288 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



solidified. This involves the chemistry of solutions of inorganic 

 compounds, chjefly silicates ; the mutual interaction of the various 

 chemical elements that appear in an analysis of the whole rock; 

 together with the possible catalytic action of constituents, notably 

 water gas, that may not become parts of the fixed compounds, but 

 may escape in greater part upon the solidification of the magma. 



Some of the minor problems, or factors, within this large one 

 may be alluded to briefly as follows : The first and most obvious 

 result of a strict correlation of the mineral composition of rocks 

 with the chemical composition of the whole mass, representing 

 the fixed components of the formerly liquid magma, is the recog- 

 nition of the nonappearance in certain kinds of rocks of some 

 minerals whose presence is necessary to satisfy the chemical require- 

 ments of the magma solutions. This is the case with completely 

 crystallized but exceedingly fine-grained lavas of particular com- 

 positions, notably andesites. 



Minerals that should be present to the extent of as much as 30 

 per cent, in some instances are not visible, are occult, and must 



99.51 100.05 



be hidden within the substance of those minerals that are visible : 

 that is, they must be held in solid solution within other kinds of 

 crystals. An example will illustrate the case. 



A magma whose chemical composition is shown by analysis I, 



