448 ELEMENTARY GEOLOGY 
The sea occupied a part, and at times possibly all of 
the area of the Mississippi valley between these three 
land boundaries. It is certain that a few islands existed 
in this sea during a part of its existence. One of these 
was in central Missouri, another in central Texas. Since 
the greater part of the Mississippi valley is occupied by 
Map showing distribution of ey 
KNOWN ARCHEAN ROCKS 
WITH HYPOTHETICAL LAND AREAS 
! 
Seerenne 
OT 87 “77 
Fic. 261. 
rocks of later age than the Archean, the record of the 
history of this ancient period in this part of the coun- 
try is buried too deeply to be read at present. 
Concerning the Canadian and western lands little is 
now known, for geological exploration has not been 
carried on extensively there We do know that even 
then these were mountainous lands. 
