ON ITS EXTERIOR. VOLCANOES. 391 



and 19 % N. lat.) cuts the axis of the general undulation 

 almost at right angles. ( 534 ) 



The form of a considerable part of the earth's sur- 

 face here indicated, and which was only begun to be 

 made out by careful measurement since 1803, is not to 

 be confounded with swellings of the surface which are 

 found included between two wall-like bounding moun- 

 tain chains, as in Bolivia around the lake of Titicaca, 

 and in Central Asia between the Himalaya and the 

 Kuen-lun. The first of these two last-named swellings, 

 the South American, which at the same time forms as 

 it were the floor of the valley, has, according to Pent- 

 land, a mean elevation of about 12,850 feet above the 

 sea; and the Thibetian, according to Captain Henry 

 Strachey, Joseph Hooker, and Thomas Thomson, an 

 elevation of about 15,000 feet. The wish expressed by 

 me half a century ago, in my very detailed Analyse de 

 1' Atlas geographique et physique du royaume de la 

 Nouvelle Espagne ( xiv.), that my profile of the high 

 plain between the city of Mexico and Guanaxuato might 

 be continued by measurements over Durango and Chi- 

 huahua to Santa Fe del Nuevo Mexico, has now been 

 completely fulfilled. The distance, allowing only one 

 fourth for the inflections, is much more than 1200 geo- 

 graphical miles ; and the peculiar characteristics of this 

 long unregarded feature of the earth's surface (the 

 gentle slope and great breadth of the undulation, i. e. 

 from 240 to 280 geographical miles) are evidenced by 

 the circumstance that the distance between Santa Fe 

 and Mexico, (a difference of latitude of fully 16 20', 

 and a distance about equal to that between Stockholm 

 and Florence,) can be travelled, on the table land, in 



