I' 



ijls 



EFFECTS OF TANGENTIAL PRESSURE 405 



limit of effective plastic flow, which should correspond with the depth of 

 the zone of isostatic adjustment, 71 to 100 miles, as stated by Hayford 

 in the article already cited. The mechanical effects of the pressure, as 

 we observe them in masses which were not very deeply buried when dis- 

 placed, are seen in numerous shearing planes on which, under appro- 

 priate stress, the masses rose as on an inclined plane. The parallel 

 mechanical effects in the depths beyond our observation, I conceive to be 

 shearing planes or their equivalents, planes of incipient displacement, on 

 which any part of the mass moves northwestward and upward when ade- 

 quate pressure is exerted from the sub-Atlantic region. Such a pressure 

 is, I postulate, now exerted, and the combined effect of very deep-seated 

 displacements is seen in the present doming of the Cretaceous peneplain. 

 Prior to the Appalachian revolution this condition did not, however, 

 exist in the subcontinental section which is now affected. 



The preceding discussion of shearing planes suggests why the surface 

 of the eastern United States is now elevated and, per contra, why the 

 region was not similarly affected before the Appalachian revolution. 

 There is another line of reasoning along which the same conclusion may 

 be reached. The ancient eastern coastal region of Appalachia, from the 

 latitude of New York to the Bahamas, lies beneath the Atlantic ocean 

 beyond our ken; but its homologue, the New England-New Brunswick- 

 Newfoundland zone, the eastern coast belt of Laurentia, is well known. 

 The latter has been intensely folded by Atlantic pressure, which was, 

 however, absorbed in the process and was not effective in the adjacent 

 districts of Laurentia. By analogy we may reason that Atlantic pressures 

 were taken up by an eastern coastal zone of Appalachia during the Pro- 

 terozoic and Paleozoic eras and the region of the mediterranean province 

 remained free from tangential stress for that reason. 



The effects of pressure of the sub-Pacific mass upon North America 

 are less well known than those of the Atlantic and they appear to be 

 more complex. The Great basin and eastern British Columbia lie within 

 the Pacific positive element, which separates them from the Pacific, pre- 

 cisely as the Appalachian folded zone lies within Appalachia. In neither 

 case is the folded zone the immediate margin of the continent. The 

 analogy of position requires a similar mechanical effect on the Pacific 

 side as on the Atlantic, and one who accepts the evidence of the displace- 

 ment of Appalachia can not well escape the inference that the Pacific 

 positive element was likewise driven in during the movements that led 

 to the folding of the Paleozoic of the Eocky Mountain trough. 



At that date the eastern limit of the folded zone, which we may take 

 as marking the effective radius of Pacific thrust, was the Wasatch dis- 



XXXV— BnLL. Geol. Soc. Am., Vol. 18, 1906 



