678 CROSS, IDDINGS, PIRSSON, WASHINGTON 



erals at a greater rate than that of augite would, in proportion 

 to the amount of femic diopside molecules converted into horn- 

 blende, because the ratio of Al 3 O s to CaO is higher, and the 

 ratio of (Mg,Fe)0 to CaO is twice as great as in augite. The 

 effect on the Si0 2 would be similar. 



The reduction of anorthite molecules by both these pro- 

 cesses would affect the character of the plagioclase actually 

 developed in the rock as compared with that reckoned as 

 standard. 



The development of mica would reduce the molecules of 

 potash feldspar or of leucite, and the formation of biotite 

 would also reduce the amount of hypersthene or olivine, and 

 that of magnetite. It would reduce the amount of calculated 

 corundum, or, in the absence of this extra A1 2 3 , would reduce 

 the molecules of anorthite and increase those of diopside or 

 wollastonite. The distribution of Si0 2 would be affected, and 

 in cases where there is no extra A1 3 3 and where (Mg,Fe)0 is 

 derived from hypersthene Si0 2 would be liberated to raise 

 lower to higher silicates, or to form quartz. 



Garnet. — In the matter of garnet it is easily seen that to form 

 simple lime-alumina garnet (grossularite), 3CaO. A1 3 3 .3Si0 3 , 

 it is necessary to combine anorthite molecules with femic wol- 

 lastonite, CaO.Si0 3 , with the liberation of silica. And it will 

 be found that this garnet is frequently developed in rocks whose 

 estimated standard minerals include wollastonite, and its crystal- 

 lization reduces the amount of anorthite. 



AN EXAMPLE OF CALCULATION. 



As an illustration of the several processes above described 

 we present the case of the Butte granite, because of its fairly 

 uniform composition as shown by five analyses, 1 its distinct 

 crystallization, which is of such size as to permit of accurate 

 microscopical measurement, and because the two aluminous fer- 

 romagnesian minerals, biotite and hornblende, have been 

 separated and analyzed. The rock from which these minerals 



1 Weed, W. H., Jour. Geol., p. 739, 1899. 



