consisting either wholly of an accumulation of volcanic substances, or in connexion with marine strata upheaved from the bottom of the sea. A great number of solitary islets likewise display the same character, and have been built up by the occurrence of violent catastrophes. The south Atlantic presents a remarkable example of this class in Ascension island, one of the most isolated solid sites above the waves of the ocean, one thousand four hundred and fifty miles from the coast of Africa, six hundred and eighty-five from St. Helena, and five hundred and twenty from the nearest particle of visible land, the island of St. Matthew. Its shore exhibits black nitrous lava. Its surface presents rugged conical hills of different kinds of lava, some with perfect craters, scoriæ, pumice, and other volcanic products being everywhere strewed in large quantities. Not a shrub was to be seen upon its first discovery on Ascension-day, in 1501, by Joan de Novo Galego, and the only vegetation consisted of some coarse grasses and ferns. There can be little doubt respecting the events denoted by the physical characteristics of this island. Probably the ocean here once rolled its waters unobstructed by any visible land, when, at some era in the past which no chronicle has marked, a grand revolution took place, from the action of that power which in recent times has invaded the dominion of the sea, and reared rocky edifices beyond the reach of its waves. The disturbing cause at length expended its energy, as it has done with reference to the peak of Teneriffe, and the extinct volcanoes of Auvergne, and an age of tranquillity ensued, marked by the ordinarily gradual and quiet operations of nature. Each of the existing continents furnishes innumerable proofs of having undergone similar grand revolutions, proceeding from some expansive power which has lifted up, broken, and overturned their masses in a thousand ways. The lowest and most level parts of the earth, says Cuvier, when penetrated to a very great depth, exhibit nothing but horizontal strata composed of various substances, and containing, almost all of them, innumerable marine productions. Similar strata, with the same kind of productions, compose the hills even to a great height. Sometimes the shells are so numerous as to constitute the entire body of the stratum. They are almost everywhere in such a perfect state of preservation, that even the smallest of them retain their most delicate parts, their sharpest ridges, and their finest and tenderest processes. They are found in elevations far above the level of every part of the ocean, and in places to which the sea could not be conveyed by any existing cause. The summits of the Pyrenees and of the Andes, at the height of 13,000 or 14,000 feet above the level of the sea, present them to our notice. These facts bear witness to the great and wonderful changes which have marked the ancient history of the earth; for it is obvious that the present continents once occupied a submarine position, from which they have been uplifted-a change analogous to that involved in the formation of new islands by a process of elevation, and probably brought about by the same agency, though acting with an incomparably greater energy.

Fatal to human life as the eruptions of the volcano have occasionally been, large views of such physical events will awaken impressions at variance with those which their detached observation often excite. He who, living on the slopes of Vesuvius, witnesses his vine-clad dwelling, or his native village, overwhelmed with the lava and ashes of the mountain, is apt to become exclusively occupied with the disaster, and will not readily reflect upon the many millions of mankind who enjoy a quiet habitation, and whose locality has never been disturbed within the period that history and tradition have chronicled such occurrences. Yet nothing is more true than that the same agency which is occasionally destructive in a few spots upon the world's ex

panse, has operated in forming or upheaving the universal crust of the globe, and has thus been the means of building up sure resting-places for unnumbered myriads of the human family. It is that protruding or elevating pow er also that has rendered the coal formations and mineral veins accessible, and thus supplied commerce with its sinews; and comparing the physical history of the globe with the career of its inhabitants, how harmless the Etnas and Cotopaxis of nature appear, in contrast with the Cæsars and Napoleons of mankind! A slight survey of the features of the external world is sufficient to show, that the tendency of their general arrangement is to minister to the happiness of man, to give him pleasure in the act of contemplation, as well as to contribute to his convenience. Its surface, so finely diversfied, is eminently calculated for the gratification of its occupiers, and expands around them in every clime an array of beauty and grandeur, sometimes apart from each other, but often blended in wild yet tasteful and imposing combinations. Wherever the traveller penetrates, he finds the terrestrial configuration so arranged in ever-varying outline, as to spread before him an inviting picture of natural scenery, which captivates, or soothes, or elevates, or excites the mind, and furnishes such pleasurable emotions as dull uniformity would not have yielded. Especially do the elevations which mark the face of the earth, whether rising to the stately proportion of mountains, or forming only the rounded, green-clad hill, give interest, grace, or sublimity, to the landscape. But the mountains perform a more important office than that of giving imposing effect and picturesque beauty to the scenery of the earth. Occupying a portion of its surface nearly equal to that which the sandy desert claims, they stand associated with political and other results of the highest importance to mankind. Where the ocean does not extend its waters to divide the families, kindreds, and tongues of the human race, the granite snow-crowned rampart is frequently the line of demarkation. Nations have thus been kept apart from each other by natural boundaries; and the difficulties connected with aggressive wars between communities thus separated, have contributed to promote peace and maintain independence. The mountains also give their aid to the clouds of heaven, attracting them to their summits, and storing up their precipitated waters in interior reservoirs, whence they issue by a thousand springs; and in the dens and caves that perforate their declivities liberty and religion have often found a secure asylum, when assailed by persecuting power and grasping ambition. "The precious things of the lasting hills" the phrase of the dying Hebrew patriarch-is not without its appro priate significancy.

