xv The Continental Area 285 



now dotted by a series of small salt lakes under the eastern 

 slope of the Sierra Nevada. The soil of the Great Basin 

 is encrusted with borax and other alkaline salts deposited 

 by the shrinking lakes. In recording their researches on 

 this region, the officers of the United States Geological 

 Survey have produced a series of the most fascinating 

 memoirs on physical geography. The volumes on the 

 exploration of the Colorado River by Major Powell, and on 

 Lake Lahontan by Mr. Russell, are especially interesting. 



366. The Appalachian Mountains, running parallel to 

 the east coast, form a broad chain of moderate height, 

 Mitchell's Peak, 6700 feet above sea-level, being the loftiest. 

 They are true mountains of elevation, the alternate anti- 

 clines and synclines forming parallel ridges and longitudinal 

 valleys, and their rocks are much more ancient than those 

 of the western heights. In the south, Carboniferous strata 

 and coal seams are laid bare in the transverse valleys, and 

 the extension north of the St. Lawrence, in the broad low 

 ridge of the Laurentides, is composed mainly of Archaean 

 rock. The Appalachians, which are sometimes called the 

 Alleghanies, form a complete minor axis, giving the east of 

 North America a short slope to the Atlantic and a long slope 

 westward. The watershed follows the eastern ridge of the 

 chain in the south, and the western ridge in the north ; the 

 Hudson River, however, cuts right across the entire chain. 



367. Mississippi Basin. One great valley, formed by 

 the meeting of the long slopes of the two mountain axes, 

 occupies the whole centre of North America. The southern 

 and northern halves of this valley dip in opposite directions 



FIG. 59. Section of North America on the meridian of '90 W. Vertical scale 

 300 times the horizontal. Sea-level marked O. 



from a broad flat transverse ridge of very slight elevation in 

 48 N. The southern south-sloping half of the valley forms 

 the basin of the Mississippi River. The Mississippi rises 

 on the crest of the gentle transverse slope, and after a 



