ALTERATION OF THE LATER ANDESITE. 



241 



The optical characteristics above described indicate that the aluminous mineral 

 is probably largely hydrargillite, a while kaolin is also very likely present. 



Abundant pseudomorphs after pyroxene consist chiefly of a pale green, very 

 faintly crystalline fibrous aggregate, which in part seems to be chlorite and in 

 part is certainly uralitic hornblende or actinolite. 



The occasional biotite crystals are bleached and contain secondary quartz in 

 seams parallel with the cleavage. 



To determine the character of the carbonates in this rock they were separated 

 and analyzed qualitatively. They were found to consist of an abundance of 

 siderite, though the larger part is calcite. No magnesium carbonate was present* 



Analyses of described lypet of later andesite. 

 [Nos. 1 and 4 by Mr. George Steiger; Nos. 2 and 3 by Dr. W. F. Hillebrand.] 



a Rosenbusch-Iddings, Microscopical Physiography of the Rock-making Minerals, 3d ed., p. 351. 



6 Determined by Mr. George Steiger, of the United States Geological Survey. 



c Not looked for. 



d Not estimated; very little. 



16843 No. 4205 16 



