REPORT OF THE DIRECTOR 1920-21 75 



in the direction of interpreting the Archean framework of the earth 

 upon which the more modern framework that Suess deciphered has 

 been superposed." 



It is an established fact that the Archean basement complex 

 (representing the Archeozoic era) has undergone not only complete 

 metamorphism but also a worldwide intense folding. The later 

 Precambrian rocks (classed formerly as Huronian, then as Algonkian 

 and more recently as Algoman, Huronian and Algonkian rocks, 

 representing Proterozoic time) have escaped metamorphism and 

 folding in some interior parts of North America but elsewhere have 

 undergone like folding as the Archean basement complex and with 

 few exceptions in the same sense; for example, in the eastern Lauren- 

 tian shield, where both are folded from the southeast. This world- 

 wide folding of the Precambrian rocks stands in striking contrast 

 to the localized folding of the earth crust in all later time. It is 

 this folding that we shall make first the object of our analysis in 

 search for criteria of Precambrian continents. We shall show that 

 this folding exhibits uniform directions over enormous tracts of the 

 earth, thereby indicating primordial masses of the earth crust that 

 responded to the folding agencies as units, and that we, for that reason, 

 consider as fundamental elements of the framework of the earth, 

 corresponding to the continents of later geologic history. 



Before we can enter upon an investigation of the directive lines 

 of these Precambrian fold systems, however, it is necessary to 

 attain clearness as to the actuality of the folds and as to their cause. 

 It is obvious from a perusal of the literature that there exists as yet 

 no consensus of opinion regarding the cause of the universal Pre- 

 cambrian folding. 



In Europe one has pointed to the closely compressed folds of 

 the Precambrian terranes, whose " strikes are tortuous and wavy 

 curves and often subcircular and even angularly broken lines " 

 (Uhlig, 1904, p. 10), such as have been mapped in the Precambrian 

 of Bohemia (Fr. Ed. Suess, 1902) and Sweden (Tornebohm, Holm- 

 quist, 1 9 10) and concluded that this close uniform folding is to be 

 explained by a uniform contraction of the entire earth crust which 

 then had a " fairly homogeneous composition " (Uhlig, 1904, p. 20). 

 Andree (1914, p. 10) and Reyer (1907, p. 156) would invoke, besides 

 the general contraction, the influence of enormous masses of eruptive 

 material as an additional factor. 



In America a certain diversity is still apparent in the literature, 

 as to whether the crumpled structure of the Precambrian rocks 

 actually represents a folding. Thus Barrell (1915, p. 511) has ex- 



