R:fisuME. 357 



2. Muscovite from biotite — The uuusual i)i'ocess by whicli biotite is replaced by a 

 mineral indistinguishable from the adjacent decomposition product of orthoclaso is 

 further illustrated in the Mount Zion (p. 324), Lincoln (p. 330), and Mosquito (p. 3128) 

 Porphyries. The intermediate stages are referred to in the text. 



3. Epidote. — This mineral undoubtedly replaces both feldspars in several rocks 

 where no intermediate stage can be seen. While the chemical replacement of ortho- 

 clase substance by epidote is not easily understood, it is a fact that the replacement 

 does occur when the conditions, whatever they may be, are favorable (p. 341). 



4. Hornblende outlines — The Gray Porphyry has fresh or partially decomposed 

 biotite, while containing evidences that hornblende was a former constituent, although 

 it is now always represented by various extreme decomposition products in areas hav- 

 ing the characteristic outline of hornblende. This hornblende must represent an early 

 Ijroduct of consolidation, destroyed in the manner commonly noticed in andesites, and 

 both its former existence and its destruction are very probably connected with the 

 fact that the Gray Porphyry sheet at Leadville is 12 miles away from the eruptive 

 channel upon l^'agle River. (See p. 331.) 



5. Rutiie and anatase from biotite — The early stages of the decomposition of biotite 

 are usually accompanied by the formation of yellow needles or of small, apparently 

 tetragonal tablets, or of both forms. The identity of the former with rutiie is ex- 

 ceedingly probable, as they are often twinned in the characteristic manner and cor- 

 respond to what have been elsewhere identified. The nature of the latter forms is 

 less easily shown, but they agree well with the descriptions of anatase byDiller,' 

 while the association with the needles seems confirmatory of this determination. 



NEGATIVE OBSERVATIONS. 



The absence of certain minerals as coustituents in some cases is worthy of note. 

 Thus in the White Porphyry no biotite or bisilicate appears, even in small quantity; 

 apatite is very rare in the same rock and seems to be wanting entirely in the Pyritif- 

 erous Porphyry; augite appears in a single rock, and oliviue-bearing types are wholly 

 wanting. 



CHEMICAL COMPOSITION. 



The simple compositiou of the Nevadite and of the White Porphyry has been 

 referred to above. The relations of the types are noteworthy and will be apparent 

 from an examination of the accompanying table, in which the analyses previously 

 given are I'eproduced. 



iDiller, J. S. Neues Jalirbuoh fur Mlueialogie, etc., I, 187. 1883. 



