This inference is still further confirmed by the great and sudden changes of level that are frequently to be seen in similar strata, faults, as they are styled by miners, in which the same bed has its level sometimes changed hundreds, nay even thousands of feet. These faults, if in greatest abundance in the more ancient rocks, are to be found even in the newest, and sometimes affect several formations incumbent on each other, of ages the most different. Thus, then, we have distinct and conclusive evidence, that as we inferred from theory, the solid crust of the globe has been shattered and fractured repeatedly, and at all the different epochs of its history. This fracturing and cracking we have shown, must, in conformity with strict mechanical laws, have been attended with the rise of the molten liquid from beneath, which ought in some cases to have formed veins and dykes, in the places where the fractures occurred. It is however possible, that the rise of the fluid from beneath, may not have taken place where the pressure occurred; but it would then have been compelled by hydrostatic pressure, to issue at some other point, breaking and tearing the weaker parts of the solid crust, in order to afford itself a vent.
The latter class of phenomena are still in action, and we have evident traces of their occurrence in all the different stages of the world's existence; of the former it will also be seen there is conclusive evidence.
The visible effects of a subterranean heat, are most frequently met with at the present day in the form of volcanoes. Of these, there are not only a great number in activity, but there are still more that have been certainly active since the last great change that the surface of the earth has undergone.
That part of the great group of mountains which we have before described, which lies in the new continent, contains many active volcanoes, and others but recently extinct. Terra del Fuego, as its very name imports, is the seat of many; Chili has several; in Peru are to be noted Arequipa, Pichinca, and Cotapaxi; while Chimborazo is obviously one that has become extinct at a period not remote. Passing the Isthmus of Panama, we find the volcanoes of Guatimala and Nicaragua almost infinite in number. In Mexico, are Orezaba, Popocatepetl, and Jorullo; the last of which first rose from beneath the surface in 1759. California has five active volcanoes; and we know, from the observations of La Perouse and Cook, that they also exist along the north-western coast of America. Mount St. Elias, in particular, was seen in a state of eruption. These mountains connect those of Mexico with the volcanoes of the Aleutian islands and of the peninsula of Alaska, which continue the system towards Kamtschatka, in which peninsula there are three of great violence. We have seen some proofs, that there are active volcanoes to the north-west of China, but none now exist in Thibet; and the action that once took place there has sought new vents, in regions more near to the present bed of the ocean. Thus, Japan has eight volcanoes, Formosa several, and, in proceeding to the south, the land of volcanic action widens, and becomes of immense extent. It embraces the Philippine, Marian, and Molucca islands, Java, Sumatra, Queen Charlotte's islands, and the New-Hebrides. The active volcanoes of Europe and western Asia are few in number; but those that are extinct form a great system, in which the active ones are included, and which seems to spread in the form of a belt, from the Caspian sea to the Atlantic. Volcanic action still occurs on the shores of the Caspian. In the chain of Elburg is a lofty mountain that still emits smoke, and around whose base are several distinct craters. Syria and Palestine abound in volcanic appearances, of which the great crater that has swallowed up the waters of the Jordan, and forms the Dead sea, is the most remarkable. Greece and the Grecian Archipelago have been, almost within historic times, the seat of a volcanic action, of great extent and violence, and which has not wholly exhausted itself. In Sicily, Ætna has burnt for 3300 years, and is yet surrounded by extinct craters of more ancient date. The Lipari islands are wholly volcanic. Vesuvius, that had long before intermitted its eruptions, and broke forth again in the great one that destroyed Herculaneum and Pompeii, is not the only volcanic mountain of Naples. An extinct one of much greater size is to be found near Roccafina. The catacombs of Rome are excavated in lava, and Tuscany contains strong evidences of volcanic action. Volcanic indications can be traced near Padua, Verona, and Vicenza, extending into Dalmatia. A district of Hungary was suspected of containing the seeds of subterranean fire, and the suspicion has been confirmed by an actual eruption. Germany and Bohemia contain a great number of extinct volcanoes, as does the south of France, and particularly Auvergne. In Spain, too, the proofs of a volcanic agency are clear and decisive.
Greenland and Iceland present a third group of volcanoes; in the latter island, a single volcano was in a state of continuous eruption for five or six years. The Azores, the Canaries and Madeiras, also contain numerous volcanoes, both active and extinct, as do the Caribbean islands.
