TRUE VOLCANOES. 383 



La Puebla de los Angeles (lat. 19° 0' 15"), 7201 feet, Ht. 



(The village of Las Vigas marks the eastern extremity of 

 the elevated plain of Anahuac, lat. 19° 37 / ; the height of 

 the village is 7814 feet, Ht.) 



Thus, though previous to the commencement of the 19th 

 century, not a single altitude had been barometrically taken 

 in the whole of New Spain, the hypsometrical and in most 

 cases also astronomical observations for thirty-two places in 

 the direction from north to south, in a zone of nearly 16^° 

 of latitude, between the town of Santa Fe and the capital of 

 Mexico have been acomplished. We thus see that the surface 

 of the wide elevated plain of Mexico assumes an undulating 

 form, varying in the centre from 5850 to 7500 feet in height. 

 The lowest portion of the road from Parras to Albuquerque 

 is even 1066 feet higher than the highest point of Vesuvius. 



The great though gentle* swelling of the soil, whose high- 

 est portion we have just surveyed, and which from south to- 

 north, from the tropical part to the parallels of 42° and 44°, 

 so increase in extent from east to west that the Great Basin, 

 westward of the great Salt Lake of the Mormons, has a di- 

 ameter of upward of 340 geographical miles, with a mean 

 elevation of nearly 5800 feet, differs very considerably from 

 the rampart-like mountain chains by which it is surmounted. 

 Our knowledge of this configuration is one of the chief points 

 of Fremont's great hypsometrical investigations in the years 

 1842 and 1844. This swellins; of the soil belongs to a dif- 

 ferent epoch from that late upheaval which we call mountain 

 chains and systems of varied direction. At the point where, 

 about 32° lat., the mountain mass of Chihuahua, according 

 to the present settlement of the boundaries, enters the western 

 territory of the United States (in the provinces taken from 

 Mexico), it begins to bear the not very definite title of the 

 Sierra Madre. A decided bifurcation,! however, occurs in 



* Compare Fremont, Report of the Exploring Exped. in 1842, p. GO; 

 Dana, Geology of the United States Expl. Exped., p. 611-613; and for 

 South America, Alcide D'Orbigny, Voy. dans V Am'crique Mcrid., Atlas, 

 pi. viii., De Geologie spiciale, fig. 1. 



f For this bifurcation and the correct denomination of the east and 

 west chains see the large special map of the Territory of Neiv Mexico, 

 by Parke and Kern, 1851 ; Edwin Johnson's Map of Railroads, 1851 ; 

 John Bartlett's Map of the Boundary Commission, 1851; Explorations 

 and Surveys from the Mississippi to the Pacific in 1853 and 1851, vol. i., 

 p. 15 ; and, above all, the admirable and comprehensive work of Jules 

 Marcou, Geologist of the Southern Pacific R. R. Survey, under the 

 command of Lieutenant Whipple, entitled Resume explicatif cVune Carte 

 Geologique des Etats Unis et d'un Profil Geologique allant de la Vallce du 



