T1IF.ni FXXI'. STRUCTURE. 
masses, are very old volcanoes, and near being extinguished, 
and that rounded tops, in the form of domes, or bells, indi- 
cate those problematic porphyries, which arc supposed to 
have been heated in their primitive position, penetrated by 
vapours, and forced up in a mollified state, without having 
ever flowed as real lithoidal lavas. To the first class be- 
long Cotopaxi, the peak of Teneriffe, and the peak of On- 
zava in Mexieo. In the second may be placed Cargueirazo 
and Pichiuclm, in tho province of Quito ; the volcano of 
Puracey, near Popayan ; and perhaps also llccla, in Ice- 
land. "In the third and last wo may rank the majestic figure 
of Chimborazo, and, (if it be allowable to place by the side 
of that colossus a bill of Europe,) the Great Sammy in 
Auvergne. 
In order to form a more exact idea of the external struc- 
ture of volcanoes, it is important to compare their perpen- 
dicular height with their circumference. This, however, 
cannot be done with any exactness, unless the mountains 
are isolated, and rising on a plain nearly on a level with the 
sea. In calculating the circumference of the peak of Tone- 
rifle in a curve passing through the port of Orotava, Gara- 
ehieo, Adexe, and Guimar, and setting aside the prolonga- 
tions of its base towards the forest of Laguna, and the 
north-east cape of the island, wc find that this extent is 
more than 54,000-toises. The height of the Peak is con- 
sequently one twenty-eighth of the circumference of its 
basis. M. von Buch found a thirty-third for Vesuvius; 
and, which perhaps is less certain, a thirty-fourth for Etna.* 
I f the slope of these three volcanoes were uniform from the 
summit to the base, the peak of Teyde would have an incli- 
nation of 12° 29', Vesuvius 12° 41', and Etna 10° 13'; a 
result which must astonish those who do not reflect on what 
constitutes an average slope. In a very long ascent, slopes 
* (filbert, Annalen tier Physik, B. 5, p. 455. Vesuvius is 133,000 
palrnas, or eighteen nautical miles in circumference. The horizontal <11.-- 
tante from Resina to the crater is 3,700 toises. Italian mineralogists 
have estimated tire circumference of Etna at 8-10,000 palmas, or 110 
miles. With these data, the ratio of the height to the circumference 
would be only a seventy-second; hut I find on tracing a curve through 
Catania, Palermo, Rronte, and Piemonte, only 62 miles in circumference, 
according to the best maps. This increases the ratio to a fifty for rfii. 
Does the basis fall on the outside of the curve that 1 assume ? 
