from the Rosita Hills, Colorado. 471 



this idea in mind, a specimen of the rock taken at the west end 



of the summit ridge was analyzed by Mr. Eakins, with this 

 result : 



Si0 2 76-22 



Ti0 2 „__ _._ 0-11 



A1 2 3 _. 19-45 



Fe 2 3 tr. 



CaO tr. 



Alk. tr. 



SO,.__ ._. 0-29 



P 2 5 0-13 



H 2 3-82 



100-02 



The alumina belonged to a mineral insoluble in most acids, 

 and infusible in alkaline carbonates. On treatment by hydro- 

 fluoric acid, which dissolves the mineral with great difficulty, 

 17*79 per cent of this substance was isolated from the rock, 

 and found to contain 84 67 per cent of A1 2 3 , with no other 

 base, while water was present in large amount. The theoreti- 

 cal composition of diaspore is : A1 2 3 85'07, H 2 14-93 = 100. 

 Microscopical examination of the rock in thin sections, and of 

 cleavage flakes of the mineral in question, showed it to possess 

 the physical and optical properties of diaspore. By reason of 

 its high refractive index its surface relief in thin sections dis- 

 tinguishes it clearly from alnnite, although both minerals occur 

 in irregular grains in the rock mass and are filled by small 

 included quartz grains. 



The alunite and diaspore of this rock having been determined 

 in the spring of 1890, the writer revisited the region in the 

 following summer, obtained further information concerning 

 the occurrence, and collected specimens of particular interest. 

 On carefully examining the material from a prospect hole sunk 

 in the quartz-diaspore rock analyzed, some specimens were 

 found containing irregular cavities an inch or more in diameter, 

 in which were groups of rather stout prismatic, colorless or 

 whitish crystals, with glistening faces, though seldom trans- 

 parent. The crystals have several planes in the prismatic 

 zone, and the low terminal planes which are clearly pyramids 

 and domes indicate the symmetry of the orthorhombic system. 

 Much of the surface rock at this end of the dike has cavities 

 with similar crystals which are dull white in color and clearly 

 much decomposed, the product being a tine micaceous mineral 

 apparently kaolin. 



On the southern slope of the dike, below the summit, a few 

 loose fragments of quartzose rock were found, with very bril- 

 liant, clear, prismatic crystals, inclined to tabular development 



