TRUE VOLCANOES. 297 



the line of inclination of the declivity, and consequently more 

 or less impede the decrease of heat in the superposed strata 

 of the atmosphere. On descending from the central plateau 

 of Mexico (whose mean elevation is 7460 feet), to the corn- 

 fields of Valladolid de Michuacan, to the charming lake of 

 Patzcuaro, with the inhabited islet Janicho, and into the 

 meadows around Santiago de Ario, which Bonpland and I 

 found adorned with the dahlias which have since become so 

 well known, we have not descended more than nine hundred 

 or a thousand feet. But in passing from Ario, on the steep 

 declivity over Aguasarco, into the level of the old plain of 

 Jorullo, we diminish the absolute elevation in this short dis- 

 tance by from 3850 to 4250 feet.* The roundish, convex 

 part of the upheaved plain is about 12,790 feet in diameter, 

 so that its area is more than seven square miles. The true 

 volcano of Jorullo and the five other mountains which rose 

 simultaneously with it upon the same fissure are so situated 

 that only a small portion of the Malpais lies to the east of 

 them. Toward the west, therefore, the number of hornitos 

 is much larger, and when in early morning I issued from the 

 Indian huts of the Playas de Jorullo, or ascended a portion 

 of the Cerro del Mirador, I saw the black volcano projecting 

 very picturesquely above the innumerable white columns of 

 smoke of the " little ovens" (hornitos). Both the houses of 

 the Playas and the basaltic hill Mirador are situated upon 

 the level of the old non-volcanic, or, to speak more cauti- 

 ously, unupheaved soil. Its beautiful vegetation, in which 

 a multitude of salvias bloom beneath the shade of a new spe- 

 cies of fan palm ( Corypha pumos), and of a new alder (Almis 

 Jorullensis\ contrasts with the desert, naked aspect of the 

 Malpais. The comparison of the height of the barometerf 

 at the point where the upheaval commences in the Playas, 



* My barometric measurements give for Mexico 1168 toises (7470 

 feet), Valladolid 1002 toises (6409 feet), Patzcuaro 1130 toises (7227 

 feet), Ario 994 toises (6358 feet), Aguasarco 780 toises (4089 feet), for 

 the old plain of the Playas de Jorullo 404 toises (2584 feet) (Hum- 

 boldt, Observ. Astron., vol. i., p. 327, Nivellement JBaromgtrique, No. 

 366-370). 



f If the old plain of the Playas be 404 toises (2584 feet), I find for 

 the maximum of convexity of the Malpais above the sea-level 487 

 toises (3115 feet); for the ridge of the great lava stream 600 toises 

 (3838 feet) ; for the highest margin of the crater 667 toises (4266 

 feet); for the lowest point of the crater at which we could establish 

 the barometer 644 toises (4119 feet). Consequently the elevation of 

 the summit of Jorullo above the old plain appeared to be 263 toises, 

 or 1682 feet. 



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