Pirsson — Petrography of Tripyramid Mountain. 425 



The discussion of the analysis really involves also the 

 question of classification and is best considered under that 

 heading. 



Classification. — In the quantitative system the norm of the 

 rock and its position may be calculated as follows : 



Norm \ Sal 7o-44_ 



3" 



0-03, 5, Germanare 



Or .--. 17-24 ^ — =— — b = 2-4, 2, Dosalane 



Ab.-.- 30-92 Fera 2823 



An 20-29 L__ 1-99 



Ne 1-99 70-44 F 68-45 



Di.--_ 10-95 K,0' + NaO' 97 A , 



Ol 8-48 — -~rr-fv^— =rz = 1"3, 3, Andase 



Mt.— 3-02 Trrv 



II 5-78 28-23 ^O " = o-40,4,Ando8e 



11,0.-- -56 Na„0' 66 ' ' 



Total 99-23 



That is, it is dosalic, perfelic, alkalicalcic and dosodic; its 

 coordinates are 2.5.3.4 and, considering its texture, it is grano- 

 andose. The attempt to classify this rock according to the 

 general qualitative system meets with some difficulty. It 

 shows some affinities to diabases, but the fabric is quite differ- 

 ent from that of a typical diabase. It would be, perhaps, 

 easiest to term it a micro-gabbro. If we consider genetic 

 relations, and the fact that it is dike rock, the case becomes still 

 more difficult. Does it belong to the alkalic clan of lampro- 

 phyres, or to the alkalicalcic (sub-alkalic) group, and, if the 

 latter, is it an aplite, Or a lamprophyre ? If we determine that 

 the whole group of Tripyramid rocks is really an alkalic com- 

 plex, and that the gabbro should be regarded as an essexite, 

 then it is either micro-essexite or, clearly, a lamphrophyre, one 

 probably complementary to the quartz-syenite aplite, and the 

 small amount of nephelite it contains would be a natural 

 feature. If a lamphrophyre we must class it as a new type, 

 or one of those aberrant camptonites mentioned by Rosen- 

 busch,* in which augite entirely replaces hornblende. In favor 

 of this view it will be seen, by reference to the table, that it 

 has an almost identical chemical composition with the dike 

 at Norway Rapids, which is described as camptonite in the 

 following section. On the other hand, the composition is quite 

 different from typical camptonites, as seen by comparing 

 with No. IV of the table ; it does not agree with them, either 

 in minerals, or chemically. From the standpoint of an alkalic 

 rock, though wanting brown hornblende, it classifies best in 

 minerals, fabric and chemical composition as a micro-essexite 

 *Mass. Gesteine, 4 te Aufl., p. 685. 



