48 GEOLOGY. 
In harmony with this fact is the occurrence of true coal measures with beds of coal, indicating 
emerged land at that epoch, north of the Colorado, at no great distance from this locality. 
Hence the theory generally received that the formation of the continent began in a nucleus 
about Lake Superior, and that the places of the Rocky and California mountains were, until the 
Tertiary period, occupied by an open sea is proved untenable. 
