638 ELIOT BLACKW ELDER 



pronounced the most important stratigraphic break in the pre- 

 Cambrian sequence. The emphasis thus placed on the mid- 

 Huronian unconformity is due to the consideration of only a limited 

 belt north of Lake Superior. When a much larger region is exam- 

 ined, this glorification of the mid-Huronian orogeny appears 

 unwarranted. The nearly parallel relations of all the Huronian 

 series south of Lake Superior proves that there the rocks were not 

 folded in the mid-Algonkian. Recently Leith' has reviewed our 

 knowledge of the mid-Huronian unconformity, showing that a con- 

 spicuous angular discordance between sedimentary beds of pre- 

 Cambrian age has been found farther east in the Sudbury and 

 Cobalt districts of Ontario, and on into Quebec. It is not certain 

 that it is the same unconformity and that it is of mid-Huronian age 

 in all of these localities, but it seems within the bounds of proba- 

 bility that such a correlation may be established in the future. 



The lack of fossiliferous strata of the ages involved still prevents 

 us from correlating the Algonkian sections of widely separated parts 

 of the continent. Were it not for this fact, we might be able to 

 ascertain in large measure the real extent of the Mesabian folding. 

 Suggestively similar relations are found in the Rocky Mountains 

 from Wyoming to Arizona, for in those states comparatively 

 unaltered late Algonkian rocks rest upon highly contorted and 

 metamorphosed sedimentary beds usually referred to the early 

 Algonkian. The outcrops of these folds are largely covered by 

 later strata and in isolated exposures the structural lines do not 

 seem to follow a single general direction. 



Penokean orogeny {late Algonkian =(?) post-Keweenawan) . — - 

 Again the Lake Superior region supplies the standard of reference. 

 After the extrusion of the Keweenawan basalts overlying the essen- 

 tially undeformed Upper Huronian strata, the region immediately 

 south of Lake Superior was compressed into folds trending nearly 

 east and west. This apparently produced the Lake Superior 

 synclinorium. In parts of northern Michigan the folding was 

 intense, leaving the strata quite schistose and in nearly vertical 

 attitude. Large bathoKtic intrusions accompanied the folding. 

 On the north side of Lake Superior, however, the deformation was 



^ Proceedings of the Internal. Geol. Congress, Toronto, 1913. 



