196 C. SCHUCHERT THE XORTH AMEEICAX GEOSYXCLIXES 



POLYGEOSYXCLIXES 



These are the more or less wide and longer- en during bnt shallow- water 

 primary geos}Ticlines, which give rise to one or more parallel geanticlines 

 and to two or more sequent geosynclines with shorter histories. In other 

 words, they are primary and greater geosynclines within a continent and 

 situated along the inner sides of borderlands, but evolving into two or 

 more sequent geosynclines separated by geanticlines. The typical ex- 

 ample is the Cordilleran primary trough, out of which have arisen the 

 Cordilleran Intermontane and Ancestral Rocky Mountains geanticlines 

 and the subsiding Pacific and Rock}' Mountain sequent geos}'nclines. 

 They may be kno^^Ti as the poly geosynclines, the prefix suggesting that 

 .several geosynclines are combined. 



Another fine example is the Andean polygeosyncline. This great geo- 

 syncline, in its size and complex history, reminds one much of the Cor- 

 dilleran trouo-h, and it was in the makino- throuo-hout the Paleozoic and 

 Mesozoic. To the west of it lay a wide borderland, first determined by 

 Burckhardt^^ for Jurassic time and fractured into the depths of the Pa- 

 cific during the Cenozoic. This downfracturing was especially active 

 during the Pleistocene and it was in keeping with the very marked up- 

 faulting of the Andes, raising the late Cenozoic peneplain at least 10,000 

 feet into the Alta Plana of present Bolivia. To the east of the Andean 

 polygeosA'ncline is the Brazilian shield, the nucleus or neutral area of 

 South America, and over its western part occasionally spread in earlier 

 Paleozoic time the waters of the western seas. As in Xorth America, the 

 thickest deposits are in the west nearest the borderland. 



It appears that the width and degree of geologic complexity in the 

 structure of the geos}Ticlines are dependent on the magnitude of the near- 

 est ocean. In other words, the greater the ocean, the wider the trough. 



In eastern Xorth America the comparatively narrow Appalachian 

 nionoo'eosvncline ceased to subside in the Pennsvlvanian, and durinof the 

 Permian the folded structure of the Appalachian polygenetic mountains 

 was completed. Xever again was this region subject to orogeny, and the 

 width of the folded tract is about 450 miles. The region has, however, 

 been arched several times since. 



The orogenic region of western North America is, on the other hand, 

 not less than 1,100 miles wide — that is, from eastern Colorado to Cali- 

 fornia — and in this area have originated, inside of the borderlands, three 

 geosynclines and two geanticlines. AVe are therefore led to believe that 



" C. Burckhardt : Beitrage zur Kenntniss der Jui-a- und Kreideformation der Cordillere. 

 -Palajontogiapbica, vol. 50, 1900, pp. 1-144. 



