DIABASE OF COBALT DISTRICT, ONTARIO 277 



found. The pink rock has not a composition intermediate between 

 the gray diabase and the intruded shales. It seems probable therefore 

 that the variations in composition are the natural consequence of the 

 cooling of a molten magma whose composition was near that of the 

 gray diabase, except that the latter does not contain the same propor- 

 tion of volatile constituents. 



The aplitic veins seem to have been formed by the last secretions 

 from the diabase magma, filling fissures which developed after most 

 of the mass had solidiiied, while the pink-colored diabase and gabbro 

 masses represent the solidification from the molten magma at an 

 intermediate stage in a process of differentiation. 



The irregular pink-colored patches and streaks were probably 

 intruded earlier into hot but viscous portions of the cooling masses. 

 In these cases it appears that there has been differentiation in the 

 diabase magma but not in situ. Some other pink portions of the 

 thick sills are probably the result of differentiation in situ. 



ROCKS OF THE NIPISSING SILVER FIELDS 



Cenozoic Recent: Clay, marl, peat. 



Pleistocene (i) Coarse unstratified material — sand, gravel, and 

 bowlders; (2) Stratified clay with some sand. 

 Great unconformity 

 Paleozoic Silurian: Grey limestone with some interbedded greenish 



shales, and at the base an arenaceous conglomerate. 

 Correlated with Niagara of New York state. 

 Great unconformity 

 Algonkian Keweenawan: Igneous intrusives only. Chiefly quartz diabase and 

 quartz gabbros with acid differentiation products. 

 Some olivine diabase and diabase porphyrite dykes. 

 Igneous contact 

 Huronian: Sedimentary rocks only. 



(i) An upper series. Probably equivalent to Middle 

 Huronian of Lake Superior. Chiefly feldspathic 

 quartzite with some conglomerate. 

 Slight unconformity 



(2) A lower series. Probably equivalent to Lower 

 Huronian of Lake Superior. Chiefly greywacke, 

 shale, conglomerate, and feldspathic quartzite. The 

 conglomerate pebbles are mostly of holocrystalline 

 igneous rocks, the matrix greywacke and grey shale. 



