YOUNGER ERUPTIVES. 



RHYOLITE. 



Among the acid orthoclastic rocks of tbe district arc a few occurrences plainly 

 distinct from any that have been referred to the group of the quartz-porphyries. 

 Their mode of occurreuce is diflercut (see p. 194) and they possess to an eminent 

 degree the habit formerly considered characteristic of the younger eruptives. 'No exact 

 data as to age are available, but they all seem to be more recent than the period of 

 folding and faulting. 



The most important body of rhyolite is that upon the northern boundary of the 

 region iinder consideration, forming the mass of Chalk Mountain. As this rock has 

 very closely the habit of that subdivision of the rhyolites recently defined as Neva- 

 dite by Messrs. Hague and Iddiugs,' that name will be applied in this description. Ac- 

 cording to the definition of the writers cited, I^Tevadite is a rhyolite "characterized by 

 an abundance of porphyritic crystals imbedded in a relatively small amount of ground- 

 mass," while liparite is a rhyolite "characterized by a small number of porphyritic 

 crystals imbedded in a relatively large amount of groundraass." These terms simply 

 designate structural extremes in a group which is so large as to need some such treat- 

 ment. They occupy about the same position as the terras " granite porphyry " and " fel- 

 site poi'phyry." 



^ CHALK MOUNTAIN NEVADITE. 



General description — This rock is characterized by the appearauce of very numer- 

 ous dark quartz crystals and clear sanidines, with but very little biotite or ore, imbed- 

 ded in a light gray groundmass. On the western and eastern parts of the mountain 

 the feldspars are nearly all small and clear, and, as in this modification there is an 

 almost total absence of biotite and ore jiarticles, the feldspars are scarcely distin- 

 guishable at first glance from the enveloping groundmass, which has under the lens an 

 exceedingly finegrained, homogeneous texture. All this only serves to bring out the 

 more strikingly the abundant dark, smoky quartz crystals, which usually present the 

 prism in very distinct development. They are here invariably fissured iu all direc- 

 tions, and fractured surfaces have an unusually brilliant, vitreous luster. In this 

 modification of the rock the quartz crystals seldom reach 1"=™ iu diameter, and the 

 feldspars, though occasionally more than 2<=™ in length, are usually less than 1<^" in 

 greatest diameter. 



'American Journal of Science, III, XXVII, j). 461, 18S4. 



