470 ELEMENTARY GEOLOGY 
the water from either side was drained. This lowland condi- 
tion in the centre, was the result of the fact that the moun- 
tain uplift raised the land near the mountains higher than that 
far removed from them. Hence a trough was constructed, slop- 
ing down to the Mississippi valley, from the base of the moun- 
tains on either side. Other valleys, like the St. Lawrence, seem 
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Sketch Map to show the el] 9 
HYPOTHETICAL EXTENSION 6 0 2h sae 
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OF LAND G 
IN THE EARLY TERTIARY 7 81 at SN 
Fic. 263. 
Shading indicates land. 
also to have been determined in earlier ages, and to have main- 
tained their approximate position, with many variations, through- 
out geological times. 
Eocene and Neocene (Tertiary) Geography. — These 
periods, which are often considered to be parts of the 
Tertiary, continued the record of development down 
nearly to the present time. In the west, mountain 
