240 N. L. BOWEN 



the light which the microscope threw on the process of crystalliza- 

 tion of their mineral constituents, which evidently took place pre- 

 cisely as it should if they were once molten. Eventually, these 

 corroborative facts came to be the criteria for the recognition of an 

 igneous or once molten rock and, at present, in actual practice it is 

 almost exclusively on the basis of the microscopic structure that a 

 rock is placed as igneous or not. Thus judged, the monomineralic 

 rocks are unquestionably to be considered as once molten, but if we 

 revert to the original criteria we find that in some respects they fail 

 to qualify. In the matter of sending tongues into surrounding 

 rocks we find them scarcely typical, and as far as occurrences as 

 lavas are concerned we find them wholly wanting. This apparent 

 discrepancy is due to the fact that we have not made our distinc- 

 tions fine enough. These rocks were formerly molten, but they 

 were never molten as such. When molten they were part of a 

 complex solution. Monomineralic rocks therefore afford the 

 strongest justification for believing that crystallization controls 

 differentiation. If differentiation took place in magmas wholly 

 liquid, it would seem that all plutonic rocks should have their 

 effusive equivalents. An examination of any table of classification 

 of igneous rocks on a mineralogic basis shows, however, a decisive 

 tendency for plutonic rocks to vary more widely than do effusives, 

 especially among basic rocks, and especially in this matter of run- 

 ning to marked richness in one mineral. This fact would have little 

 significance if it were a fairly common feature of plutonic rocks to 

 lack an effusive equivalent, but it becomes of the greatest signifi- 

 cance in connection with the manner of origin here advocated for 

 the monomineralic rocks when it is realized that in this respect 

 the monomineralic rocks stand alone. 



VOLUME AND AGE RELATIONS OF" MONOMINERALIC ROCKS 



Of the monomineralic rocks anorthosite is the only one that 

 occurs in any great amount. The actual volume of pyroxenite and 

 peridotite exposed at the surface of the earth appears to be insig- 

 nificant. 1 On account of the exceptional period required for the 



1 Daly's figures would indicate the order of magnitude (Igneous Rocks and Their 

 Origin, p. 44). 