SUBJECT IX.

CAVERNS AND SUBTERRANEAN PASSAGES.

HEN we reflect upon the manner in which the solid crust of the earth appears to have been formed, upon the powerful upheaving force by which its elevated sites have been raised, and the posterior agency of subterranean gases, volcanoes, and earthquakes, it is natural to expect chasms in the surface of tremendous depth, spaces also in the interior which have not been filled up with masses of stone similar to the materials of the earth itself, but by water, air, or vapor, with those cavities of grotesque and romantic appearance that are found in mountainous regions. There are few natural objects which have more awakened curiosity, or more strongly affected the imagination, than the hollow places, of various forms and sizes, common in districts which have been subject to great physical disturbance. Their seclusion and gloom-their fantastic architecture-the effect of torch-light upon their numerous crystallizations-the augmentation of sound and its reverberation-together with their unknown extent in many cases-all these causes contribute to invest the cavities of the earth with exciting interest; nor is it strange to find them interwoven with the traditions and mythologies of unenlightened nations. On account of their sombre interior and strange outline being adapted to impose upon an ignorant populace, and give effect to religious observances, the priesthoods of antiquity localized in caverns their false divinities, and celebrated sanguinary rites upon the natural altars found in their recesses. A cave, with a priestess seated upon a tripod at its mouth, pretending to inhale a vapor from the interior which inspired a knowledge of future events, the gift of Apollo, was the original Delphian oracle, reverenced by the mind of Greece, and resorted to by the proudest monarchs of the ancient world. The cavern, along with the deep forest, commended itself to the primitive inhabitants of northern Europe by its mystery and gloom, as an appropriate spot for the performance of a barbarous worship, and many local titles of such sites preserve the memory of their former uses An instance of this we have in Thor's cave, or, as Darwin calls it,

"The blood-smeared mansion of gigantic Thor,"

a broad excavation on the face of a huge rock in the limestone district of Derbyshire, divided into two chambers, one beyond the other, with a detached stone at the further extremity, where the light of day is very much subdued. But in India the largest use has been made of caverns for reli gious purposes, and immense pains have been taken with their adornment, extension, and architecture, at Elephanta, Salsette, and Ellora, where there are elaborately-wrought temples constructed, probably out of small natural crevices in the rock. We shall now refer to a few of those cavities which

are entirely the workmanship of nature, with whose form man nas not intermeddled, and notice the principal phenomena which they exhibit.

