576 DESCRIPTIVE GEOLOGY. 



of large cavities filled with agates and chalcedonies. They range from the 

 size of a pea to a foot in diameter ; and the surface of the flat slopes toward 

 Pine Valley is literally covered with lenticular masses of chalcedony, which 

 have weathered out from the easily disintegrated basalt. It is only rarely 

 that these chalcedonies entirely fill the cavities in the basalt. They usually 

 form a linins: from half an inch to three or four inches in thickness, with a 

 botryoidal surface toward the interior. They are both of uniform chalce- 

 dony, and formed by repeated deposition of agate layers. On the lower 

 sides of the cavity^ there is found a considerable deposit of delessite, which 

 sometimes plays a prominent part in the agate layers. 



The erosion subsequent to the outflow of these basaltic tables has been 

 comparatively limited : the ravines cut in them are of sloping sides, and not 

 usually over a couple of hundred feet deep. Everj^thing points to a very 

 slight erosion since the basaltic period, whereas it is equally evident that 

 the whole range had been subjected to severe changes or modifications of 

 form before the volcanic period. 



An obscure outcrop in the diorites of Agate Canon, which, by the 

 presence of quartz, biotite, hornblende, and feldspar, seems to be related to 

 the diorites, proves, on examination, to be a granite, containing titanite, 

 resembling, in this particular, the Jurassic granites of the Sierra Nevada. 

 The hornblendes are peculiarly cleavable, and the biotite is present in pro- 

 portions smaller than in the neighboring diorite. The feldspars are highly 

 altered, so that the proportion between orthoclase and plagioclase is not to 

 be recognized. Professor Zirkel says:^ "The product of this decomposition 

 is curious, consisting of broader or narrower colorless, prismatic rays, which 

 are either massed confusedly together like felt, or are heaped together in 

 the form of stars and bunches, presenting a beautiful aggregate polarization." 

 The occurrence of this granite is very obscure. It is entirely surrounded 

 with diorites, and may be either older or a Jurassic granite dike subsequent 

 to the diorites. It is not to be connected with the heavy mass of granite 

 which forms the north slope of the range' about 4 miles northeast of Agate 

 Pass. 



This is a broad mass of granite, extending 5 miles in a northeast and 



1 Microscopical Petrograpby, vol. vi, 48. 



