from the Rosita Hills, Colorado. 469 



positive, and the cleavage is parallel to the basal plane. Sec- 

 tions at right angles to the cleavage show stronger polarization 

 than in feldspar, and sharp extinction parallel to the cleavage 

 lines. The rough crystals in the cavities have the same optical 

 characters, and the more perfect of them have a hexagonal 

 outline caused by apparently rhombohedral planes. A chem- 

 ical analysis of the average rock was made by Mr. L. G-.' 

 Eakins, in the laboratory of the U. S. Geological Survey, with 

 the following results : 



Molec. ratio. 



Si0 2 65-94 



A1 2 3 12 95 4- 102 -127 3*26 



KO' 2-32 4- 94 -025 



Na 2 1-19 4- 62 -019 f '° 44 1<13 



S0 3 12-47 4- 80 -156 4- 



H 2 -_.. 4-47 4 18 -248 6*36 



Fe 2 3 etc. 0*55 



99*89 



From the above ligures it is seen that the constituents of 

 alunite are present in very nearly the molecular proportion 

 required for that mineral, with a slight excess of w T ater and of 

 bases. It is quite probable that there was a small amount of 

 kaolinite in the material analyzed. The sulphate present is 

 slowly soluble in H 2 S0 4 , and after slow roasting alum can be 

 extracted with water. This rock then is made up of quartz, 

 two-thirds, and alunite, one-third, aside from insignificant 

 impurities. The specimen analyzed seems to be representative 

 of the entire upper part of the hill, though the percentages of 

 quartz and alunite doubtless vary somewhat. Rhyolite occurs 

 on the lower slopes about it and also underneath the summit of 

 the hill, as shown by tunnels. A transition from rhyolite to 

 alunite rock has not been observed at this place, but no good 

 reason is known for doubting that rhyolite was the original 

 rock here as at the other locality to be described. The observed 

 limitation of the alunite rock in depth no doubt corresponds 

 with the horizon at which sulphureted hydrogen gave rise to 

 sulphurous acid on oxidation near the surface. 



III. The alunite-diaspore rock of Mt. Robinson. 



Mt. Robinson is the highest point of the Rosita hills, although 

 the summit is but six hundred feet above its southern base. 

 Its slopes are smooth, owing to the soft andesitic material 

 beneath, but the top is a projecting ridge, a quarter of a mile 

 long, with a cliff of from fifty to seventy-five feet in height on 

 the south, while a growth of aspens come close up to the crest 

 on the north. The jagged crest is fifty feet or more in width, 



