664 CROSS, IDDINGS, P IRS SON, WASHINGTON 



the sum of the constituents will represent the chemical compo- 

 sition of the rock. From this the salic and femic minerals may 

 be calculated as in the first instance, and the norm deter- 

 mined. 



In many cases it will not be necessary to reduce all of the 

 minerals to their chemical constituents in order to determine 

 the norm. For the minerals may be largely salic or femic as 

 they occur in the rock. This is the case with the feldspars, 

 feldspathoids (lenads), and quartz, with orthorhombic pyroxenes, 

 olivine, diopside, magnetite, etc. 



It is chiefly the aluminous ferromagnesian minerals that 

 require reduction. And for first approximations the proportions 

 of salic and femic constituents contained in these minerals are 

 given in Tables XII, XIII, and XIV at the end of this Part. 



From these it is seen that in aluminous pyroxenes the pro- 

 portion of the salic component is often o.io, and does not 

 exceed 0.23. The femic component is 0.90 in most cases, and 

 rarely 0.77. 



For hornblende and closely related amphiboles the salic 

 component is from 0.20 to O.34, and in the more sodic amphi- 

 boles it is about 0.10. 



For micas the two components are nearly equal, that is, 0.50 

 each. 



When the amounts of salic and femic components are known 

 the Class of the rock is established. For rocks of the first three 

 Classes the next step is the determination of the relative 

 amounts of quartz, feldspar, and feldspathoid. For persalane 

 rocks this is comparatively simple, since the other minerals are 

 present in small amounts. But as the amount of mica, amphi- 

 bole, and augite increases it becomes necessary to determine 

 more accurately the nature of the salic component involved in 

 each, in order to adjust the silica before reckoning the propor- 

 tions of quartz, feldspar, or feldspathoids. In such cases the 

 first method must be followed, or the aluminous ferromagnesian 

 minerals must be resolved into aluminous and non-aluminous 

 portions and the silica adjusted to these portions and to the 



