PAH-TSON MOUNTAINS. 779 



also near the head of Grass Canon, and in the canon descending to the 

 eastward from' Granite Peak, where it occurs well developed, but in very 

 small crystals, in a rock apparently rich in triclinic feldspars. In thin sec- 

 tions, under the microscope, Zirkel has detected, in addition to the above, 

 apatite, specular iron, and, in one instance, magnetite, completely surrounded 

 by minute prisms of apatite. 



The rock from the summit of Granite Peak may be taken as a typical 

 granite of the region. It differs but little, if any, from the description given 

 above, being, perhaps, somewhat lighter in color, as it contains less horn- 

 blende than is found in the greater part of the granite body. It has been 

 subjected to chemical analysis by Prof. Thomas M. Drown, of Lafayette 

 College, with the following result: 



Silica ' 64.02 



Alumina 17.60 



Ferrous oxide 4.03 



Manganous oxide 0.16 



Lime 4.38 



Magnesia 1.27 



Soda - 4.79 



Potassa 2.62 



Loss by ignition 0.80 



99.67 



It will be noticed that, although standing within the usually prescribed 

 limits of silica percentage, it is nevertheless a very basic granite, and that 

 the large amount of soda present would indicate the predominance of oligo- 

 clase over orthoclase. 



In the depression lying between the Pah-tson and Kamma Mountains 

 occur several small isolated outcrops of granite, rising above the Tertiary 

 and Quaternary deposits. Apparently they possess little geological interest 

 except as indicating an extension of the granite body to the northward, now 

 concealed beneath the andesites and rhyolites of the Kamma group. 



The rhyolites of the Pah-tson Mountains are found along the highest 