In comparing together volcanoes that are in present activity, and others in which the crater and the streams of emitted lava are too distinct to permit a doubt of their having arisen from the same cause, differences are observed that only have arisen from great differences in the circumstances under which the eruption has taken place. In many of the ancient volcanoes, we find the emitted streams are arranged in prismatic forms, constituting basalt, and frequently passing into what under other circumstances would be styled trap by the Wernerians. Now, we know that when streams of lava enter the sea, they spontaneously assume the prismatic structure. Hence we may infer, that these ancient volcanoes originally gave vent to their craters beneath the level of the sea, at a time when the rocks through which they penetrated, and over which their streams have passed, were beds of the primitive ocean. The trap rocks themselves may have been formed in a similar manner, by upward pressure of the igneous fluid beneath, through the veins and fissures formed on the breaking of the solid crust. Trap traverses, in dykes of unknown depth, many formations, and is occasionally seen forming beds between successive strata. It frequently occurs in faults, and sometimes in extensive overlying masses. Close observation, and a just course of analogy, lead to the irresistible conclusion, that all the trap rocks, however situated or arranged, grow out of the same great cause, the rising of the liquid interior of the earth to its surface. An action sometimes taking place through veins and fissures in the solid crust, and sometimes by the eruption of volcanoes, both occurring during the pressure of water upon the surface. One of the most extensive groups of trap-rocks is to be seen in the north-eastern part of the state of New-Jersey. The Hudson is bordered for nearly forty miles by a great ridge of columnar rock, lying upon sandstone. When this is surveyed with an eye to its analogy to volcanic action, it appears as if it were the outpourings of a crater, whose basin is now occupied by the lake in which the Hackensack river takes its rise, and whence a great stream of lava has run over the sandstone rock, as far as the strait that separates Staten Island from the main land. The two Newark mountains are ridges of the same description, of even greater extent; other smaller ridges of the same kind are also distinctly visible, and the whole of this last system appears to have proceeded from a crater now filled by the alluvion of the Passaic, but which is bordered by a ridge still occupying two-thirds of a circle, and showing conclusive marks of igneous action, that goes by the name of the Hook mountain. The phenomenon of a dyke of trap is well exhibited in the quarries near Hartford in Connecticut, where this rock has been laid bare for a considerable depth, as it rises through a sandstone rock, instead of overlying it, as it is seen to do on the Hudson.
The trap-rocks, which are, generally speaking, of the character called by mineralogists greenstone, vary in this district of New-Jersey, from a compact basalt of homogeneous structure, to one of regular and distinct crystallization, not distinguishable in hand specimens from primitive syenite. A rock of this last character is to be found in the mountain that extends from Morristown to Mount Kemble, which is columnar in its structure, but almost identical, in mere external characters, with stratified rocks of gneiss containing hornblende, that are found in the primitive ridges within a few miles.
Thus then the older volcanic rocks gradually pass in character into those which, under the general name of granitic, form the apparent nucleus of gneiss and mica slate mountains, and penetrate them, and the primitive limestones, in veins. One of the best instances of veins of granite with which we are acquainted, are those which occur in the quarries of white marble at Kingsbridge, which are traversed in every direction by thin veins of a rock, principally composed of a white fetid felspar, mixed with spangles of silvery mica, and small grains of quartz, interspersed with occasional masses of tourmaline. The famous locality of chrysoberyl, beryl, and other interesting minerals, at Haddam, in Connecticut, is said to occur in a granitic vein passing through strata of gneiss.
In all these cases we cannot fail to see evidence of igneous eruptions, taking place, however, under circumstances widely different from those of our present terrestrial volcanoes, or of the submarine craters of more remote dates, but which can be readily explained by supposing, either that the penetration took place when the surface of the earth was so intensely heated as to admit of the injected veins being slowly cooled, and therefore more perfectly crystallized; or that the issuing mass was so great as to retain its heat for a great length of time.
It might at first sight appear difficult to explain how volcanic energies should still continue in activity, now that the mean temperature of the earth has become constant, and the outer crust can be no longer subject to the shrinking, and consequent cracking which it must have undergone while cooling. The phenomena that attend volcanic eruptions furnish a full explanation of this, for they are attended in almost all cases with the evolution of great quantities of gaseous matters, and steam, which must therefore exist in a state of intense compression, and at elevated temperatures, in the mass whence the volcanic flood issues. Their elastic energies are sufficient to account for all the striking effects that attend the action of volcanoes.