306 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



The mode (Rosiwal method) is approximately: 



Alkaline feldspar (soda orthoclase) : 32-6 



Labradorite 27-9 



Augite 20-1 



Biotite 16-9 



Magnetite 1*2 



Apatite - 8 



Quartz -5 



100-0 



(Specific gravity calculated from mode = 2-848, closely agreeing with observed 

 specific gravity). 



According to the Norm classification the rock enters the sodipotassic sub- 

 rang, kentallenose, of the alkalicalcic rang, camptonase, in the salfemane 

 order, gallare; but it is also very close to the sodipotassic subrang, shoshonose, 

 of the alkalicalcic rang, andase, in the dosalane order, germanare. According 

 to the older classification the rock is evidently a typical (basic) augite-biotite 

 monzonite. 



Lamprophyric Dikes and Sills. 



In the Selkirk mountain system many of the formations older than the 

 Kossland voloanics are cut by lamprophyric dikes and thin sills which are 

 sometimes very abundant. Both sills and dikes are generally highly inclined, 

 approaching the vertical, and are bodies of quite moderate size; widths of either 

 dikes or sills are seldom as much as 20 feet and average only a few feet. The 

 larger number of these differentiation products are minettes but there are also 

 representatives of the kersantites, camptonites, and odinites. The dikes are 

 specially numerous in the Pend D'Oreille schists, quartzites, and limestones 

 where these rocks crop out in the canyon of the Pend D'Oreille river and on 

 the west side of the Columbia. Others cut the large masses of Pend D'Oreille 

 marble, the Wolf grit, Irene conglomerate, and doubtless other members of the 

 Summit series. Still others transect the different belts of rock in the Priest 

 River terrane. 



The minettes, as the most abundant lamprophyres in the region, have 

 merited most attention. On account of their fine grain and degree of alter- 

 ation their diagnosis merely through microscopic study was not to be entirely 

 trusted. For that reason as well as on account of their intrinsic interest a 

 number of chemical analyses have been made of the minettes. With the help 

 of the analyses and rather numerous thin sections the conclusion was reached 

 that four different types of minette occur more or less abundantly in the 

 Boundary belt. The types are augite minette, mica minette (biotite the only 

 femic essential), augite-olivine minette, and hornblende-augite minette. 



Porphyritic Mica Minette. — The type which for distinction may be called 

 mica minette was found in the form of a three dikes cutting the Pend D'Oreille 

 series near the railroad bridge over the Pend D'Oreille river. The dikes run 



