THE 



JOURNAL OF GEOLOGY 



APRIL-MAY, 1898 



CHEMICAL AND MINERAL RELATIONSHIPS IN 

 IGNEOUS ROCKS. 



The attempt to correlate the mineral composition of igneous 

 rocks with the chemical composition of their magmas, that is, of 

 each rock as a whole, is rendered difficult by the chemical char- 

 acter of the rock-making minerals themselves, and by the fact 

 that no fixed association of minerals necessarily results from the 

 crystallization of an igneous rock magma, the association in a 

 given case being affected to a greater or less extent by the 

 physical conditions attending the solidification of the magma. 



The pyrogenetic rock-making minerals are mostly silicates of 

 several elements which may enter in different proportions into 

 the composition of distinct minerals ; so that the chemical dif- 

 ference between a number of these minerals lies in the propor- 

 tions of their chemical components rather than in the kinds. 

 Among the more important rock-making minerals, including the 

 chief silicates, together with quartz and magnetite, there is no 

 element found only in one mineral. Each constituent may enter 

 several of them. Nevertheless there are limitations to the kinds 

 of elements constituting certain minerals, as well as more or 

 less definite proportions to their amounts in each case. But it is 

 to be remembered that with the exception of quartz none of them 

 has an absolutely fixed composition, but each belongs to an 

 isomorphous or morphotropic series, that is, represents a more 

 or less variable mixed salt or crystal. Variations in the chemical 

 constituents of rock magmas will affect the chemical composi- 

 Vol. VI, No. 3. 219 



