370 POSITIONS OF ANCIENT CONTINENTS 
base of North America a uniform structure consisting of a north¬ 
east system of folds in the east, that however does not abut, as in 
Asia, against the northwest system, but gradually swings into it. 
In Africa large areas of the Pre-Cambrian basement complex 
are exposed in the Sahara Desert and south, throughout central 
Africa, to the Red Sea, and across it into Arabia. In the Sahara 
and in western Africa, as far as the Kalahari Desert, the strike of 
the folds is reported to be uniformly north-south, as far as known 
today, with variations in the east to northnorthwest. Likewise in 
West Australia the direction of the folded areas of metamorphosed 
Pre-Cambrian rocks is north-south with a slight trend to west; 
and, as far as we could find, the northerly direction is also present 
in Madagascar with variations to northeast. It thus appears that 
there was a third immense tract of uniform Pre-Cambrian folding 
extending from West Africa to Australia. 
There are a number of smaller areas of Pre-Cambrian rocks 
known that have not been folded since Pre-Cambrian time. These 
are in Asia, found in Cambodia, Borneo, and India. The latter 
shows in the Arvali Mountains Pre-Cambrian folds striking north- 
northeast (N 36° E) and extending within 65 kilometers of the 
Himalaya Mountains, which, from the north, have overridden the 
Pre-Cambrian Mountain system. Farther south there is also a 
younger group of folds with east-west direction. These Indian 
Pre-Cambrian folds, especially as seen in the Arvali Mountains, 
are entirely* distinct from the Asiatic system and probably are 
referable to the African system. There is likewise in Bohemia a 
Pre-Cambrian complex, striking northeast for the most part, that 
is independent of the Russian plate and the Baikal direction. In 
America, the Colorado mass, exposed in the Grand Canyon and 
south, represents a body of Archean and Proterozoic folds with a 
northeast direction and distinctly outside of the Laurentian system 
of North America. 
Close relation obtains between the Pre-Cambrian continent and 
the Paleozoic ones. From a survey of the Pre-Cambrian folding 
we have arrived at the distinction of similar units for Pre-Cam¬ 
brian 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 
