75o 



AUSTIN F. ROGERS 



which does not have a fixed chemical composition. We may, 

 however, assume the following percentages which are average 

 values for biotite Si0 2 =37 per cent, Al 2 3 =i6 per cent, Fe 2 3 =6 

 per cent, FeO= 15 per cent, MgO= 12 per cent, K 2 0= 10 per cent, 

 H 2 0=4 per cent. 



The recalculated chemical analysis of the rock given is as follows : 



Orthoclase 



Albite 



Corundum 



Biotite 



Baddeleyite 



Total 



Si0 2 . 

 A1 2 3 

 Fe 2 3 

 FeO. 

 MgO. 

 CaO. 

 Na 2 

 K 2 0. 

 H 2 0. 

 Zr0 2 . 



3° 



2. 1 

 0.9 



°-3 

 0.9 



0.7 



0.6 

 o. 2 



°-5 



44-1 



43-7 



0.3 



0.9 



0.7 



4-3 

 5-0 

 0.2 



o-S 



26.2 



36-4 



30.9 



5-7 



o-S 



99-7 



Chemically, this is a peculiar rock on account of the high alumina 

 and low silica content. It may be called a corundum-syenite. In 

 order to place this rock in the new quantitative classification it is 

 necessary to convert the percentage compositions of the oxids into 

 percentages of the standard minerals, which in this case are nearly 

 the same as the actual minerals. In other words, the mode and the 

 norm agree closely, biotite being practically the only critical mineral. 

 The calculated norm of the rock is shown in table on p. 751. 



All the potash goes into the orthoclase molecule, all the ferric 

 iron and an equivalent amount of the ferrous iron go to make the 

 magnetite molecule. The remaining ferrous oxid goes with all 

 the magnesia to form the hypersthene molecule which requires an 

 equivalent amount of silica. The silica remaining after deducting 

 that required for the orthoclase, hypersthene, and zircon would 

 naturally go into the albite molecule, but it is found that there is too 

 much soda for this amount of silica so that the silica and soda must 

 be distributed between the albite and nephelite according to the 



equations : x 



x-\- y = molecules of Na 2 

 6x-\-2y = available S1O2 



1 Quant. Class, of Igneous Rocks, 194 (1903). 



