INSULATED VOLCANOES. 319 



pembling crevices, have been widened by running waters ; 

 but these hypotheses of successive erosions cannot well be 

 applied to the completely enclosed basins of Titicaca and 

 Mexico. These basins, as well as those of Jauja, Cuenca, 

 and Almaguer, which lose their waters only by a lateral and 

 narrow issue, owe their origin to a cause more instantaneous, 

 more closely linked with, the upheaving of the whole chain. 

 It may be said that the phenomenon of the narrow declivities 

 of the Sarenthal and of the valley of Eysack in the Tyrol, is 

 repeated at every step, and on a grander scale, in the Cor- 

 dilleras of equinoctial America. We seem to recognize in 

 the Cordilleras those longtitudinal sinkings, those "rocky 

 vaults," which, to use the expression of a great geologist,* 

 " are broken when extended over a great space, and leave 

 deep and almost perpendicular rents." 



If, to complete the sketch of the structure of the Andes, 

 from Tierra del Fuego to the northern Polar Sea, we pass 

 the boundaries of South America, we find that the western 

 Cordillera of New Grenada, after a great depression between 

 the mouth of the Atrato and the gulf of Cupica, again rises 

 in the isthmus of Panama to 80 or 100 toises high, aug- 

 menting towards the west, in the Cordileras of Veragua and 

 Salamanca,f and extending by Ghiatimala, as far as the con- 

 fines of Mexico. Within this space it extends along the 

 coast of the Pacific, where, from the gulf of Nicoya to 

 Soconusco (lat. 9^ 16), is found a long series of vol- 

 canos,J most frequently insulated, and sometimes linked to 

 spurs or lateral branches. Passing the isthmus of Tehuan- 

 tepecor Huasacualco, on the Mexican territory, the Cordillera 



* Von Buch, Tableau du Tyrol meridional, p. 8. 1823. 



f If it be true, as some navigators affirm, that the mountains at the 

 N. W. extremity of the republic of Columbia, known by the names of 

 Silla de Veragua, and Castillo del Choco, be visible at 36 leagues distance, 

 the elevation of their summits must be nearly 1400 toises, little lower 

 than the Silla of Caracas. 



J See the list of twenty-one volcanos of Guatimala, partly extinct, 

 and partly still burning, given by Arago and myself, in the Annuaire du 

 Bureau des Longitudes pour 1824, p. 175. No mountain of Guatimala 

 having been hitherto measured, it is the more important to fix approxi- 

 mately the height of the Volcan de Agua, or the Volcano of Pacaya, 

 and the Volcan de Fuego, called also Volcano of Guatimala, Mr. Juar- 

 ro* expressly aavg, that this voicaao which by torrents of water and 



