662 NORMAN L. BOWEN 



surfaces. One especially fine example lies east of Davidson Lake 

 where a dike about 200 feet wide, cutting the Archaean complex, 

 shows phenocrysts from wall to wall. Only one dike, definitely 

 determinable as such, was seen in the sedimentaries and this was near 

 the basement Archaean. This was of the porphyritic type. The 

 phenocrysts carefully determined in oriented sections are mainly a 

 uniform andesine, Ab^^An^j. A narrow outer rim usually shows 

 zonal growth with zones of labradorite, Ab^^An^^, and andesine 

 alternating. In some cases the phenocrysts contain idiomorphic 

 crystals of olivine near their outer edges. 



The ground-mass consists of plagioclase, augite, olivine, apatite, 

 and iron ores. 



The plagioclase is mainly labradorite sometimes showing zonal 

 growth, with acid andesine forming the outer zones. In a few cases 

 the core is andesine with a zone of labradorite surrounding it and then 

 again andesine as the outer layer. 



The olivine is in small grains, usually very fresh. It is optically 

 negative and therefore belongs to the iron-rich olivines. Most of it 

 appears to have crystallized before the feldspar of the ground-mass. 

 A brownish augite fills interspaces of the ground-mass between the 

 feldspar laths in perfect ophitic structure. Apatite is unusually 

 abundant, in very long needles, which penetrate all the constituents. 

 No quartz or micropegmatite were found. Black iron ores are rather 

 abundant. 



GRANOPHYRE AND RELATED ROCKS 



None of the dikes show evidence of differentiation as far as can 

 be determined in the hand specimen. The sills, however, are not 

 always entirely composed of the dark-gray diabase. In places we 

 often see little pink spots, found to be areas of micropegmatite (quartz 

 and albite). This material may increase in amount until it forms 

 quite the whole of the rock, giving rise to "red rocks" or granophyres. 

 Moreover pink aplitic veins are often numerous in the sills. To the 

 development of these "red rocks" and their relations to the diabase 

 and inclosing sediments attention will now be given. 



DAVIDSON LAKE SILL 



Close to the west shore of Davidson Lake is a sill about 50 feet 

 thick cutting the arkoses of the Middle Huronian (Fig. i). 



