-1859] WRITINGS OF JOSEPH HENRY, 67 



ever an average elevation in the principal part of about 

 4,000 feet, and the lowest notch or pass in the ridge on the 

 eastern side is 5,717 feet above the ocean. Along the 35th 

 parallel the vertical section across the mountain system is 

 considerably greater in width and elevation. The general 

 height above the ocean is at least 5,500 feet, and the lowest 

 pass of the principal ridge is here 7,750 feet. The section 

 of the system between the parallels of 38° and 40° has an 

 elevation of 7,500 feet, and the lowest notch in the principal 

 ridge is 10,032 feet above the level of the sea. From this 

 section, as we pass to the north, the altitude and width 

 decline; and along the parallel of about 47° the mountain 

 base is much contracted in breadth, and has a general alti- 

 tude of 2,500 feet. The lowest pass however of the most 

 elevated ridge of this section is 6,044 feet. We have no 

 definite information as to the mountain base north of this 

 line. It appears however to continue at a lower elevation, and 

 consequently to produce less influence upon the climate of 

 the country to the east of it than the portion within the 

 boundary of the United States. 



From the eastern edge of what we have called the moun- 

 tain system — that is from the foot of the Rocky Mountain 

 chain to the Mississippi river — a space comprising, as was 

 said before, about one-third of the whole breadth of the 

 United States, the surface consists of an extended inclined 

 plain, which slopes eastward to the Mississippi and south- 

 ward to the Gulf of Mexico, having at the greatest elevation, 

 near the intersection of the parallel of 40° and longitude 105°, 

 a height of upwards of 5,000 feet, whence it gradually de- 

 clines to the Mississippi river to about 1,000 feet. At the par- 

 allel of 35° it has very nearly the same elevation; and thence 

 it slopes to the bed of the Mississippi to about 450 feet, and 

 south to the level of the sea at the Gulf of Mexico. This 

 extended plain is traversed by a number of approximately 

 parallel rivers flowing eastward and southward to the Missis- 

 sippi river and the Gulf of Mexico, which have their rise 

 principally in the mountain system, and are chiefly supplied 

 by the melting of the snow and the precipitation of vapor 



