1 903 J Petrography of some Igneous Rocks. 87 



consisted of an olii^^oclase with the composition Abj An,. There 

 are also aggregates of epidote and chlorite, frequently having de- 

 finite outlines, which give them the appearance of being secondary 

 after augite. Calcite is also present as a secondary mineral ; and, 

 as accessory constituents, ilmenite, leucoxene and sphene are found 

 in considerable quantities. 



On account ot the very considerable quantity of augite 

 which was present in the original rock, as well as on account of 

 the large proportion of iron ore present, the rock possesses a 

 distinctly basic character, and consequently is better classed as a 

 basalt than as an augite andesite. 



No. 125. — From the Medal Mineral Claim, Aspen Grove. 



" It is an extension of a dyke of from 10 ft. to 12 ft. wide, of 

 lighter colour than the general rock, inclined to be porphyritic in 

 structure and containing much lime ; there is an impregnation of 

 copper sulphide, but not of important quantity." 



The hand specimen shows fine-grained reddish-looking rock 

 containing greenish chlorite aggregates and a little chalcopyrite. 



Under the microscope, it is seen to be of volcanic origin, 

 being composed of idiomorphic plagioclase containing zonally 

 arranged alteration products and some individuals of pale green 

 augite. The rock in some places shows a brecciated structure, 

 the phenocrysts having broken outlines, while elsewhere may be 

 seen aggregates of epidote individuals, apparently representing 

 inclusions of some highly altered foreign rock. There are also 

 certain forms now outlined in hydrated oxide of iron and filled with 

 decomposition products, highly suggestive of the former presence 

 of olivine. Biotite is noticeable, but is in nearly all cases wholly 

 or partially altered to chlorite. 



Native copper is present in the slides and, from its appearance, 

 seems to be of secondary origin, it borders the plagioclase 

 crystals in narrow strings and also occurs in bunches, running 

 off" in little strings which pass through the cracks. It sometimes 

 occurs filling spaces which were once occupied by olivine. 



The rock was probably a basalt. 



No. 138. — From the Magpie Mineral Claim, lying to the west of 

 the Big Sioux. 



