THE GRANITIC ROCKS OF THE SIERRA NEVADA I 57 



older granite are supposed to be older than the potash-aplites of 

 the granodiorite series. 



PARTIAL ANALYSIS OF SODA-APLITE (NO. I^30). ANALYST, STOKES 

 Si0 2 - - - 76.17 



CaO - - - - - 1.64 



K 2 - ... 2.48 



Na 2 - - .... 4 . 54 



No. 1730 is a dike in gneiss from the north bank of the 

 Mokelumne * a little below the mouth of Blue Creek in the Big 

 Trees quadrangle. This rock is made up chiefly of quartz and 

 feldspar. In addition there is a little biotite present. For the 

 purpose, however, of a rough calculation this biotite can be 

 ignored and all the lime ascribed to anorthite, all the soda to 

 albite, and all the potash to orthoclase or microcline. Nearly 

 all the alumina of the rock is contained in the feldspar molecules. 

 The amount of alumina may therefore be calculated, and equals 

 .1289 molecules, or by weight 13.15 per cent. The free silica 

 can be estimated by deducting from the total silica the amount 

 in the feldspar. 



Total silica - - 1.2695 



Silica in feldspar .... .6562 



Free silica - - - - -6133 



The molecular composition of soda-aplite No. 1730 is then 

 approximately as follows. 



Molecules Percentage 



Potash-feldspar - - .2112 13.83 



Soda-feldspar .5856 38.34 



Lime-feldspar - - .1172 7.67 



Quartz - .6133 40.16 



1.5273 100. 



The albite and anorthite molecules together form plagioclase, 



the ratio being Ab 5 An t ; hence the plagioclase is acid oligo- 



clase. The potash-feldspar is chiefly or entirely microcline. 



The relative abundance of the constituents of soda-aplite No. 



1 See Seventeenth Ann. Rep. U. S. Geol. Surv., Part I, p. 700-705, for other notes 

 about these gneisses. 



