ANDESITIC ROCKS NEAR SILVERTON, COLORADO 7 



ceed farther than this stage, hut at other times complete alteration to chlorite and 

 calcite is fonnd. Hematite is also seen deposited on the cleavage cracks, and 

 epidote occasionally is a secondary product. 



Augite in small colorless or pale green crystals occurs at times. The amount, 

 liowever, never seems to he great. The mineral is bounded by (110), (010), and 

 (100) in the prismatic zone, as usual. Cleavage after (110) is present, hut not well 

 developed. Twins after (100), sometimes polysynthetic, are found. Extinction in 

 sections near (010) is 42 degrees. The augite seems to alter more readily than horn- 

 blende to chlorite, calcite, and epidote, so that the maximum amount of augite can 

 never be ascertained. It seems certain, however, that this mineral plays a less 

 important part in tjiese rocks tlmn the hornblende. 



Magnetite is always present in considerable quantity in both phenocrysts and 

 groundmass. It occurs in well defined octahedrons, sometimes twinned after the 

 Spinel law. The mineral decomposes to hematite and limonite. In advanced 

 stages of alteration the presence of leucoxene rims around the surfiices, as well as 

 along the parting planes, proves that the mineral is titaniferous. Occasionally the 

 shape of the crystals and amount of leucoxene is such as to indicate ilmenite, but 

 the fact that the substance is so strongly magnetic bespeaks magnetite. 



Apatite in short hexagonal prisms, terminated evidently by base and pyramid, 

 is generally present in and near the magnetite. 



A mineral having the properties of zircon is found very sparingly. 



The groundmass, which megascopically is always aphanitic, is, so far as could 

 be observed, holocrystalline. It consists mainly of plagioclase microlites, both 

 twinned and untwinned, often woven into a felt-like or pilotaxitish structure, but 

 sometimes possessing a parallel arrangement due to flowing. In rock number I, 

 which was separated by means of the Klein solution, a quantit}'^ of the ground- 

 mass was found in the portion having the specific gravity 2. 684-2. 04 at 22 de- 

 grees centigrade. However, most of the groundmass seems to be included in the 

 quantity having the specific gravity 2.64-2.(504 at 22 degrees, while a very small 

 portion ranges from 2,604-2.578 at 22 degrees, w'hich was the lightest portion 

 found in the rock. Although these masses were not pure, still a good idea is given 

 of the limits within which the groundmass is confined. Extinction angles in sec- 

 tions normal to (010) and angles measured with reference to the long directions 

 of ther microlites gave values from degrees up to 7-9 degrees. It seems safe, even 

 from the specific gravities, to conclude that the groundmass consists of oligoclase- 

 andesine, perhaps with some albite, in contradistinction to the andesine-labra- 

 dorite of the phenocrysts. The specific gravity 2.578, at which all particles of the 

 rock had slowly fallen, shows that there could be no glass present in the ground- 

 mass. Besides plagioclase, the groundmass contains innumerable specks of what 

 seem to be magnetite, together with a few other minerals in small amounts, which 

 are largely secondary products, like calcite, epidote, and chlorite. 



Chemical Composition* 



The analyses w'ere made in the chemical laboratory of Case School of Applied 

 Science, under the direction of Doctor A. W. Smith, by the students of the college. 



♦Messrs E. W. Gebhardt and W. G. Haldane, who made analyses II, III, IV, V, VI, VIII, and 

 IX, chose the chemical study of these rocks as a subject for the degree of bachelor of science. 

 Analysis I was executed Vjy Mr E. O. Cross, while VII was analyzed by Mr E. B. Willard. To all 

 of these gentlemen my thanks are due for their work. 



