434 W. Cross — Topaz and Garnet in Rhyolite. 



The rock of the other ridges mentioned is distinctly porphy- 

 ritic in structure, showing small glassy sanidines and dark 

 smoky quartz crj^stals imbedded in a predominant dull gray 

 groundrnass. Small lithophyses occur in this groundmass, 

 sometimes exhibiting a very delicate concentric arrangement 

 of white films, which, like those of the previously described 

 rock, are covered by small quartz crystals. 



Minerals of the cavities.— The outer walls of the lithophyses, 

 as well as the concentric shells, when present, are primarily 

 formed of a pure white mineral which is but seldom developed 

 in recognizable form. Sometimes a frost-like structure caused 

 by the interpenetration of delicate blades is seen, and quite fre- 

 quently round or botryoidal masses of the white mineral occur, 

 the surface usually showing minute crystal facets. A micro- 

 scopical examination of these surfaces shows the mineral to be 

 sanidine in stout crystals of common form. Earely, clear crystals 

 1 or 2 mm in length are developed, and then a beautiful blue 

 color parallel to ^--l may be detected.* In a single cavity in 

 the porphyritic rock the feldspar assumes delicate stalactitic 

 forms, somewhat branching at the ends and clear only at the 

 very tips. The round feldspathic masses are often fissured in 

 a manner clearly showing shrinkage. A silica determination 

 upon a small quantity carefully selected, gave 66 per cent, 

 showing that but little free quartz was included. Alumina 

 and alkalies are also present about as in sanidine. 



Upon all surfaces of feldspar, and occasionally upon garnet 

 and topaz, are clear doubly terminated quartz crystals, often 

 not much below I™™ in length and of the form already men- 

 tioned. Although careful search has been made no tridymite 

 has been found. 



Minute opaque ore particles are very sparingly distributed 

 through the lithophyses. Some of them are developed in tablets 

 about O'b 1 ^ 11 across and these at least are probably to be re- 

 ferred to hematite. None are entirely free from dull coatings, 

 making identification of faces difficult. 



Garnet occurs in isolated crystals of a maximum diameter 

 of about l cm , the average being 2*5 mm , of dark red color, 

 clear and transparent, and with finely polished faces. The pre- 

 dominant form is that of 2-2 (211) with small facets of i (110). 

 The faces of 2-2 usually show a striation parallel to the edge of 

 i, due to the oscillatory combination of undetermined faces. 

 There are also indications of another trapezohedron in which 

 the value of m can be but little less than 2. 



The optical properties of this garnet are anomalous. Thin 

 sections were prepared parallel to the cube, dodecahedron and 



* For fuller statements concerning this color and its cause, see Bulletin 20, 

 TJ. S. Geological Survey, p. 75, 1885. 



