"volcanic mountains: such are those of Laach, near the abbey of the same name, a few leagues from Andernach; such also is the little lake,

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perfectly circular, of Paven in Auvergne. And "besides volcanic soils, we meet with many sorts "of mountains, especially those which are of a "calcareous or gypseous nature, which contain great caverns and cavities; and it is very na"tural to think, that the concussions of earthquakes, "when they are violent, may occasion the rupture " and downfal of the masses which are above them."

But, what is the immediate cause which gives action to these powerful agencies? "We have seen, that the volcanos which are in activity, are situated in islands, or on coasts not far

from the sea. Those which we find in the "interior countries of the earth, are all extinct. "These observations naturally lead us to con"clude; that the vicinity of the SEA, is a condition essential to the existence of vOLCANOS: they "further lead us to think, that the water of the sea, penetrating into the volcanic cavities, is a cause of

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eruptions. But, how should water penetrate "into volcanic cavities? If it penetrates in great

quantity, (and it would seem that it cannot be "otherwise of the sea-water,) would it not rather "tend to extinguish the volcanic fire, than to "increase its action? This certainly is a question, "the solution of which is difficult; but, though

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it may be much complicated, its solution is not

"impracticable. I shall not enter into any details

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upon this subject; but I shall confine myself to the statement of the known fact, that the presence of water, and in great quantity, is incontestable in volcanic phenomena. We know the astonishing power of this fluid, when reduced to vapour or steam; but, our steam-engines can hardly convey to us an idea of the power which it "is capable of acquiring in caverns whose sides are "several thousand yards in thickness, and which "sustain the mountains of Etna and Chimboraço: "heat may extend its elasticity to a point, of which it "is is difficult to form any idea. Nor is water re"duced to vapour the only elastic fluid which "exercises a power in volcanic foci1."

And, what evidence do we discover, of the ancient action of volcanic power?" In considering "the different volcanic soils, with relation to their different epochas, we distinguish, among "the productions of extinct volcanos, some which,

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speaking by geological comparison, are of a "recent epocha; since they are of a date posterior "to the excavation of valleys. But we discover "others of a much more ancient epocha, since

they are anterior to the formation of valleys; "these are lodged upon the summits, while the "former rest upon the low ground.-Those of "the most ancient epocha, are almost entirely composed of basalt3. This substance was emit

"ted from the earth in the form of streams of "matter in a state of fusion, which ran and spread themselves upon a soil already existing. "Those flowing masses, sometimes of several leagues in length, and of more than a league in breadth, often assumed the form of layers, or beds, one above the other. The basaltic matter, in cooling, experienced condensation ❝ or contraction; it separated; and the crevices

being perpendicular to its surface, as must "have been the case, divided it into prisms more ❝or less regular. The most celebrated assémblage of the columnar prisms of basalt, is that "which is seen on the north coast of Ireland, " and which is known by the name of the Giant's Causeway. The volcanic soil which forms the "north of Ireland, constitutes also the soil of the "Hebrides; in one of which islands, that of Staffa, is the celebrated grotto of Fingal, the "finest basaltic monument known, according to "M. Faujas."

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Powerful, however, as these tremendous agents are, some eminent geologists have under taken to pronounce them unequal to the production of effects so vast as those which we are investigating. But then, they have drawn their conclusion from the limited. effects of the conical volcanos now operative upon the globe. M. Humboldt, who has

2 CUVIER, § 17. D'AUBUISSON, tom. i. p. 254-267.

accurately observed the volcanos of what is called the New World, remarks: "that the domain of "volcanic action has been too much limited;" that we ought "no longer to restrict the idea of "volcanic action to the effects produced by the "craters of our burning volcanos ""-that "the "action of volcanic fire by an insulated cone,

by the crater of a modern volcano, differs essentially from the action of that fire across "the ancient fissured crust of our planet." It is one thing, to calculate the power of a volcano, and another thing, to calculate the power of volcanic action; to compute the actual effects of an individual volcano, whose focus is limited to one point, and which from that point has effected perpendicularly a channel of discharge; and to compute the possible effects of volcanic power, rendered general within the globe, and acting simultaneously against its solid crusts, without any actual vent to determine its issue. No sound inference can be drawn from the former, to limit the power of the latter; on the contrary, the former, only furnishes a datum by which we are enabled to form a judgment of the multiplied power of the latter. The former, exhibits a particular application of the latter; but the latter, in its principle, is the proper subject of our inquiry. We may securely pronounce, that the power or principle of action which we contemplate in an eruption of

1 Superp. of Rocks, p. 157, 8.

* Ib.

P. 408. See above, vol. i. p. 218.

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Ætna or Vesuvius, is physically capable of being extended as much beyond the effects which we witness in that eruption, as the principle of action which blasts a rock, or blows up a fortress, is extended beyond the action which we witness in the spoutings of a gerbe, or a Roman candle, And this is admitted by the writer last quoted, when he observes: "that heat can increase the elastic "force of vapour or steam to a point, of which it " difficult (he would better have said, impossible)

to form an idea 1." We cannot therefore, philosophically, limit the general question of volcanic action to the measure of the action exercised in the particular case proposed. And, of the insufficiency of the rule, we have experimental evidence; because, we are utterly unable to form an

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"When I reflected upon the almost infinite power that is some"times displayed in the eruptions of Mount Vesuvius, throwing up incalculable masses of matter into the very clouds; I was induced "(says the skilful Mr. Perkins) to consider, how this immense power "could be generated. How is it, that this power is so wonderfully great? "Is it not high elastic steam? The thought struck me, that it must be "owing to the water being confined by pressure, until it got sufficiently "charged with heat, to enable it to rend asunder whatever confined it, "thereby driving every thing before it. If we wanted further proof "of the tremendous power of steam, we have only to inquire of many "practical iron-founders, what it is that has sometimes caused the liquid "iron to leave its mould and pass off through the roof of the foundry, "in a metallic shower? The answer would be; that a small quantity "of water had accidentally found its way into the bottom of the mould; " and it might also be added, that a thousand times that quantity "thrown on its heated surface would be perfectly harmless.” (An Account of the Concentrating Steam Engine, 1824, p. 2, 3.) This is the action, which M. Humboldt denominates dynamical.