IKSULATEB VOLCANOES. 
319 
«embling 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 Ahnaguer, 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 tlie whole chain. 
It may be said that the phenomenon of the narrow declivities 
of the Sarcuthal 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. W e seem to recognize in 
the Cordilleras those lougtitudinal 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 Euego to the northern Polar Sea, we pass 
the boundaries of South America, we find tlmt the western 
Cordillera of New Glrcnada, after a great depression between 
the mouth of the Atrato and the gulf of Guinea, again rises 
in the isthmus of Panama to 80 or 100 toises high, aug- 
menting towards the west, in the Cordileras of \eragua and 
Salamanca,t and extending b^v Gluatimala, as far as the con- 
fines of Mexico. IVithin this space it extends along the 
coast of the Pacific, where, from the gulf of Nicova to 
Soconusco (lat. 9^°— 10°), is found a long series of vol- 
canos.J most frequently insulated, and sometimes linked to 
spurs Dr lateral branches. Passing the isthmus of Tehuan- 
tc'pecor Huasacualco, on the Mexican tei'ritory, the Cordillera 
* Von Buch, Tableau du Tyrol meridional, p. 8. 1823. 
+ If it be true, assume navigators affirm, that the mountains at the 
K. \V. extremity of the rcpulilic of Columbia, known by the names of 
Silla de Veragua, and Castillo del Choco, be visible at 36 leagues distance, 
th« elevation of their summits must be nearly ] 400 toises, little lower 
than the Silla of Caracas. 
i See the list of twenty-one volcanos of Guafimala, partly extinct, 
and partly still burning, given by Arago and myself, in the Annuaire du 
Bureau des Longitudes pour 1824, p. 1/5. No mountain of Guatimala 
having been hitherto measured, it is the more important to fix approxi- 
mately the height of the Volcan de Agita, or the Volcano of Pacaya, 
and the Vo/caH de J^ego^ called also Volcano of Guatimala, Mr. Juar- 
ros expressly says, that this voicaao, which by torrents of water and 
