REPORT OF THE DIRECTOR I92O-2I 11/ 



From our survey of. the Precambrian folding we have arrived 

 at the distinction of similar units for Precambrian time namely, 

 (1) the North American-Greenland unit, (2) the Eurasian unit, 

 (3) the African- Indian- Australian unit. South America remained 

 undefined as a unit through lack of evidence of original Precambrian 

 folding, but the Paleozoic folds of the east coast, if posthumous in 

 character, suggest its relation to the African-Indian unit. 



We shall now compare these units, which we have considered as 

 Precambrian continental units, with the Paleozoic continents in 

 general and the early Cambrian continents in particular. 



Fig. 2 Continents in Lower Cambrian time, after Freeh, Arldt, Bolsche, 

 Schuchert, Ulrich and Holtedahl. 

 A Lower Cambrian transgression of eastern Asia. 



Walcott (1891) has published the first map of North America 

 in Lower Cambrian time, on which the principal fact of the wide 

 extent of the continent and of the presence of two interior con- 

 tinental seas that filled the two long troughs in the east and west 

 was clearly brought out. These invasions did not occur along the 

 continental margins but entered the continental platforms. Schu- 

 chert (19 10) has through the accumulation of many new facts, 

 mainly by Walcott, improved on the first map and Ulrich (19 19, 

 fig. 4) has lately published a map suggesting that the long arms of 

 the sea distinguished by Walcott and Schuchert, were much more 

 restricted as assumed before, and the eastern arm was separated 

 into three different minor invasions. However that may be, all 



