HORNBLENDE 323 



crystals mentioned above. Here again the hypothesis offered is that of 

 gravitational assemblage. At ordinary temperatures hornblende is 20 per 

 cent denser than quartz and feldspar, the dominant minerals of the rock. 

 If the same ratio obtains at the temperatures at which the magma con- 

 gealed, the phenocrysts of hornblende might sink through the viscous 

 magma without requiring such advantage of size as the feldspars possessed. 

 The locality represented in the illustration, plate 43, is on the east base 

 of mount Silliman, in latitude 36° 39', longitude 118° 41'. Another 

 locality is on the north slope of the dome called Liberty Cap, at the head 

 of Yosemite valley. 



Banding 



A third phenomenon with which I am disposed, though less confidently, 

 to associate gravity is a banding of granite. About one mile south of 

 Cooper meadow, just to the left of the trail leading to Upper Relief val- 

 ley (in latitude 38° 13', longitude 119° 49'.3), there is a body of granite, 

 some scores of feet in thickness and some hundreds of yards in length, 

 which is conspicuously banded throughout. The rock is of rather fine 

 texture and composed of quartz, feldspar, mica, and hornblende. The 

 bands are alternately dark and pale, the color of dark bands being given 

 by the dominance of hornblende and mica, that of the pale bands by the 

 dominance of feldspar and quartz. They range in width from one inch 

 to nearly or quite one foot. Some of the dark bands are darker than 

 others; some of the pale bands are paler than others. The transition 

 from a pale band to a dark may take place in a quarter of an inch or be 

 diffused through an inch. The more abrupt transitions are from a pale 

 band below to a dark band above. Within both dark and pale bands the 

 attitudes of minerals seem to be wholly irregular. There is no parallel- 

 ism of orientation and nothing about the rock suggests schistosity. 



Several instances of unconformity were observed, as though the various 

 bands had been successively deposited and the history of deposition had 

 been interrupted by temporary erosion. Such a plane of unconformity 

 appears in plate 44, opposite the man's wrist. At another locality, the 

 southeast base of Goat mountain, in the Kings Eiver basin (latitude 36° 

 51'. 3, longitude 118° 34'.2), unconformity is associated with a discord- 

 ance of dip of more than 20 degrees. I did not there see the rock in situ, 

 but the banding is fully and characteristically displayed in a large boul- 

 der. Figure 1, based on a diagrammatic field sketch, represents a portion 

 of the boulder. 



I think that these bauds are not only apparently but actually the result 

 of deposition, and that the unconformities have been caused by erosion 



