128 NEW YORK STATE MUSEUM 



continents and oceans. These principal, still active, belts are 

 the latitudinal (Libbey's) belt and the circum-Pacific belt* The trans- 

 Atlantic belt of Paleozoic time, connecting Europe and the 

 Appalachian geosyncline has become extinct; and new belts that 

 originated later are the longitudinal belts of the Atlantic and Indian 

 oceans. 



Probable Causes of the Worldwide Extension and the Principal 

 Directions of the Precambrian Folding 



We have, in the preceding chapters, described the uniform direction 

 of the Precambrian folding over vast areas; and assuming, as a 

 working hypothesis, that this wide parallelism of the trend lines 

 over these immense tracts indicates a uniform reaction of the latter 

 as units to orogenic forces and thereby proves their character of 

 entities in the framework of the earth; we have acclaimed these 

 large units as primeval or arch-continents. By comparing their 

 location and general form with that of the early Cambrian continents, 

 as well as by considering the character of the Precambrian sediments 

 we have shown that this tentative inference of the continental 

 character of the large uniformly folded areas is well-supported by 

 independent evidence. 



The questions of the cause of this worldwide Precambrian folding 

 and the interlocking one of the significance of the directions that 

 the folding exhibits in different segments of the earth, arise as 

 a natural sequence of the study of the parallelism of the trend lines; 

 but their solution is not of critical importance for the problem of 

 the Precambrian continents; the recognition of the latter being 

 simply based on the grand arrangements of the trend lines as 

 indicating segmental units of ' a continental order of magnitude. 

 Nevertheless, the direction of the Precambrian trend lines is so 

 intimately connected with that of the major axes of extension and 

 the distribution of the arch-continents that it seems proper to inquire 

 into the probable causes of the worldwide Precambrian folding 

 that are suggested by the trend lines; an inquiry that had to be 

 postponed until the folding itself had been traced over the earth 

 and its general trend lines recognized. 



The Precambrian worldwide folding may be due to one or several 

 of three groups of causes. It may have originated as: (i) local 

 folding by terrestrial forces that persisted through immense intervals 

 of time and gradually involved the whole earth; (2) simultaneous 

 worldwide folding by terrestrial forces; (3) worldwide folding by 

 cosmic forces. 



