EFFECTS OF TANGENTIAL PRESSURE 407 



ered. There is a folded zone in Grantland extending from Lincoln sea 

 to the Sverdmp islands and involving Triassic and older rocks.* Beyond 

 the fact that it is there, we know but little about it. In Alaska the 

 mountains north of the Yukon plateau are composed of moderately 

 folded Paleozoic and Mesozoic sediments,! the structure and stratigraphy 

 of which are but imperfectly known. 



But though there is 3fet much to learn of the striicture of Arctic North 

 America, we may nevertheless compare it with northern Siberia as a 

 region in which tangential pressure has been but feebly exerted, at least 

 since the beginning of the Paleozoic. The Arctic ocean being but a rela- 

 tively small and not exceedingly deep basin, this result is consistent with 

 the h3rpothesis that the pressures were exerted from it upon the adjoin- 

 ing positive elements. 



In the compressed folds of the iron ranges, the Lake Superior region 

 offers a problem of tangential pressure which is rendered obscure by 

 antiquity and isolation. If we consider it by itself, we may say that 

 during the Huronian period the zone between Isle Wisconsin and Lau- 

 rentia was alternately the scene of deposition, folding, and erosion 

 until three thick, distinct series had accumulated. These phenomena 

 Avere accompanied and followed by intrusive and extrusive igneous 

 activity. The region has since then been very deeply eroded and rem- 

 nants of the deepest troughs alone contain the sediments. The trend 

 of the folds shows the dominant north-south direction of pressure; but, 

 so far as the local conditions go, there is no evidence as to whether the 

 applied thrust was exerted toward the south or toward the north. 



There is a broader vicAv of the northern regions, which gives the events 

 of Huronian time a setting in a worldwide phenomenon : the grouping 

 of vast areas of highly schistose, so-called Archean rocks about the Arctic. 

 The Canadian protaxis, Siberia, Eussia, and Greenland, constitute masses 

 of great magnitude, which are composed chiefly of very ancient gneisses 

 and schists, that had been forced from gieat depths to the surface before 

 the Proterozoic era approached its end. Their rising was in part deter- 

 mined probably by isostatic adjustment, but the condition of the rocks 

 shows that in the depths from which they have come they were subjected 

 to intense tangential stress. That this stress was exerted in a general 

 way from south toward the north we may infer from the following con- 

 siderations : We have seen that there is reason to conclude that those por- 



* Sverdrup : The New Land. 



t A reconnaissance in northern Alaska, across the Rocky mountains, by F. C. Sclirader. 

 Professional paper no. 20, U. S. Geological Survey, 1904. Geography and geology of 

 Alaska, by A. H. Brooks. Professional paper no. 45, U. S. Geological Survey, 1906, pi. 

 xxvii. 



