VOLUME XXV NUMBER 3 



THE 



JOURNAL OF GEOLOGY 



APRIL-MAY 1917 



THE PROBLEM OF THE ANORTHOSITES 



N. L. BOWEN 

 Geophysical Laboratory, Carnegie Institution of Washington 



STATEMENT OF THE PROBLEM 



Seldom can it be truly said that the puzzling feature of any 

 object is its simplicity, yet of all the problems that the anorthosites 

 present to us for solution the most difficult is their simple minera- 

 logical composition. Bunsen long ago taught geologists to think of 

 rock magmas as solutions, and the so-called solution theory 1 of 

 magmas has now gained general acceptance. We have been 

 enabled to understand many features of magmas that without the 

 aid of the theory of solutions would remain incomprehensible. We 

 understand why the order of separation of mineral from a magma 

 is not simply the order of their fusibility. We understand also why 

 a rock magma remains liquid at temperatures far below the tem- 

 peratures of fusion of the individual minerals that enter into the 

 magma and, therefore, why magmatic temperatures are compara- 

 tively moderate. It is because the individual minerals exist in the 

 magma in mutual solution and therefore have their specific 

 properties modified. But when we turn to anorthosites we find 



1 Nowadays it is scarcely proper to speak of the solution theory of magmas, for 

 magmas are solution in virtue of the definition of a solution rather than by theory. 

 On the other hand, to speak of the theory of solutions as applied to magmas is, of 

 course, entirely permissible. 



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