86 The Ottawa Naturalist. [August 



No. 117. — Taken from Big Watchman Mine. 



" . . . • A rock which has the appearance of being a volcanic 

 agglomerate or breccia containing considerable lime." 



Hand specimen is a fine-grained greyish rock, showing a few 

 stains of green carbonate of copper on the joint planes. 



Under the microscope, the rock is seen to be composed of a 

 fine-grained groundmass of plagioclase felspar, in which are 

 imbedded large porphyritic crystals of felspar twinned according to 

 Albite and Carlsbad laws, some untwinned individuals and some 

 microperthitic intergrowths of albite and orthoclase. There are 

 also some forms which are now entirely filled with magnetite and 

 chlorite, but which, from the shape, once belonged to some ferro- 

 magnesian constituent, in all probability hornblende. There are 

 areas in the section which consist of angular and subangular 

 fragments, giving to it a brecciated appearance and which once 

 evidently belonged to some closely related volcanic rock caught 

 up by this one while it was in a molten condition. 



The rock is an altered andesite, which in some parts of the 

 mass probably passes into an andesitic tuff or breccia. 



No. 120. — From Big Sioux Mine, Aspen Gro\/e. 



" The country rock appears to have been of igneous origin and 

 is somewhat similar to that of the rest of the camp ; but at this 

 point it has been considerably altered and now approaches ser- 

 pentine." 



The hand specimen shows a massive fine-grained greenish- 

 coloured rock, having green carbonate of copper stains in the 

 cracks and on the weathered surfaces. 



Under the microscope, the rock is seen to be very much 

 altered and to consist of a groundmass and phenocrysts. The 

 former makes up a very small proportion of the rock and consists 

 of small plagioclase crystals. The phenocrysts consist of felspar, 

 in large laths, showing twinning according to both the Albite and 

 Carlsbad laws, some individuals having these two combined, thus 

 allowing of the determination of their composition by Michel 

 Levy's admirable method.* By this it was found such individuals 



* See Becker, G. F., on the Determination of Plagfioclase Felspars in 

 Rock Sections. Amer. Jour, of Sc, May, 1898. 



