TIME AND CHANGE 
tent, have gone down and have not yet come up, if 
they ever will. The great Mississippi Valley was 
under water and above water time after time during 
the Paleozoic period. The last great invasion of 
the land by the sea, and probably the greatest of all, 
seems to have been in Cretaceous times, at the end 
of the Mesozoic period. There were many minor in- 
vasions during Tertiary times, but none on so large 
a scale as this Cretaceous invasion. At this time a 
large part of North and South America, and of Eu- 
‘ rope, and parts of Asia and Australia went under 
the ocean. It was as if the earth had exhaled her 
breath and let her abdomen fall. The sea united the 
Gulf of Mexico with the Arctic Ocean, and covered 
the Prairie and the Gulf States and came up over 
New Jersey to the foot of the Archean Highlands. 
This great marine inundation probably took place 
several million years ago. It was this visitation of 
the sea that added the vast chalk beds to England 
and France. In parts of this country limestone beds 
five or six thousand feet thick were laid down, as well 
as extensive chalk beds. The earth seems to have 
taken another hitch in her girdle during this era. 
As the land went down, the mountains came up. 
Most of the great Western mountain-chains were 
formed during this movement, and the mountains of 
Mexico were pushed up. The Alps were still under 
the sea, but the Sierra and the Alleghanies were 
again lifted. 
110 
