394 REACTION OF THE INTERIOR OF THE EARTH 



throughout a zone of more than 16 of latitude, from 

 Santa Fe to the city of Mexico, all hypsometrically 

 determined, and the greater part having also their geo- 

 graphical positions fixed astronomically. In looking 

 over this zone we see the surface of the high broad 

 Mexican plain undulate between 5500 and 7000 French 

 feet (5862 and 7460 English). The lowest portion of 

 the route from Parras to Albuquerque is still more 

 than a thousand feet higher than the highest part of 

 Vesuvius. 



This great but gentle swelling ( 539 ), the highest part 

 of which we have now been considering, increases so 

 much in breadth from its tropical portion to the paral- 

 lels of 42 and 44, that the breadth of the " Grreat 

 Basin," west of the Mormons' Salt Lake, is upwards of 

 340 geographical miles, with a mean elevation of 4260 

 feet. It is everywhere very distinct from the rampart- 

 like chains of mountains which surmount it. The 

 knowledge of this form is one of the principal fruits of 

 Fremont's great hypsometrical investigations in 1842 

 and 1844. The swelling belongs to a different and 

 earlier epoch than the comparatively late elevation of 

 the mountain-ranges and systems of varying direction. 

 At the point where, about the 32d parallel of latitude 

 according to the present boundary-determinations, the 

 mountains of Chihuahua enter the western territory of the 

 United States, (the provinces which have been separated 

 from Mexico,) the mountains have already assumed the 

 somewhat indefinite name of Sierra Madre. It is not, 

 however, until near Albuquerque that a decided bifur- 

 cation ( 54 ) takes place. At this bifurcation the western 

 branch retains the general name of Sierra Madre, and 



