DIABASE OF COBALT DISTRICT, ONTARIO 273 



properties of rhombic pyroxenes; but their composition has not been 

 definitely determined. 



Feldspar. — The feldspars generally show green or gray tints and 

 occasionally there is present a pink-colored variety. The measure- 

 ment of angles of extinction by the Michel-Levy method indicates 

 that much of the feldspar is of highly calcic varieties, and chemical 

 analysis of the rock indicates the same. The pink-colored feldspar is 

 more highly sodic, and in aplitic veins showing no ferromagnesian 

 mineral has the composition of albite. The lime-soda feldspars are 

 rarely fresh, partial alterations being indicated by white opaque 

 portions and small grains and flakes of brightly polarizing minerals, 

 the exact nature of which has not been definitely determined. Small 

 grains of epidote are characteristic components of the aggregate. The 

 sodic feldspars, unlike the calcic, are very free from evidences of 

 decomposition. 



Ilmenite occurs in typical forms showing the skeleton-like arrange- 

 ment, and alteration along crystallographic directions to leucoxene. 

 The powdered mineral is not noticeably attracted by the magnet, but 

 large rock masses influence the magnetic needle. 



Quartz occurs frequently intergrown micrographically with feld- 

 spar, and in the gray diabase is rare otherwise. In the aplitic veins 

 it is often found with the feldspar in typical panidiomorphic structure. 



Biotite occurs frequently as dark-brown pleochroic plates, which 

 are conspicuous in the powdered rock, though their weight is com- 

 paratively insignificant. 



Hornblende. — A green hornblende is a common constituent, 

 especially in specimens which show alterations in the pyroxenes. 

 It is usually in fibrous aggregates and is probably secondary. Some 

 few specimens in which pyroxenes are quite fresh show well-formed 

 green hornblende which is probably primary. A partial analysis 

 of the red rock from James Township (Anal. 4) showed less than i per 

 cent total alkalies, and as part at least of this was due to incomplete 

 separation from feldspars, the hornblende in this sodic portion is not 

 an alkali variety. 



Olivine is typically absent in the large diabase masses but occurs 

 as yellowish-green grains in varieties containing no micropegmatite. 

 In small intrusives olivine is a common constituent. 



