1892.] Mineralogy and Petrography. 163 



rhyolites of the Silver-CHH-i:..sit:i mining district in Custer County, 

 Colorado, where spherulites occur of all sizes, up to ten feet in diam- 

 eter. All are products of the consolidation of a magma, whose coin- 

 position is* 



SiO a AL0 3 Fe 2 3 FeO MnO CaO MgO K.0 Xa o H.0 

 171.56 13.10 .61 .28 .16 .74 .14 4.06 B.77 5.52 

 or about 5 alkaline feldspar and '. free silica, from which nearly all of 

 the Ca, Mg, etc., had been separated as phenocrv.-ts of plagioclax> 

 before the formation of the spherulites. The oldest of the spherulites 

 are miuute bodies, in some of which a granophyric growth ii detecta- 

 ble. The large ones are found in many generations. Some contain 

 internal cavities, while others are compact. The hollow >phenililes are 

 composed of radiating branching orthoc lax s,.\vith opal and other forms 

 of silica between the fibres, forming a mass through which are scat- 

 tered minute balls of tridymite or grains of quartz. Another type of 

 spherulite is the trichitic, in which the feldspars are branched and 

 curved to an unusual degree, forming a radiating bunch parallel to 

 whose radii trichites of magnetite are arranged. Both hollow and 

 trichitic spherulites are often surrounded by a supplemental growth in 

 which the feldspar is in very delicate needles. The various genera- 

 tions of spherulites locally make up the entire rock, but usually there 

 is a little residual material consisting of glass, of another radiate 

 growth, or of a combination of both. Compound spherulites are com- 

 posed of regular orientations of successive growths. The many spher- 

 ulites of quartz that have been described are thought by the author to 

 be largely feldspar and quartz aggregates, in which the orthoclase is 

 elongated parallel to c with the abnormal optical orientation, and thus 

 have a positive double retraction, when they are with difficulty distin- 

 guished from quartz microlites. Cross has traced unmistakable pris- 

 matic orthoclase down into fibres, and so seems warranted in stating 

 that determinations of the character of the material of spherulites 

 based entirely on the character of the double refraction of the fibres 

 are worthless. Some of the spherulites of the Colorado occurrence 

 consist entirely of positive feldspar, while others are composed of mix- 

 tures of this with a negative variety. With reference to the origin of 

 spherulites, Cross reaches the same conclusion as that reached by 

 Iddings; the mass in which spherulitic growth was set up must have 

 come to rest and consequently must have been pasty, since fluidal lines 

 cross the spherulites undisturbed in their courses. During the forma- 

 tion of some of the spherulites the mass again became pasty, and in 



