THE PROBLEM OF THE ANORTHOSITES 235 



raised in mind. An individual cannot do more than state the 

 problem and leave its suggested solution to confirmation or refuta- 

 tion at the hands of those acquainted with various anorthosites in 

 the field. 



Of anorthosite in general it can be said, however, that no 

 effusive equivalent has hitherto been found anywhere. This must 

 be regarded as a very surprising fact if there were ever masses of 

 molten plagioclase adequate to furnish such great exposures of 

 anorthosite as occur in various parts of the earth. On the other 

 hand, if these anorthosite masses were merely collections of plagio- 

 clase crystals effusive anorthosites are scarcely to be regarded as 

 possible. 



MONOMINERALIC ROCKS IN GENERAL 



Enough has been said incidentally in the foregoing to make it 

 clear that the problem of any monomineralic rock is, in its essen- 

 tials, the same as the problem of the anorthosites. There are no 

 more promising methods of obtaining pure molten olivine or 

 pyroxene than there are of obtaining molten plagioclase. On the 

 other hand, the collection of crystals to give substantially solid 

 masses of nearly pure olivine or pyroxene does not seem out of the 

 question. 



A survey of the domain of igneous geology lends considerable 

 support to the possibility that peridotites and pyroxenites are so 

 generated. In making the test of their occurrence as small dikes 

 we find that this is perhaps the most characteristic manner of 

 occurrence of peridotite, but on closer examination it appears that 

 this fact may be due rather to the elastic nature of the term perido- 

 tite, which may be applied to a rock containing considerable plagio- 

 clase, pyroxene, hornblende, or mica, or all of these, as well as its 

 olivine. Typical dunite, or nearly pure olivine rock, however, 

 probably does not occur as dikes, if we except, again, its occurrence 

 in such form in closely related and essentially contemporaneous 

 igneous rocks. The same statement may be made of rock types 

 excessively rich in pyroxene. As to the question of their occurrence 

 as effusive types it is found that peridotite has an effusive equiva- 

 lent in picrite, but picrite is far from being a pure olivine rock. 

 Dunite itself has apparently no effusive equivalent. With the 



