POSITIONS OF ANCIENT CONTINENTS 371 
original Pre-Cambrian folding, but the Paleozoic folds of the east 
coast, if posthumous in character, suggest its relation to the Afri- 
can-Indian unit. 
We shall now compare these units, which we have considered 
as Pre-Cambrian continental units, with the Paleozoic continents 
in general and the early Cambric continents in particular. 
Walcott (1891) has published the first map of North America 
in Early Cambric time, on which the principal fact of the wide 
extent of the continent and of the presence of two interior conti¬ 
nental 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 (1910) has through the accumulation of many new facts, 
mainly by Walcott, improved on the first map, and Ulrich (1910) 
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 
these authors agree, according to their maps, in the assumption 
of a former greater extent of the continent in early Cambric time. 
Schuchert states that “from the extent and position of the Lower 
Georgic invasions, it is inferred that the North American continent 
was larger during the late Proterozoic than at any subsequent 
period.” There is no doubt that North America extended not only 
considerably to the east of the Atlantic states in “Appalachia,” but 
also in Arcadia, and especially in the far northeast, where it 
reached over Greenland to Europe. Holtedahl’s map extends the 
Early Cambric continent over Iceland, around Scotland, to south¬ 
ern Norway. In the west the continent also reached beyond the 
boundaries of the present continental mass, as is evidenced by the 
absence of massive Cambric deposits in the costal regions. 
In his “Paleogeography of North America,” Schuchert has 
repeatedly pointed out that the Pre-Cambrian era closed with a 
great continental elevation. He states, “Laurentide revolution.— 
This was one of the ‘critical periods’ of the earth when the seas 
were withdrawn from North America for a very long time. During 
this interval, of which only the later or eroding portion is known, 
the continent was larger than at present, possibly as great as at the 
close of the Paleozoic, or even greater than at that time.” 
