58 REPORT UNITED STATES GEOLOGICAL SURVEY. 



aifects the surfaces of the hornblende crystals more than the interior, 

 and where a crystal so altered is exposed by fracture it might readily 

 be taken for a pseudomorph of hematite or limonite after hornblende. 

 A section through the crystal, however, generally discloses a core of 

 unaltered or only partially changed hornblende. In the cases of smaller 

 crystals or hornblendic dust and microlites, the alteration is often com- 

 plete and the color of the groundmass is changed from the normal dark 

 gray or green of hornblende andesite to a light yet strong red. Id the 

 fine-grained varieties this has produced a strongly deceptive appear- 

 ance, and any one familiar with volcanic rocks would have been led after 

 macroscopic inspection to pronounce such rocks either acid trachytes or 

 even rhyolites. The thin sections, however, show abundant plagioclase 

 and very little sanidin; they also reveal abundant traces of the disso- 

 lution of the hornblende. The cause of this extensive alteration may 

 possibly be found in the action of hot-spring waters so abundant 

 throughout this district. It appears to be decidedly different in its 

 effects from the ortliuary action of atmospheric waters. 

 I give herewith a diagnosis of the various specimens : 



"Eo. 1. liHYOLITE. 



This specimen contains a very few small crystals of sanidin and still 

 fewer and smaller quartzes, disseminated in a hyaline or perlitic base, 

 showing indistinct iluidal texture. The field is occasionally traversed 

 by lines or seams which appear to be almost pure quartzite ; also spots 

 or miculi of the same. There is considerable ferritic matter disseminated 

 throughout the base, imparting a pale reddish color. A few dark brown 

 spots occur, which may be decomposing hornblende. 



Locality. — Eastern spur of Sepulchre Mountain, about one mile above 

 Mammoth Hot Springs. 



No. 2. Ehyolite. 



Contains very few fragmental crystals of sanidin and quartz, the quan- 

 tity of crystaline matter being quite small. The base has very strongly 

 marked fluidal characteristics, and exhibits figures which are axiolitic 

 (very similar to that given by Zirkel in Fig. 1, Plate YI, Survey Fortieth 

 Parallel, Vol. VI). 



Locality. — Summit of Mount Evarts, opposite Mammoth Hot Springs. 



"No. 3. Ehyolite. 



An extremely fluidal base, showing a web of coarse, glassy fibers, form- 

 ing tangled meshes without any definite direction. Base wholly glassy. 

 Crystals very few and very minute, chiefly sanidin, with curious inclu- 

 sions of long, slender needles of feldspar. 



Locality. — Base of sheet capping Mount Evarts. 



No. 4. Ehyolite. 



Shows a mottled or damosene base, which is axiolitic. Crystals few 

 and fragmentary. Several plagioclase crystals noted. 



Locality. — Low hill to the left of Elk Creek crossing, Junction Valley. 



No. 5. Ehyolite. 



Holding fragmental and distorted crystals of quartz and sanidin; 

 scanty and small. The base shows the fluidal structure to an extreme 

 degree, in very irregular forms, however, as fibrous or drawn out in the 

 direction of flow in wavy lines, consisting of a thoroughly transparent 

 glass surrounded by a border of nearly opaque glass, probably charged 

 with ferrite. Whenever this peculiar groundmass borders upon a crys- 



