144 



H. W. TURNER 



Peak quadrangle. It will be noted, however, that the descrip- 

 tion of the rock by Lindgren indicates that it approximates in 

 texture to a granite-porphyry, and that the chemical composition 

 is nearer that of an aplite than of the biotite-granite to the south of 

 the Pyramid Peak district. I myself have not observed the latter 

 to pass into porphyritic forms with a fine-grained groundmass. 

 The calculation of the mineral composition is made in the 

 following way. All of the phosphorous pentoxide (P 2 5 ) is 

 ascribed to apatite. All the magnesia (MgO) is ascribed to 

 biotite. The molecular ratio of the oxides in the biotite is cal- 

 culated from an analysis of biotite separated from biotite-granite 

 No. 2136, and as the biotite in all the biotite-granite is optically 

 similar, it is fair to assume that it has, in all three of the granites, 

 averaged, sensibly the same composition. After deducting the 

 titanium oxide (Ti0 2 ) required for the biotite, the remaining 

 titanium oxide is ascribed to titanite. All the zirconium oxide 

 (Zr0 2 ) is calculated as zircon. After deducting the potash 

 (K 2 0) in the biotite, all the remainder is calculated as in potash- 

 feldspar. All the soda (Na 3 0) in the rock is supposed to be 

 present in the albite. The chemical analysis T of the biotite, 

 however, shows that it contains a little soda (0.38). In calcu- 

 lating the ratio of the oxides in the biotite, the soda was placed 



Composition of the biotite-granite, deduced from the average analysis 

 above given 



Pyramid Peak. 164. 

 Lindgren 



Quartz 



Potash-feldspar 

 Soda-feldspar . 

 Lime feldspar . 



Biotite 



Magnetite .... 



Titanite 



Apatite 



Zircon 



Water 



Per cent. 



39.80 



28.17 



25.09 



2.47 



3.10 



.61 



•35 

 •25 



•3i 

 100.15 



Am. Jour. Sci., l5 



