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University of California. 



LVOL. 3 



covered by a bed of white sandy shale in a single stratum from 

 four to ten feet in thickness. This series of rocks dips at an 

 angle of 15° to the west. The crest of the ridge above the quarry 

 is bare of overlying sedimentary rocks for a hundred yards back 

 from the canon. 



The age of this second series of sandstones and shales is also 

 uncertain, due to lack of sufficient fossils. What shells were 

 found, however, taken in connection with the stratigraphic 

 relations, would make the age probably late Pliocene. 



The plutonic rock itself is broken and shattered in all direc- 

 tions, showing the great stresses and movements to which it has 

 been subjected. 



PETROGRAPHICAL TYPES. 



The plutonic rock, as a mass, is of constant character, but 

 contains both acid and basic modifications in the form of dykes 

 and inclusions. The basic phase, consisting of a darker, finer- 

 grained variety than the main mass, is the oldest portion of the 

 whole, and contains intrusions of all the other phases. The acid 

 variety occurs only in dykes, from the fraction of an inch up to a 

 foot in width, and cuts both the other phases of the rock. How- 

 ever, the basic phase appears to occur chiefly in the west of the 

 exposure, while the acid dykes are more numerous in the east. 

 The dykes are both pegmatitic and aplitic. These four types of 

 the rock will each be taken up in turn and discussed. 



THE MAIN MASS . 



Macroscopical Characters. — In a hand specimen this rock is 

 seen to consist of black hornblende and a glassy feldspar, of 

 about equal proportions. The hornblende shows a well developed 

 prismatic cleavage, and a slight tendency toward idiomorphic 

 forms. Some of the crystals are 12 mm. in length, but average 

 about 5 mm. The feldspar is in more or less irregular grains, 

 averaging 4 mm. in size. A few of these show cleavage faces. 

 Besides these essential minerals there occur some small flakes, 

 1.5 mm. in size, of lustrous brown biotite, scattered very sparingly 

 throughout the rock. Wherever the biotite occurs, the surround- 

 ing minerals are stained greenish, from the slight alteration of 

 biotite to chlorite. Original pyrite also occurs in small grains. 



