VOLUME XXXI NUMBER 2 



THE 



JOURNAL OF GEOLOGY 



February-March IQ2J 



THE PROBLEM OF THE ANORTHOSITES AND 

 OTHER MONOMINERAL IGNEOUS ROCKS 



F. LOEWINSON-LESSING 



Polytechnical Institute, Petrograd 



The story of monomineral igneous rocks, which occupy today 

 such a prominent place in the problems of petrogenesis, may be 

 outlined in a few words as follows. Formerly no special attention 

 was paid to these rocks. When "simple" rocks were considered 

 sediments, the monomineral silicate rocks were thought to belong 

 to the sedimentary or the metamorphic series. That was, for 

 instance, the case of the Canadian and other North American 

 anorthosites, and Adams^ was obHged to argue, in 1893, ^^^ their 

 igneous origin. Several years afterward the present writer con- 

 sidered the monomineral igneous rocks as derived from those 

 extremely pure magmas, unable to split farther, .toward which the 

 differentiation of a complex magma tends. From my own and 

 Brogger's point of view on magmas as solutions of minerals, where 

 the differentiation consists not in the wandering of separate oxids, 

 as was then maintained by Iddings, but in the transfer of those 

 groups of oxids and siHca which correspond to the minerals of the 

 future rock, I was led to admit that every igneous rock had its feld- 



^ F. Adams, "tJber das Norian oder Ober-Laurentian von Canada," Neues Jahrb.f. 

 Miner., Beil.-Bd. VIII (1893), p. 419. 



