356 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



MUTUAL BELATIONS OF BOOK TYPES. 



The large number of porphyrites constitute a series connecting the most dis- 

 tinctly plagioclastic forms with those in which orthoclase assumes a very prominent 

 place by virtue of its abundant large crystals. The full significance of this transi- 

 tion will be shown in a forthcoming report upon the Ten-Mile mining district, which 

 lies immediately north of the Leadville region. 



BOOK CONSTITUENTS. 



During the study of the eruptives which have been described several constitu- 

 ents were found to possess unusual development, while some of great rarity were 

 noticed. 



1. Lustrous sanidine The sanidine of the Chalk Mountain Nevadite is charac- 

 terized by a delicate but perfect parting, parallel to a plane in the orthozone, deter- 

 mined approximately as *f- P^ . When this parting is highly developed it causes a 

 brilliant satiuy luster parallel to the plane of parting. (See p. 348.) 



2. Zircon Minute but highly perfect crystals of nearly colorless zircon are regu- 

 larly, and sometimes abundantly, scattered through nearly all of the rocks described. 



3. Allanite The main group of the quartz-porphyries and porphyrites contains 

 allanite regularly, but sparsely, distributed through it. With the exception of the 

 contemporaneous identification by Mr. Idclings, no instance of the occurrence of this 

 mineral in such rocks is known to the writer. (See p. 329.) 



4. Topaz This does not appear as a rock constituent proper, but is found in 

 drusy cavities in the Nevadite. It is associated here with quartz, sanidine, and biotite 

 crystals and seems to be a sublimation product. (See p. 347.) 



5. Orthoclase fragments in a dark porphyrite containing abundant hornblende 

 and biotite and occurring as a dike in the Archean, were found numerous pebble-like 

 fragments of orthoclase, each belonging to a single individual and unlike anything 

 observed in other rocks of the region. These rounded pieces are analogous to worn 

 fragments of foreign rocks often seen in neighboring dikes, but their true nature could 

 not be definitely established. (See p. 339). 



DECOMPOSITION OF KOCK CONSTITUENTS. 



Notwithstanding the uniform and simple composition of the rocks described, a 

 few points of great interest were observed in connection with the decomposition of 

 their constituents. 



i. The Sacramento Porphyrite illustrates the tendency to the formation of a siugle 

 end product from all the chief decomposable elements, to a degree hitherto unknown 

 to the writer, either in literature or in personal experience. Some specimens of this 

 rock show epidote replacing hornblende, biotite, orthoclase, and plagioclase, all other 

 secondary products being comparatively insignificant in these cases. In certain other 

 -.specimens of the same rock a common result of the decomposition of biotite, ortho- 

 clase, and plagioclase is muscovite, epidote and all other alteration products being 

 here subordinate. (See p. 341.) 



