66o NORMAN L. BOWEN 



wide axial angle. The slide presents no difference whatever from 

 the diabase of the Cobalt area. Biotite, black iron ores, and apatite 

 are the accessory minerals. Small areas of micropegmatite of quartz 

 and an indeterminate feldspar are always present. 



GABBRO 



In places the diabase has moderately coarse phases with augite 

 in stout prisms showing one perfect cleavage face, the diallagic part- 

 ing, which determines the fracturing of the rock. The cleavage face 

 is nearly always bent, sometimes into a considerable arc. This bend- 

 ing is a constant character of the augite of the coarse phase from 

 widely separated points. Under the microscope this phase shows a 

 nearly simultaneous crystallization of augite and plagioclase, the 

 feldspar in broad areas generally inclosing the augite. 



The feldspar is an acid labradorite, Ab^^An^^, approximately that 

 of the outer zones of the crystals of the normal diabase. Some zonal 

 growth was shown in a few examples, the outer zones being slightly 

 more acid. 



The pyroxene is augite throughout, with cleavage parallel to loo 

 and a lamellar structure parallel to the base. Enstatite is absent. 

 The augite has- often gone over, partly, to uralite. Both augite and 

 plagioclase are in stout prisms of about 3 mm. average length. There 

 is no evidence of granulation of any of the constituents, so the bending 

 of the augite must be attributed to disturbance during crystallization. 

 A little iron ore occurs, and moderately coarse micropegmatite inter- 

 stices in small amount. The feldspar of these could not be deter- 

 mined. Where micropegmatite is in contact with iron ore and augite, 

 secondary biotite has sometimes been built. The rock is a gabbro, 

 near augite diorite. 



No definite relation of the gabbro to the sill boundaries could be 

 made out. There is usually a gradual passage from diabase to 

 gabbro, but in some cases small dikelike masses of the gabbro were 

 found in diabase. The gabbro probably represents the more slowly 

 crystallized, slightly more acid parts of the sills. This phase is well 

 developed in the area west of Logan Lake. In places in this area the 

 gabbro becomes very coarse, with pyroxenes up to three inches in 



