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But he, at the same time, clearly saw that there was no evidence of the occurrence of great deluges of lava along such fissures; he showed how the most remarkable plateaux, composed of successive lava sheets, might be built up by repeated and moderate ejections from numerous isolated vents; and he expressly insists upon the rapidity with which the cinder-cones around the orifices of ejection and the evidences of successive outflows of lava would be obliterated by denudation.
As the greater part of these cones are ancient, many of them being almost effaced by the rain or buried beneath the ejections which have surrounded their bases since the time they were formed, we are led to believe that many thousands of them have been formed during the history of the volcano. The history of these subsidiary cones appears to be connected with the lava caves noted above.
At present the floor on which the ruin stands appears to be gradually sinking, though the rate of movement is very slow. Another evidence that the ejections may travel for a great distance underground on their way to the vent is afforded by the fact that Vesuvius and Ætna, though near three hundred miles apart, appear to exchange activities that is, their periods of outbreak are not simultaneous.
Now, it is remarkable that these microscopic shells of Diatoms are not unfrequently found in the ejections of volcanoes; while it is generally supposed that, in the case of those situated near the sea, eruptions are caused by the formation of explosive steam consequent on the access of sea-water to the reservoirs of molten lava lying underground.
But amongst these vast ejections, these truncated fossil craters were embedded masses of the rare self-luminous stone that made the City of Light. Chapman told me how in pockets or huge amygdaloidal cavities, this white phosphorescent substance was quarried, brought up bodily perhaps in the slow upheaval of the region from the deep-seated sources of this mineral flood.
These are the springs of petroleum and of thermal waters, the igneous meteors, and the ejections of muddy substances attended with explosions, of which I acquired a knowledge in the vast provinces of Venezuela, whilst travelling over a space of two hundred leagues from east to west.
They are only partially exposed, being covered up by modern ejections; from this cause, I was unable to trace their junction with the trachyte, or to discover whether they had flowed as a stream of lava, or had been injected amidst the overlying strata. There are three principal beds of obsidian, of which the thickest forms the base of the section.
We estimate the importance of Sulla much too highly, or rather we dispose of those terrible proscriptions, ejections, and restorations for which there never could be and never was any reparation on far too easy terms, when we regard them as the work of a bloodthirsty tyrant whom accident had placed at the head of the state.
Those of California exist chiefly in Napa Valley, north of San Francisco, in a canon or defile. Their waters are impregnated not with silica, but with sulphur, and they thus approach more nearly in their character to mud-volcanoes, whose ejections are, in like manner, much impregnated with that substance.
On the dryest parts of the prairie, miles from any permanent stream, his ejections of mud may be found. Shallow or deep, his borings always reach water. He is always at home, but less accessible to callers than the ant. To the smaller birds he is forbidden fruit.
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