THE OROGENIC EPOCHS IN NORTH AMERICA 639 



so mild that on the Mesabi Range and in Thunder Bay the Upper 

 Huronian beds are still only gently inclined. Westward the 

 Penokean orogenic disturbance can be traced with some confidence 

 to the Cuyuna Range of central Minnesota, and southward perhaps 

 as far as the Baraboo district of southern Wisconsin, where rocks 

 usually referred to Middle or Upper Huronian are highly folded. 

 Eastward it extended an unknown distance into Ontario. 



In reference to the late Algonkian orogeny, as to its predeces- 

 sors, it is not yet possible to trace the extension of the folding much 

 beyond the Lake Superior district because of the present uncer- 

 tainty as to the ages of the strata. It is worth noting, however, 

 that rocks believed to be of late Algonkian age, more or less 

 strongly folded before the deposition of the Paleozoic beds, exist in 

 the Black Hills of South Dakota, in Colorado, Montana, Utah, 

 Arizona, and perhaps other western states and Canadian provinces. 

 These beds are now so generally covered by younger rocks, that no 

 continuous system of mountain folds has been worked out. The 

 two Algonkian epochs of folding in the Rocky Mountains have 

 been discussed by S. F. Emmons.' 



Taconic orogeny {late Ordovician). — Because its effects are most 

 evident in New England, the "Taconic revolution" was recognized 

 more than fifty years ago by that group of geologists among whom 

 Dana was ever a leader. Even today it would be difficult to 

 improve upon the description, given in the early editions of Dana's 

 Manual of Geology, of this signal event in the history of eastern 

 United States. Because of its relation to fossiliferous strata, it is 

 the first of the deformational epochs of which the age can be 

 determined with a high degree of accuracy in the geologic scale and 

 traced with confidence over a large area. Its effects in closely folded 

 and even metamorphosed strata have been traced from the vicinity 

 of Quebec (city) in Canada south through Vermont and western 

 Massachusetts to New York City. Strong suggestions that it 

 extended southwestward as far as Virginia have already been 

 cited by Dana and others, but it is doubtful whether we shall soon 

 know how far in that direction the crumpling was felt. The 



' S. F. Emmons, "Orographic Movements in the Rocky Mountains," Bull. Geol. 

 Soc. Am., I (1890), 245-86. 



