DIFFERENTIATION OF KEWEENAW AN DIABASES 437 



the aplites of the Cobalt area may be due to the tact that the magma 

 from which they separated was on the whole more basic. The 

 ophitic texture of the diabases indicates a magma early saturated 

 with calcium. The alkalies, being in small quantity, were mostly 

 left over until the end of the crystallization period and then united 

 with the remaining aluminium and silica to form potassium or sodium 

 feldspars, while the small excess of silica occurring in a few places 



Fig. 4. — Acid dikes filling columnar joint fractures in diabase. 



went to form quartz. It is thus assumed that the magma became 

 saturated with the more basic materials first, and the remaining acid 

 materials, still liquid, were in some cases crowded toward the lower 

 and central portion of the mass to escape into the fissures when 

 opened, and form dikes. 



In the case of the pegmatite dike in which the feldspar is largely 

 calcic and occurs with the silica, it is probable that the excess of 

 magnesium and iron caused the rocks to become saturated with 



