2 74 REGINALD E. HORE 



Chemical composition of the diabase. — The examination of expo- 

 sures in all parts of the district proves that most of the diabase is of 

 the medium-grained gray type and the microscopic examination shows 

 that there is great uniformity in the mineral composition of this rock. 

 Chemical analysis of specimens from one locality may therefore be 

 taken as typical of many square miles of the diabase. The following 

 analyses (Nos. i, 2, and 3) are of specimens taken from the diabase 

 in the silver-producing area at Cobalt. All three specimens were 

 taken a few inches from the surface and in them are evidences of 

 alteration — especially in the feldspars. In reddish portions of the 

 diabase, analysis (see No. 4) shows a higher percentage of silicon 

 and a marked increase in sodium, and microscopic examination of 

 such specimens shows a higher percentage of feldspar-quartz inter- 

 growths and less of pyroxene. 



Diabase dikes. — In the diabase there are dikes and veins of various 

 types. The darker-colored dikes are usually very fine grained, dis- 

 tinctly ophitic in texture, high in content of iron oxides, and frequently 

 show olivine. They vary in width from a few inches to several feet. 

 Dikes more than one hundred feet wide are usually very similar in 

 composition to the larger masses. A comparatively small number of 

 the dark-colored dikes show little or no olivine and are characterized 

 by large phenocrysts of white or gray plagioclase among which lab- 

 radorite has been recognized. 



The sodic aplitic veins.- — Lighter-colored dikes and veins are in 

 some cases composed almost entirely of sodic feldspars and quartz. 

 Most of the other fillings have quartz or calcite as the chief mineral, 

 and where both are present the quartz is distinctly older than the 

 calcite. These light-colored fissure-fillings are in many cases very 

 irregular in shape, but are generally less than three feet in width. 

 They have various marginal characters. Some are not well marked 

 off from the diabase, grading into it by an interlocking of crystals 

 that leaves no definite contact, thus closely resembling the " contem- 

 poraneous veins" of Teall and Geikie. Others show decided lines of 

 demarkation and a few have a soft green aphanitic selvage. In James 

 Township there are numerous such aplitic veins, usually but a few 

 inches in width and commonly gray, flesh-colored, or greenish gray. 

 The former are chiefly composed of feldspar and quartz, while the 



