-1859] WRITINGS OP JOSEPH HENRY. 65 



The continent of North America is traversed in a north- 

 erly and southerly direction by two extensive ranges of moun- 

 tains — the Alleghany system on the east and the Rocky 

 Mountain system on the west. We give the latter name to 

 the whole upheaved plateau and all the ridges which are 

 based upon it. These two systems separate from each other 

 more widely as we pass northward, and between them is the 

 broad interval which, within the territory of the United 

 States, is denominated the valley of the Mississippi; but in 

 reality the depression continues northward to Hudson's Bay^ 

 and even to the Arctic ocean, giving free scope to the winds 

 which may descend from that inhospitable region. It how- 

 ever may be divided into two great basins, one sloping 

 towards the south, comprising the basin of the Mississippi, 

 and the other sloping to the north, including the basins of 

 Mackenzie's river and of Hudson's Bay, the dividing swell 

 which may be traced along the heads of the streams having 

 an elevation of about 1,200 feet. Our remarks must be prin- 

 cipally confined to the portion of the continent south of the 

 49th degree of latitude. 



The swell of land or watershed, on which the Alleghanies 

 are situated, has an average elevation of at least 3,000 feet, 

 although the ridges and mountains based upon it rise to a 

 much higher elevation. The loftiest point is Clingman's 

 Peak, of the Black Mountains in North Carolina. It has 

 lately been measured by Prof. Guyot, and is found to have 

 a height of 6,702 feet. The next greatest elevation is Mount 

 Washington, the highest peak of the White Mountains, in 

 New Hampshire, which, according to the same authority, 

 has an elevation of 6,285 feet. The lowest depression in this 

 watershed, with the exceptions to be next mentioned, is in 

 Pennsylvania, and has an elevation of a little less than 

 2,000 feet. Further north the whole system is cut through 

 by the valley of the Hudson nearly to its base, and also by 

 the valley of the St. Lawrence. The latter, together with 

 the basins of Lakes Ontario and Erie, forms a narrow trough 

 between the Atlantic and the Mississippi valley, along which 

 the flow of air may locally affect the climate. The position 

 6-2 



