Pirsson — Petrography of Tripyramid Mountain. 409 



These analyses shotf that the Tripyramid rock agrees in 

 composition very closely with other syenites of alkalic type. 

 The magnesia, lime and total alkalies have proportions very 

 similar to those in the original umptekite of Ramsay, the chief 

 difference being in the relative proportions of the alkalies. It 

 is not very unlike one of Brogger's nordmarkites. All of 

 these are alkalic, and to emphasize this Washington's analysis of 

 the well-known syenite of Plauen, which is of subalkalic type, 

 is introduced for comparison. The difference in proportions 

 and amounts of the alkalies, lime and magnesia is striking. 



Classification. — Considering the fact that this is an alkalic 

 syenite, with an alkalic hornblende as the predominant ferro- 

 magnesian mineral, it must be classed with those types which 

 have been termed umptekite. This, notwithstanding the small 

 amount of labradorite present, which is an unusual feature in 

 these rocks, and which a somewhat higher percentage of mag- 

 nesia might have prevented by uniting with the lime to form 

 more hornblende, or pyroxene. 



In the quantitative system the position of the rock is shown 

 in the following calculation of its norm and place from the 

 analysis: 



Sal 88-38 _ „ 



7-3, Fersalane 



Or .... 



28-36 



Ab.... 



55-54 



An.... 



3-34 



Ne.... 



1-14 



Di .... 



6-37 



Hy — - 



0-60 



11 



1-52 



Mt.... 



3'25 



Ap.... 



0-34 



Total 



99-46 



Fern 12-08 



L 1-14 1 



-j^=—z <T— , Canadare 



F 87-24^7 



KO' + NaO' 161 



CaO' ~ 12 



K.O' 51 



13, Nordmarkase 



Na„0' 110 



0-46, Nordmarkose 



The rock is, therefore, persalic, perfelic, peralkalic, and doso- 

 dic, and its coordinates are 1, 5, 1, 4. The six per cent of 

 hornblende is not sufficient to make the mode abnormative and 

 it may therefore be termed grano-nordmarkose. 



MONZONITE (MONZONASE). 



Megascopic. — As exposed in the bare rock surfaces of the 

 " Y," and for a long distance down Slide Brook, and corre- 

 spondingly on Avalanche Brook up to the limits mentioned on 

 the North Slide, this type has the following megascopic char- 

 acters on a freshly fractured surface. Phanerocrystalline ; 

 medium to coarse grain ; dominantly feldspathic ; whitish, but 

 dotted with blotches consisting of grouped anhedra of horn- 

 blende and some biotite ; feldspars generally anhedral to sub- 



