POSITIVE ELEMENTS OF NORTH AMERICA 393 



epochs of diastrophic activity. During the periods of inactivity the dis- 

 tinctions between heterogeneous elenients become less obvious and may 

 become obscured by extended peneplanation and marine transgression. 



The critical epochs for North America occurred at intervals during the 

 Proterozoic, distinguished only in the Lake Superior region ; during the 

 Silurian and Devonian; at the close of the Paleozoic; at the close of the 

 Cretaceous, and on the Pacific coast during the Jurassic, late Tertiary, 

 and Quaternary. Intervening among these were the Cambro-Ordovician, 

 the lower Carboniferous, and the Cretaceous transgressions, which oc- 

 curred during long periods of quiescence. 



Positive Elements of North America 



Examination of a geologic map of North America shows that there are 

 several areas characterized by tlie presence at the surface of pre-Cambrian 

 rocks which exhibit a schistose structure developed under great pressure, 

 ];)robably at considerable depth. This fact indicates elevation. The sub- 

 jacent masses no doubt have at times subsided toward the earth's center 

 along with the continent as a whole; they may have been depressed rela- 

 tively to adjacent areas to some extent, but the algebraic sum of vertical 

 movements has been upward, and has been positive as compared with 

 other parts of the continent and the neighboring ocean bottoms. Whether 

 they be regarded as horsts or as protrusions resulting from radial elonga- 

 tion, their movement is positive and they may fitly be called positive 

 elements. 



The geologic characteristics of a positive element are deep denudation, 

 an absence of sediments of critical periods, and the corresponding pro- 

 longed duration of the sum of unconformities. Let us attempt to apply 

 these criteria to an analysis of North America. 



The Canadian shield, the protaxis of Dana, which is also called Lau- 

 rcntia, is at once the largest and most readily distinguished positive ele- 

 ment of the continent. Dana described it as V-shaped, but the shallow 

 Hudson bay, which forms the V, is but a small epicontinental sea sub- 

 merging a part of the element. The true boundary may be traced along 

 the Saint Lawrence valley into the deep of Baffins bay, and thence north 

 of the Arctic archipelago (which is scarcely to be separated from Green- 

 land), across the Arctic ocean and back to the mouth of the Mackenzie. 

 Beneath the Cretaceous of western Canada the margin of the element lies 

 hidden. It ranges past lake Winnipeg toward and around the isle Wis- 

 consin, and thence follows the shore of the Paleozoic mediterranean east 

 to the Adirondacks and the Saint Lawrence, 



