REPORT OF THE CHIEF ASTRONOMER 307 



SESSIONAL PAPER No. 25a 



from three to six feet in width and seem to be composed throughout of this one 

 type* though they are associated with dikes of augite minette. The mica 

 minette is a dark gray, fine-grained, highly micaceous rock usually showing 

 phenocrysts of biotite up to 2 mm. or more in diameter. The ground-mass is 

 the common hypidiomorphic aggregate of biotite, orthoclase, with little 

 labradorite; the accessories are apatite, titanite, and magnetite, with a little 

 interstitial quartz which may be secondary. The alteration products are the 

 same as in the augite minette, from which this rock differs mineralogically 

 only in the fact that the pyroxene is here absent. The mica minette repre- 

 sented in all of the collected specimens is rather badly .altered — so much so as 

 to discourage the idea of chemical analysis in their ease — but there seems to 

 be little question that both augite and olivine were absent from this rock in its 

 original condition; in any case they were present in but accessory amounts. 

 The specific gravity of a typical specimen was found to be 2-790. 



Augite Minette. — A large proportion of the lamprophyric intrusives belong 

 to the species, augite minette. This rock was found in dikes in the Pend 

 D'Oreille group as exposed on both sides of the Columbia river and at many 

 points along the walls of the Pend D'Oreille canyon. The freshest specimen 

 collected was, however, taken from a 60-foot dike cutting biotite-spangled and 

 garnetiferous miea schist in Belt F of the Priest River terrane on the summit 

 of the ridge two miles E.N.E. from the peak of North Star mountain. The- 

 dike is nearly vertical and strikes north and south. 



The rock is dark greenish to slate-gray and is porphyritic, with conspicu- 

 ous, lustrous phenocrysts of brown biotite measuring 5 mam. or less across the 

 foils. In thin section, idiomorphic prisms of a nearly colourless, diopsidic 

 augite are seen to be yet more abundant phenocrysts than the mica. The 

 prisms range from 0-5 mm. to 1-5 mm. or more in length. The ground-mass 

 is a fine-grained hypidiomorphic-granular aggregate of minute augite and 

 biotite crystals with abundant orthoclase. The last often encloses the femic- 

 minerals poikilitically. Apatite, magnetite, and a little quartz, which is inter- 

 sertal between the feldspars, are the primary accessories. The orthoclase is 

 somewhat kaolinized, while chlorite, epidote, and calcite have been secondarily 

 developed, but, for a minette, this rock must be regarded as unusually fresh. 



Mr. Connor's analysis of the same specimen (No. 900) resulted as follows : — 



Analysis of augite minette. 



Mol. 



Si0 2 53-32 -889 



Ti0 2 -90 -Oil 



A1 2 3 14-16 -139 



FeA 2-15 -013 



FeO fc 5-08 -071 



MnO -10 -001 



MgO 7-90 -198 



CaO 7-12 -127 



SrO -05 



BaO -12 -001 



25a — vol. ii — 20 i 



