GNEISS AND LIMESTONE CONTACT PHENOMENA 



279 



In other words: the larger the quantity of the mafic minerals, 

 the smaller is their Fe-quotient. In the most acid types of gneiss, 

 rich in quartz, we find biotite and hornblende, and often clino- 

 pyroxene also, extremely rich in iron. In the mafic minerals of 

 the "dark bands" the amount of iron in proportion to magnesia 

 is much less. 



At the same time as the Fe-quotient decreases, the amount 

 of anorthite in the plagioclases increases. This relation is somewhat 

 obscured by the epidote, a substitute for the anorthite. 



The percentage of the iron compound in the epidotes does not 

 show quite regular relations with that in the mafic minerals. 

 This is probably connected with the later origin of the epidote. 



This correspondence between the Fe : Mg ratio and the relative 

 proportion of salic and femic minerals is a special case of what is 

 inherent in any cognate series of igneous rocks which have been 

 derived from a common magma by the processes known as mag- 

 matic differentiation. Its occurrence in the banded gneiss proves 

 that the band structure is in itself a product of differentiation. 

 In other words, the "dark bands" are those in which the minerals 

 containing solid solutions are richest in the highest melting com- 

 pounds or, more exactly, compounds least soluble in the residual 

 magmas. The mafic minerals of these most "basic" members of 



