II. Cross — s '"///' secondary minerals, etc. 



Aki. XI.Y. — Noti on sonu secondary minerals of tfu Amphi- 

 bol< ((nt/ Pyroxem groups; by Whitman Cboss. 



In the course of tin' microscopical examination of some 

 rocks from Ouster County, Colorado, the writer bas observed 

 two peculiar minerals of secondary origin, one an amphibole 

 and the other a pyroxene, which arc of interest on accounl <>j* 

 their unusual properties, relationships and mode of formation. 

 One of the observed minerals is a clear 1)1111* amphibole occur- 

 ring as a pseudomorphic replacement <>f common hornblende or 

 of augite, and also as enlargements of the Bame, with a pleochro- 

 ism much like that of glaucophane, while the orientation of 

 the ellipsoid of elasticity brings the variety into close relation- 

 ship with the new species riebeckite. The pyroxene is of a 

 bright green color and seems at first to he orthorhombic in 

 crystal form, hut it is thought more probable that ft is related 

 to segirite, in which there is a close approximation to the optical 

 orientation of an orthorhombic species. 



The blue amphibole appears most prominently in a dike 

 rock cutting sharply across upturned Arclmean gneisses, and 

 best seen at the northern base of the eruptive Rosita hills, 

 about five miles east of the mining town of Silver Cliff. This 

 dike rock is completely decomposed at its outcrop and the pro- 

 duct, which is a mixture of calcite, quartz, barite, limonite, 

 etc., with local impregnations of galena and other metallic sul- 

 phides, has been considered a mineral vein and several mining 

 claims have been located upon it. It is known for a distance 

 of about one mile with a course north w T est to southeast, Pros- 

 pect shafts on this "vein" have usually passed into a hard and 

 apparently fresh rock at depths of less than fifty feet. This 

 material is dense and tough, of apparent porphyritic structure, 

 showing many bright green grains of pyroxene often more 

 than one cm. in diameter, and smaller glistening prisms of 

 brown hornblende, both being imbedded in a bluish gray mat- 

 rix which usually makes up about half the rock nniss. The 

 microscope shows the matrix to consist of quartz and calcite 

 with minute blue or green amphibole needles penetrating and 

 coloring it. The pyroxene and hornblende are often regularly 

 intergrown and these two are the only primary constituents of 

 this rock now remaining, for there is no indication of the 

 character of the mineral or minerals replaced by the quartz- 

 calcite mass. This decomposition has already attacked the 

 pyroxene grains and they rarely show crystal outlines, but the 

 hornblende 18 better preserved and even the terminations are 



ae times distinct. 



Am. Jouil 8ol— Thud Svbdsb, Vol. XXXIX. N'<>. 233.— Mat, 1890. 



