1922] Whitman: Genesis of the Ores of the Cobalt District 275 



undulations, however, is dubious, being possible only if the magma 

 advanced slowly. In any case, a secondary warping of the surface 

 of injection in the first period of heat deformation would probably 

 have been parallel with the chief structural axes and coincident with 

 the original undulations of the diabase sheet, since those undulations 

 represented directions of maximum length of the sheet, and axes of 

 minimum resistance to folding. 



In the first heat period, deformation must have been caused chiefly 

 by reaction against the lateral expansive force generated by the dis- 

 tension of the wall-rocks. It would have behaved in every way as 

 an external compressive force exerted parallel with the major struc- 

 tural axes ; and the folds induced by it in the diabase and wall-rocks 

 must have possessed the usual characteristics of folds due to com- 

 pression. But this folding would have deformed the plastic diabase, 

 and would therefore have left no record in it. When the temperature 

 of the immediate walls had become approximately the same as the 

 interior of the sheet, expansion may, perhaps, be said to have attained 

 its maximum. At that juncture the walls would have reached their 

 maximum content of heat, while the diabase would, some time since, 

 have been undergoing contraction. This condition must have pro- 

 duced true cooling cracks in the diabase ; and some of these might 

 have formed avenues of escape for small quantities of aplitic and 

 pegmatic material. This in fact appears to have occurred since the 

 few vein-dikes of this material which have been found are in fissures 

 which seem to have, as a rule, erratic orientations. 



From this point onward the sole influence in diabase and wall- 

 rocks would be contraction. Not only would an arch or sag in the 

 heated zone tend to draw in upon itself, matching its tensile strength 

 against its own shearing strength, but the shrinkage of the entire 

 length and breadth of heated rock would have borne in upon the 

 regions of yielding, consisting of the initial undulations, tending to 

 accentuate them by folding. This period of deformation would have 

 left its traces in the diabase because this would have been solid and 

 resistant. 



The foregoing suppositions are offered as an explanation of the 

 observed fact that the folding of the district, affecting both the 

 adjacent sediments and the diabase itself, developed innumerable large 

 and small surfaces of shearing in both formations parallel with the 

 surfaces of folding, which in their turn paralleled the original undula- 

 tions of the diabase sheet. This phenomenon has been observed also 

 to some extent in the South Lorrain and Casey areas. 



