W. C? , oss — Topaz and Garnet in Rhyolite. 435 



octahedron of the most suitable crystals which could be pro- 

 cured and the optical action seen, so far as it possesses regu- 

 larity, corresponds to the observations of Klein.* The action 

 is weak and its regularity is no doubt disturbed by the numer- 

 ous inclusions of quartz at the base of all crystals. 



In the porphyritic rhyolite the garnets are usually very small 

 but perfect and possess a somewhat lighter color than in the 

 more felsitic rock, appearing brownish red or even approaching 

 to a cinnamon color. 



In chemical composition this garnet proves to be typical 

 spessarlite, that is, an alumina garnet in which the lime is al- 

 most wholly replaced by manganese with some ferrous oxide. 

 The analysis given below was made by L. Gr. Eakins, in the 

 Denver laboratory of the U. S. Greological Survey. The ma- 

 terial analyzed was of the common dark red variety from the 

 main locality, first described, and was obtained from about 30 

 crystals, which were crushed and the powder carefully exam- 

 ined under a lens, only the transparent and perfectly pure par- 

 ticles being appropriated for analysis. 



Sp. Gr. 4-23 at 18° C. 



Si0 2 .35-66 



Al„O s 18-55 



Fe 9 0, -32 



FeO 1425 



MnO 29-48 



CaO 115 



KoO -27 



Na a O -21 



HO -44 



100-33 



Topaz appears in the lithophyses of all modifications of this 

 rhyolite, though much less abundantly than the garnet. Its 

 crystals are prismatic, clear and transparent, either colorless, 

 pale bluish or distinctly wine yellow, and show a fine develop- 

 ment of all faces represented. The form is the usual one for 

 this mineral, the faces, in order of prominence, being: 



J(U0), z-2 (120), 2(221), 0(001), 2-? (021), 44(041), 



i-fc(lOO), £-3(130), and 2-i(201). 



The largest crystals which have been found (not seen by the 

 writer), are said to have measured about one-half inch parallel 

 to the brachy-axis, but such specimens are very rare and the 

 average diameter is not more than one-eighth of an inch. As 

 the prisms are attached in all positions double terminations are 



*C. KleiD. "Optische Studien am Granat." Nachr. v. d. k. Ges. d. Wiss. 

 zu Gottingen, 1882, 457. 



