GEOGRAPHICAL DISTRIBUTION. ^QS 



rise, but little zoological information has been 

 received. 



2. The Central Region, or vast plain extending from 

 the Rocky ISIountams on the -west, to the Appala- 

 chian Mountains on the east. Bemg watered by 

 the JSIississippi 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, rismg 

 near the western extremity of Lake Superior, and 

 running southwards to the Gulf of Mexico, divides 

 it into two parts. 



3. 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 



