Pirsson — Petrography of Tripyramid Mountain. 411 



ponent minerals by the Kosiwal method, but a rough approx- 

 imation, gained by study of a hand-specimen and the thin 

 sections, is as follows : 



Alkalic feldspar 40 per cent. 



Andesine - 40 " " 



Hornblende 12 " " 



Diopside 3 " " 



Other minerals 5 " " 



Total 100 percent. 



Chemical Composition. — Although the classification of the 

 rock appears quite clear, a chemical analysis would be desira- 

 ble for several reasons, especially for comparison with the asso- 

 ciated types ; but as all attempts to obtain material, which 

 would be fresh enough to warrant the labor of a good analysis, 

 were unavailing, it has not been undertaken. The actual min- 

 eral composition shows, however, that it would not be essentially 

 different from other monzonites, such as those of Monzoni and 

 central Montana, if regard is paid to its more salic nature. . 



Classification. — In the quantitative classification it is easily 

 seen that this rock is dosalic but near to persalic, it is clearly 

 perfelic and, from the feldspar relations mentioned above, it is 

 also domalkalic. This would place it in the rang monzonase, 

 but in which of the three middle subrangs it belongs is uncer- 

 tain, though probably in monzonose. In any event it is on the 

 edge of the group and very close to the persalic line, on 

 account of the abundant feldspar. In this respect it is like the 

 monzonose from Yogo Peak, which I described under the 

 name of syenite,* and which has since been classified as a felds- 

 pathic monzonite by Rosenbusch,f and included under mon- 

 zonose by Washington.^: 



In current classification this rock of Tripyramid Mountain 

 is certainly a feldspathic monzonite, whether one uses this term 

 according to the broad definition of Brogger, or the more 

 restricted one of Rosenbusch, since its genetic affinities are 

 shown by its association with the syenite. 



JS"orite (Andose). 



The occurrence of this rock on Slide Brook, between the 

 gabbro and monzonite, has been discussed in the foregoing- 

 paper. The material selected for investigation came from the 

 slabs exposed in places in the brook bed, sufficiently far from 

 the contact with the gabbro to be uninfluenced by it. 



*This Journal, 3d series, vol. v, 1895, p. 471. 

 fMass. Gestein., 4th Aufl., 1907, p. 170. 

 X Anal, of Igneous Eocks, p. 254, 1903. 



