BY H. I. jensp:n. 



911 



Warrumbungle Mountains, to grey laurvikite from Laurvik 

 (Norway), to uraptekite, Red Hill (New Hampshire), and to 

 rhombenporpliyr, as the following table shows. 



The rock under discussion differs from laurvikite in containing 

 less lime and alumina; it contains relatively less alkali than 

 umptekite, and less silica than typical pulaskite. It is best con- 

 sidered to be a basic facies of pulaskite. What is particularly 

 striking is that it appears that the piarent-magma of the Nandeivar 

 alkaline rocks is the same as that of the Warrumbungle rocks. 



The Quantitative Classification of Igneous Rocks. 



Of late years there has been a good deal of discussion as to the 

 merits of the quantitative classification and magmatic nomen- 

 clature devised by Iddings, Washington, Pirsson and Cross. A 

 few remarks on this subject will not be out of place here. 



The quantitative system has done excellent work in waking up 

 petrologists to the value of rock-analysis. Analyses of rock-types 

 and of rocks difficult to classify are essential both for correct 

 identification and for arriving at conclusions regarding magmatic 

 differentiation. Further, for pui poses of comparison, the calcula- 

 tion of the norm is invaluable. Take, for instance, the anal3'ses 

 of N.30 and N.ll in Table i. The microscope revealed affinities 

 between these rocks, sufficiently to enable one to say that both 

 belong to the alkaline group. The analysis brings their affinity 

 into more marked prominence. But it is only when we compare 

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