220 JOSEPH P. IDDINGS 



tion of several minerals in any case, as well as the relative pro- 

 portions of all the minerals constituting the rock. 



The relations between the minerals crystallized from a rock 

 magma and the physical conditions attending its solidification 

 have been scarcely more than recognized in a general way, 

 though many examples have become well known. But enough 

 has already been learned to warrant the conclusion that the 

 attendant physical conditions exert a definite control over the 

 grouping of the chemical elements in the molten magma, whereby 

 the kind and character of the minerals crystallizing from it are 

 affected. And it may be confidently predicted that careful com- 

 parison of the chemical composition of rocks with their exact 

 mineral composition and texture, and with their mode of occur- 

 rence as geological bodies, will eventually lead to the discovery 

 of these relationships. 



Realizing the difficulties in the way of a complete correlation 

 of the mineral and chemical composition of igneous rocks, and 

 the limitations of our present knowledge both as to the relation- 

 ships just mentioned, and as to the actual chemical composition 

 of many rock-making minerals, it may still be possible to make a 

 beginning of the correlation by attempting to state certain rela- 

 tionships between the theoretical molecules of the chief rock- 

 making minerals and the chemical composition of igneous rock 

 magmas. 



Composition of the rock-7naking minerals. — Owing to the com 

 plexity of even this preliminary correlation, it is advisable to 

 consider only the more important rock-making minerals, leaving 

 the less frequent ones for future elaboration of the discussion. 

 The former may be grouped as follows : Quartz, the feldspathic 

 minerals : the feldspars proper, with leucite, nephelite and the 

 sodalites ; muscovite, forming a link between feldspathoid min- 

 erals and ferromagnesian minerals ; biotite and the other ferro- 

 magnesian minerals : olivines, pyroxenes, amphiboles ; besides 

 magnetite. Minerals necessary to a fuller discussion are meli- 

 lite, garnet, titanite, perofskite, apatite, zircon, and others. 



The chemical composition of each of these minerals, or groups 



