In order to form a more exact idea of the external structure of volcanoes, it is important to compare their perpendicular 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 Teneriffe in a curve passing through the port of Orotava, Garachico, Adexe, and Guimar, and setting aside the prolongations of its base towards the forest of Laguna, and the north-east cape of the island, we find that this extent is more than 54,000 toises. The height of the Peak is consequently 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.* (* Gilbert, Annalen der Physik B. 5 page 455. Vesuvius is 133,000 palmas, or eighteen nautical miles in circumference. The horizontal distance from Resina to the crater is 3700 toises. Italian mineralogists have estimated the circumference of Etna at 840,000 palmas, or 119 miles. With these data, the ratio of the height to the circumference would be only a seventy-second; but I find on tracing a curve through Catania, Palermo, Bronte, and Piemonte, only 62 miles in circumference, according to the best maps. This increases the ratio to a fifty-fourth. Does the basis fall on the outside of the curve that I assume?) If the slope of these three volcanoes were uniform from the summit to the base, the peak of Teyde would have an inclination of 12 degrees 29 minutes, Vesuvius 12 degrees 41 minutes, and Etna 10 degrees 13 minutes, a result which must astonish those who do not reflect on what constitutes an average slope. In a very long ascent, slopes of three or four degrees alternate with others which are inclined from 25 to 30 degrees; and the latter only strike our imagination, because we think all the slopes of mountains more steep than they really are. I may cite in support of this consideration the example of the ascent from the port of Vera Cruz to the elevated plain of Mexico. On the eastern slope of the Cordillera a road has been traced, which for ages has not been frequented except on foot, or on the backs of mules. From Encero to the small Indian village of Las Vigas, there are 7500 toises of horizontal distance; and Encero being, according to my barometric measurement, 746 toises lower than Las Vigas, the result, for the mean slope, is only an angle of 5 degrees 40 minutes.
In the following note will be seen the results of some experiments I have made on the difficulties arising from the declivities in mountainous countries.*
(* In places where there were at the same time slopes covered with tufted grass and loose sands, I took the following measures:—
5 degrees, slope of a very marked inclination. In France the high
roads must not exceed 4 degrees 46 minutes by law;
15 degrees, slope extremely steep, and which we cannot descend in a
carriage;
37 degrees, slope almost inaccessible on foot, if the ground be
naked rock, or turf too thick to form steps. The body falls
backwards when the tibia makes a smaller angle than 53 degrees with
the sole of the foot;
42 degrees, the steepest slope that can be climbed on foot in a
ground that is sandy, or covered with volcanic ashes.
When the slope is 44 degrees, it is almost impossible to scale it, though the ground permits the forming of steps by thrusting in the foot. The cones of volcanoes have a medium slope from 33 to 40 degrees. The steepest parts of these cones, either of Vesuvius, the Peak of Teneriffe, the volcano of Pichincha, or Jorullo, are from 40 to 42 degrees. A slope of 55 degrees is quite inaccessible. If seen from above it would be estimated at 75 degrees.)
Isolated volcanoes, in the most distant regions, are very analogous in their structure. At great elevations all have considerable plains, in the middle of which arises a cone perfectly circular. Thus at Cotopaxi the plains of Suniguaicu extend beyond the farm of Pansache. The stony summit of Antisana, covered with eternal snow, forms an islet in the midst of an immense plain, the surface of which is twelve leagues square, while its height exceeds that of the peak of Teneriffe by two hundred toises. At Vesuvius, at three hundred and seventy toises high, the cone detaches itself from the plain of Atrio dei Cavalli. The peak of Teneriffe presents two of these elevated plains, the uppermost of which, at the foot of the Piton, is as high as Etna, and of very little extent; while the lowermost, covered with tufts of retama, reaches as far as the Estancia de los Ingleses. This rises above the level of the sea almost as high as the city of Quito, and the summit of Mount Lebanon.
The greater the quantity of matter that has issued from the crater of a mountain, the more elevated is its cone of ashes in proportion to the perpendicular height of the volcano itself. Nothing is more striking, under this point of view, than the difference of structure between Vesuvius, the peak of Teneriffe, and Pichincha. I have chosen this last volcano in preference, because its summit* enters scarcely within the limit of the perpetual snows. (* I have measured the summit of Pichincha, that is the small mountain covered with ashes above the Llano del Vulcan, to the north of Alto de Chuquira. This mountain has not, however, the regular form of a cone. As to Vesuvius, I have indicated the mean height of the Sugar-loaf, on account of the great difference between the two edges of the crater.) The cone of Cotopaxi, the form of which is the most elegant and most regular known, is 540 toises in height; but it is impossible to decide whether the whole of this mass is covered with ashes.
TABLE 3: VOLCANOES:
Column 1: Name of the volcano.
Column 2: Total height in toises.