GEOGRAPHICAL DISTRIBUTION 103 
rise, but little zoological information has been 
received. 
2. The Central Region, or vast plain extending from 
the Rocky Mountains on the west, to the Appala- 
chian Mountains on the east. Being watered by 
the Mississippi River and its tributary streams, it 
is commonly known as the great valley or basin of 
the Mississippi. It is for the most part underlaid 
with horizontal strata of secondary limestone, and 
in its eastern and northern parts contains coal 
formations of great extent. Its rivers are of great 
volume and length, and the Mississippi River, rising 
near the western extremity of Lake Superior, and 
running southwards to the Gulf of Mexico, divides 
it into two parts. 
8. The Atlantic Region, extending from the Appa- 
lachian chain on the west, to the Atlantic Ocean. 
This is for the most part a gently sloping plain, ex- 
tending from the base of the mountains to the 
ocean. The northwestern, or more elevated portion, 
is based principally on primary strata, while the 
division nearer the sea is underlaid by horizontal 
cretaceous and tertiary, composed of marls, cal- 
careous clays, and sand. 
As the physical characteristics which we have indi- 
cated, rather than described, are very marked, and dis- 
tinguish a country of very great extent, they probably 
afford as many facts tending to show the influence of 
this class of causes, as are to be found elsewhere; and 
