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Updated: May 15, 2025
Hence the points of eruption must have been situated westward of the present range, in the direction of the main Cordillera: as, however, the flanks of the Cordillera are entirely composed of various porphyries, chiefly claystone and greenstone, some intrusive, and others belonging to the porphyritic conglomerate formation, but all quite unlike these submarine lava-streams, we must in all probability look to the plain of Uspallata for the now deeply buried points of eruption.
It is not uncommon for lava-streams to require more than ten years to cool in the open air; and where they are of great depth, a much longer period. The melted matter poured from Jorullo, in Mexico, in the year 1759, which accumulated in some places to the height of 550 feet, was found to retain a high temperature half a century after the eruption.
Owing to a general similarity in their materials, volcanic cones in all parts of the world have very similar curvatures; but older volcanic mountains, in which lava-streams have broken through the cone, secondary cones have arisen, or portions have been blown up, are more irregular in outline and more gradual in inclination.
That is to say, the surfaces of the lava-streams are not at all, or only slightly, decomposed into soil suitable for the growth of plants, except in rare instances. E. G. Hull, "On the Domite Mountains of Central France," Scien. Proc. Roy. Dublin Society, July 1881, p. 145. Dr. Hull determined the density of the domite of the Puy de Dôme to be 2.5, while that of lava is about 3.0.
The lava-streams at the eastern foot of the Portillo are interesting, not so much from the great denudation which they have suffered at a comparatively late period as from the evidence they afford by their inclination taken conjointly with their thickness and compactness, that after the great range had assumed its present general outline, it continued to rise as an axis of elevation.
I remember having been much struck when viewing volcanic islands, which have been worn by the waves and pared all round into perpendicular cliffs of one or two thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean.
The people had stormed the Bastile with arms, killed the governor, and for the first time the dreadful cry "To the lamp-post!" was heard in the streets of Paris; for the first time the iron arms of the lamp-posts had been transformed to gallows, on which those were suspended whom the people had declared guilty. Meanwhile the lava-streams of revolution had not yet flowed out as far as Versailles.
Almost to the summit, whence issue perpetually faint wreaths of smoke, it is clothed with vegetation, and looks calm and beautiful, although beneath are hidden fires which occasionally burst forth in lava-streams, but more frequently make their existence known by the earthquakes which have many times devastated the town. I brought letters of introduction to Mr.
Such breached craters are to be found in all parts of the world, and are not confined to any one district, or even continent, so that they may be considered as characteristic of the class of volcanic crater-cones to which I am now referring. In the case of the moon, however, we fail to observe any decided instances of breached craters, with lava-streams, such as those I have described.
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