Vol. II, 1891, pp. 67-69. 



