468 W. Cross — Alunite and Diaspore 



rock masses. As a further proof that typical volcanic action 

 has occurred here stand the masses of decomposed rhyolite 

 which are to be described, for they can only be explained on 

 the supposition that the rhyolitic outburst, known to have been 

 of violently explosive character, was followed by a period of 

 sulphurous gaseous exhalations whose products are identical 

 with those of well known volcanic regions. There were two 

 vents in particular which thus became true solfataras, and 

 about them the rhyolite has suffered change into a rock-mass 

 affording unusual resistance to atmospheric agencies and now 

 forming rugged cliffs and projecting outcrops in a region of 

 prevailingly gentle slopes and rounded contours. These two 

 localities are in Democrat Hill and Mount Robinson, both situ- 

 ated in the inner part of the group of the Rosita Hills. 



II. The alunite rock of Democrat Hill. 



Democrat Hill is situated in the center of the Rosita Hills 

 and in the acute angle between two gulches. It is of general 

 rounded shape at the base, with a diameter of 1,500 feet and 

 rises only 400 feet. On the north, or opposite the forks of the 

 gulch, it joins on to a long ridge of andesite, and indeed all to 

 the north is andesite, excepting later dike rocks, while to the 

 south the prevalent rock is rhyolite, chiefly in the form of 

 flows, some of which issued from the conduit below the hill. 



The upper three hundred feet of Democrat Hill stands out as 

 a rough massive knob whose projections are somewhat rounded, 

 though split by Assures and presenting occasional cliff faces. The 

 color is slightly reddish and the whole presents a strong resem- 

 blance to certain outcrops of massive granite. The lower 

 hundred feet of the hill are covered by great angular blocks 

 which have fallen from the cliffs above. A close examination 

 of the rocks shows it to be cellular, the cavities being of irreg- 

 ular shape and varying in size, with a maximum diameter of 

 several inches, and an average of about one inch. The cells 

 are lined by rudely tabular crystals some of which are com- 

 posite and all are obscured by the minute quartz crystals 

 deposited upon them. The mass of the rock is an irregular 

 aggregate of imperfectly tabular grains of a mineral closely 

 resembling orthoclase in luster, hardness, and general appear- 

 ance. A pronounced cleavage runs parallel to the dominant 

 planes of the tablets. The only other constituent of the rock 

 is quartz, which forms a very evenly and finely granular mass 

 between the tablets, and its grains are also abundantly included 

 in the latter. A small amount of snow-white kaolinite is 

 sometimes seen in the cavities. 



Microscopical examination of thin sections of the rock, and 

 of cleavage flakes, shows the questionable mineral to be uniaxial, 