That extensive cavities exist in the interior of the crust of the globe is evident from the phenomena of volcanoes and earthquakes. They are not accessible to observation, but the repeated tremblings of the soil in various places, and experiments made of oscillations of the pendulum, point to the conclusion, that there are large underlaying hollows, at no great distance from the surface, of which the superficial land forms the roof. The table-land of Quito, and the burning mountain of Jorullo, each surrounded by the most powerful volcanoes upon the earth, are supposed to be examples of this. Condamine believed that a considerable portion of the former mountain was to be regarded as the dome of an enormous vault; and Parrot has shown it to be highly probable, by a careful calculation, that a cavity of at least a cubic mile and a half exists beneath its surface. The rumbling noise, like that of distant thunder, which, on the testimony of Humboldt, usually precedes and accompanies the eruption of its volcanoes, affords evidence in favor of this supposition, and as an increase of the subterranean vacuity must be the necessary consequence of every outbreak, it is not at all an improbable event that the blooming landscape will ultimately fall in, and this piece of table-land become an immense depression. The quantity of material scooped out of the interior of the earth by volcanic action is immense, and calculated to produce vacuities in which the largest mountains would have ample space. It has been estimated that Etna, in one of its last most important eruptions, that of 1769, threw out a mass of lava equal in volume to a cone 5,820 feet in height, and 11,640 feet in breadth, or nearly four times larger than Vesuvius. Fourteen such eruptions would produce a mass equal to Mont Blanc, reckoning from the level of the sea, and twenty-six such large eruptions have occurred since the twelfth century. In 1783, when the earthquake of Calabria occurred, the Skaptar volcano, in Iceland, poured forth a stream of lava fifty miles long, between twelve and fifteen broad, and from

one to six hundred feet in thickness, which must have been equal to six times the mass of Mont Blanc, and two and a half times that of Chimborazo. From the discovery of America to 1759, the plain of Malpais, a volcanic district in Mexico, had remained undisturbed, and was covered with plantations of indigo and sugar-cane at the latter period. In June. a succession of earthquakes commenced, and on the night of September the 28th a tract not less than from three to four miles in extent, rose up in the shape of a dome; and six great masses suddenly appeared, having an elevation of from 1,312 to 1,640 feet above the original level of the plain. The most elevated of these is the volcano of Jorullo, which is continually burning, the projection of which, with its kindred masses, must have created a considerable subterranean vacuity, and probably the whole dome-shaped plain of Malpais is hollow. Hence, it is a common event, in countries subject to great volcanic activity for portions of the surface to fall in, the subsidence frequently becoming the bed of a lake. A part of the forest of Aripas in the Caraccas thus subsided in 1790; a lake was formed nearly half a mile in diameter, and from eighty to a hundred yards in depth, and for several months after the trees of the forest remained green under the water. In the same year, in Sicily, at Santa Maria de Nisremi, a portion of the country three Italian miles in circumference sunk thirty feet deep. Occurrences of the same kind appear to take place in the depth of the sea, the falling in of its bed being indicated on the surface of the waters by their sudden retreat, and violent agitation on their return. A remarkable example of this phenomenon took place at Marseilles, on June 28, 1812, when the water in the harbor suddenly sank, then rushed out with great rapidity, and returned with equal violence; a movement which was repeated several times, till the equilibrium was restored, occasioning considerable damage to the shipping. Instances of similar events are innumerable, which serve to prove the existence of cavities, both in the interior of the expost crust of the earth, and those parts of it over which the oceau rolls.

To Humboldt we are indebted for a large amount of information respecting the cavities which appear upon the surface, the chief differences of their form, the beds in which they are found, and the causes which may have originated them. In the primary rocks, caverns are relatively fewer than in the later deposites, while the oldest masses of the granite and gneiss formations are particularly destitute of them. The principal are wide fissures, sometimes of unknown depth, and those hollow passages which occur in Switzerland and Dauphiné, called crystal caves, owing to their walls being richly furnished with pillars of rock crystal. Similar vacuities occur in the gneiss of the Pine mountain in the neighborhood of Wiesenthal, but they are not important. In Sweden and Norway, the granite presents fissures and caves of extraordinary extent, and perfectly unexplored hitherto; such as the cave of Marienstadt, the end of which is not known, and the enormous deep hole at Frederickstall, where a stone thrown in only gives the echo of its fall in a minute and a half or two minutes; an observation which, if well founded, would give, on the calculation of Perrit, a precipitous depth of 59,049 feet, the highest estimate, or 39,866 feet, the least; that is, from twice to three times the height of Chimborazo. It is the primitive limestone that supplies the most numerous examples of caves and grottoes in the primary rocks; and if these yield in point of size to the later limestone formations, this arises from the inferior extent of the primitive limestone, rather than from its incapacity to form caves. In the transition mountains, and those of stratified structure, it is still the limestone in which the more extensive caves are found of which those of the Hartz, the splendid caverns of Derbyshire,