86 NEW YORK STATE MUSEUM 



This northwest and north direction of the Precambrian rocks 

 in the western part of the Canadian shield, can, like the east-west 

 direction of the middle part, be traced southward into the middle 

 of the continent; for we find in the Archean basement complex of 

 the Ozark mountains in Missouri a N 50 W strike (Van Hise & 

 Leith, p. 734), in that of the Black Hills (ibid., p. 728, 732, 733; see 

 also Sundance Folio) a northwest to north direction; farther 

 south in Oklahoma a west-northwest ■ — east-southeast direction 

 (ibid., p. 738), and in the important inlier of Precambrian (Archean 

 of earlier writers, probably Algonkian according to Paige) rocks in 

 Burnet county, Texas, above Austin the folds form two major anti" 

 clinal axes that have a general northwest-southeast trend (Sidney- 

 Paige, 1912; Comstock, 1891, p. 5 5 3 ff ; Suess v. 3, pt 2, p. 82; Van 

 Hise & Leith, 1909, p. 746). 



vSuess (v. 3, pt 2, p. 284) defines Laurentia as comprising not 

 only the Canadian shield but also all the Precambrian basement 

 complex on which the Paleozoic rocks rest in flat position. He thus 

 includes Greenland and possibly the North Atlantic and extends 

 Laurentia to its " natural boundaries " which are, in his view, the 

 Caledonian and Appalachian mountains in the east, the Rocky 

 mountains in the west and the United States range in the north. 

 Although he cites the n~ountain ranges of Oklahoma as western con- 

 tinuations of the Appa 1 achians (western Altaid system of moun- 

 tains), he includes the inlier in Burnet county into Laurentia, con- 

 sidering it as a monadnock of the old abraded Precambrian surface ; 

 and he thus extends Laurentia southward as far as 300 n. I., 

 suggesting (ibid., p. 282) that also the Colorado plateau may repre- 

 sent a part of Laurentia. 



Suess (ibid., p. 290) also held that Laurentia, although a very ancient 

 unit that acts as a block or foreland against all younger folds, has 

 no uniform structure, but, as shown in the Lake Superior region, 

 is cemposed of various systems of folds, unconformities and trans- 

 gressions. With the later evidence here brought forward in regard 

 to the folded structure of Laurentia, it seems to us to possess a 

 remarkably uniform structure, the entire system of Precambrian 

 folds forming a single curve of northeast folds in the east, of east- 

 west folds in the middle and northwest folds in the west. This is 

 clearly true of the Canadian shield and can be traced through the 

 middle of the continent to the southern extremity of Laurentia, 

 as defined by Suess. 



We will now turn to the eastern and western boundaries of Laurentia, 

 in whose determination Suess agrees with the earlier conception 



