Pirsson — Petrography of Tripyramid Mountain. 415 



For the sake of comparison analyses of two other norites are 

 added. Unfortunately not many good reliable analyses of this 

 rock, made by the latest approved methods, are available for 

 this purpose. In this connection the reader is invited to com- 

 pare Nos. I and IV ; Brogger's rock is a lamprophyric dike- 

 rock, one of the clan attendant upon the nephelite syenite 

 (laurdalite) intrusions of South Norway. He gives the com- 

 position as follows : plagioclase of the composition Ab 2 Anj, 44*7 

 per cent, augite 25*3 per cent, bronzite 8*6 per cent, lepido- 

 melane (biotite) 14*6, iron ore 5*0 and apatite 1*5 per cent = 99*7. 

 Tt is to be noted that, while the minerals are essentially the same 

 as those in our norite, their relative proportions are somewhat 

 different. Chemically, the two magmas are similar ; save for 

 the differences in the ferrous iron, magnesia, and the lime, 

 they are remarkably alike. The bearing of this on genetic 

 relationships is discussed in a later place. 



Classification. — In the quantitative system the position of 

 this rock is shown by the calculation of its norm and place. 



Or 10-01 Sal 66-41 _ . 



av. qo-/tQ ^ — = =2*2 = 2, Uosalane 



Ab 32 49 F em 33.35 ' 



An 23*91 66*41 



Di..... 5*59 £i±Q=_2_=o, = 5, Germanare 



Hy 10*24 F 66*41 ' ' 



Mt 7*19 — V, ^> =— = 0-9 = 3, Andase 



II 3-95 Ca0 86 



Ap 4*37 33*35 K,0' 18 



H s O ... 0*34 N^O' = 6¥ = °' 29=4 ' And ° Se 



Total 100*10 



It is, therefore, dosalic, perfelic, docalcic and dosodic, with 

 coordinates 2.5.3.4. Comparing the norm with the mode pre- 

 viously given, no critical mineral appears in notable amount ; 

 the mode is therefore a normative one, and since the texture 

 is granular it may be termed grano-andose. In qualitative sys- 

 tems the rock is a norite, of feldspathic character. 



Gabbro (Hessose). 



The material selected for investigation is from the Black 

 Cascade on Slide Brook. The gabbro on Avalanche Brook is 

 of somewhat finer grain, but the sections show that it is petro- 

 graphically similar. 



Megascopic. — Holocrystalline ; medium to coarse grain ; 

 general color dark grayish to black ; dominantly composed of 

 feldspar of a deep smoky-gray color in roughly tabular crystals, 

 10 mm across the cleavages of h (010) and lath shaped, 10 mm by 



