I36 NEW YORK STATE MUSEUM 



tion of the " pulsations " or alternations of revolutionary and quies- 

 cent periods in the geologic history, now postulated on good grounds 

 by geologists in this country, notably Chamberlin, Schuchert and 

 Ulrich, and Stille (191 1, see Andree p. 83) and others in Europe. 



It is thus the more recent trend of opinion to consider the shorten- 

 ing of the earth's crust as resulting from great compressive forces 

 that originate in the depths of the earth from recrystallization 

 and condensation and that lead to movements of the nature of 

 convection currents in the zone of rock flowage which in their turn 

 carry forward the upper portion of the crust, or the zone of folding 

 and fracture. 



While these movements of the deeper zone are of irresistible 

 power, they are also very slow, but may at times become so extensive 

 and quickened as to lead to the revolutionary periods of earth history. 

 In Precambrian time these revolutions owing to the more rapid 

 contraction of the earth, would seem to have been more extensive, 

 or even world-embracing, and more violent. The result was the 

 worldwide intensive folding, foliation and intrusion of the Precam- 

 brian rocks, which was for the most part carried out in the deeper 

 zone, that of rock flowage; while the later more localized folding 

 is that of the outer zone of faulting and folding. This latter, how- 

 ever, is also due to the carrying forward of the outer crust by the 

 movement of the deeper zone. Observations like those of Holtedahl 

 (1920, p. 23) of the wavelike motion of areas in Scandinavia during 

 Paleozoic time, or the rhythmus observed between transgressions 

 and emergences by Karpinsky in Russia and by Chamberlin, 

 Schuchert and Ulrich in North America, suggest that this process of 

 flowage continues on a smaller or less rapid scale almost perpetually, 

 though with distinct pulsations and changes of direction. 



Applying these views to the character and arrangement of the 

 Precambrian folds and associated features, we infer that there has 

 taken place a broad movement of the material of the zone of rock 

 flowage toward the arch-continents of North America (Laurentia) 

 and Archi- Eurasia from the sites of the Pacific and middle Atlantic 

 oceans, which has led to the orderly arrangement of the Precambrian 

 folds of those continents on a huge scale. 



We have seen in the preceding chapters, first, that there exists 

 evidence indicating a very slow folding process that gradually 

 extended from the coast lines inward, and then that there is still 

 more evidence suggestive of a more rapid folding during periods 

 of revolutions and of more active contraction of the earth crust. 

 A fair conclusion from these two groups of evidence combined would 



