REPORT OF TEE CHIEF ASTRONOMER 405 



SESSIONAL PAPER No. 25a 



The ground-mass feldspars never display rhombic outline and form either 

 thin tabular crystals or stouter individuals of rectangular sections. Many of 

 them are twinned on the albite law but the twinning lines are seldom clean-cut, 

 straight, and continuous through the crystal-section as in the case of the soda- 

 lime feldspars. Both anorthoclase (in these twinned individuals) and sodiferous 

 orthoclase seem to be represented. The difficulty of diagnosing the often very 

 minute microlites is great and there is no certainty that soda-lime feldspar may 

 not also occur. The microlites of augite and biotite react optically like the 

 corresponding phenocrystic minerals. 



The glass in which all the crystalline constituents are embedded is colour- 

 less to pale brownish. It may be quite isotropic but, in most cases, there are- 

 faint changes of colour as the slide is rotated between crossed nicols and under 

 the gypsum plate. This anisotropic behaviour is apparently attributable to 

 incipient zeolitization of the glass. It may be noted that, in the thirty or more 

 thin sections of this intrusive porphyry, not one shows outlines of analcite in 

 crystallized individuals. It will be seen that in a closely associated extrusive 

 lava, primary analcite forms one-third of the rock. In the porphyry of the 

 chonolith, however, the isotropic base is, seemingly, an entirely interstitial and 

 amorphous substance, a glass. If analcite is present in the base it must be 

 secondary and without crystal form. 



Chemical Composition and Classification of the Rock. — Professor Dittrich 

 has chemically analyzed a specimen nearly representing this central phase of 

 the chonolith. The specimen was taken from a ledge on the northern brink 

 of Rock creek canyon, about 2,500 yards up-stream from the confluence with 

 Kettle river. The chonolith here contacts with two heavy masses of conglo- 

 merate which formed part of its roof. The specimen was collected at a 

 point 200 feet, measured perpendicularly, from the conglomerate. Even at that 

 distance the ground-mass of the rock carries a considerable amount (perhaps 20 

 per cent by volume) of glass. The specific gravity of the analyzed specimen is 

 only 2-621. From the careful study of many thin sections, the writer has con- 

 cluded that the completely holocrystalline phase must give an almost identical 

 analysis. Professor Dittrich's work shows the following proportions among the 

 oxides (specimen No. 1054) : — 



Analysis of principal phase, Rock Creek chonolith. 



Mol. 



Si0 2 51-83 .864 



Ti0 2 -86 -Oil 



A1 2 0, 18-25 -178 



Fe-A 4-26 -027 



FeO 1-46 -020 



MnO tr. 



MgO 3.28 -082 



CaO 4.08 -073 



SrO -42 -004 



BaO -43 -003 



Na 2 4-68 -076 



K.0 5-75 -061 



