« ПретходнаНастави »
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.
The earthquake is a phenomenon connected with volcanic eruptions, and arising from the same great cause ; but while the latter are confined to certain mountains, and restricted within narrow limits at the present day, an earthquake is sometimes found to prevail over a very large portion of the earth's surface. To omit the more usual phenomena of earthquakes, we shall speak of but one, which has in some cases been observed, that throws a great light upon the manner in which the stratified rocks have had their levels changed, and been dislocated and distorted in the manner we now find them. We allude to the sudden raising of countries of greater or less extent. Of this we shall quote three several instances from a paper of Arago’s.
“During the night of the 28th September 1759, a district of three or four square miles, situated in the Intendency of Valladolid, in Mexico, was raised up, like an inflated bladder. The limits where the elevation ceased may still be determined at the present day, by the fracture of the strata. At these limits the elevation of the ground above its primitive level, or that of the surrounding plain, is no more than thirty-seven feet; but towards the centre of the lifted district, the total elevation is not less than five hundred feet.
“ This phenomenon had been preceded by earthquakes that lasted nearly two months ; but when the catastrophe occurred, all seemed tranquil ; it was an. nounced only by a horrible subterranean noise, that took place at the moment when the ground was lifted. Thousands of little cones, of from six to ten feet in height, called by the natives ovens, arose in every direction ; finally six great projections were suddenly formed along a great crevice lying in a north-east and suuth-west direction, all of which were elevated from 1200 to 1600 feet above
the adjacent plains. The greatest of these small mountains has become a true volcano, that of Jorullo, and vomits forth lava.
“It will be seen that the most evident and well characterized volcanic phenomena accompanied the catastrophe of Jorullo ; that they were perhaps its cause ; but this did not prevent an extensive plain, old and well consolidated, upon which the sugar-cane and indigo were cultivated, from being, in our own days, suddenly raised far above its primitive level. The escape of inflamed matter, the formation of the ovens and of the volcano of Jorullo, far from having contributed to produce this effect, must on the contrary have lessened it ; for all these openings must have acted like safety valves, and permitted the elevating cause to have dissipated itself, whether it were a gas or a vapour. If the ground had opposed a greater resistance ; if it had not given way in so many points, the plain of Jorullo, instead of becoming a simple hill five hundred feet in height, might have acquired the relief of the neighbouring summits of the Cordilleras.
“ The circumstances that attended the formation of a new island near Santorin, in the Greek Archipelago, seem to me also well fitted to prove that subterranean fires not only contribute to elevate mountains by the aid of ejections furnished by the craters of volcanoes, but that they also sometimes lift the already consolidated crust of the globe.
“On the 18th and 22d May 1707, there were slight shocks of an earthquake at Santorin.
“On the 23d, at sunrise, there was seen between the great and little Rameni (two small islands) an object that was taken for the hull of a shipwrecked vessel. Some sailors proceeded to the spot, and on their return reported, to the great surprise of the whole population, that it was a rock that had risen from the waves. In this spot the sea bad formerly a depth of from 400 to 500 feet.
“On the 24th, many persons visited the new island, and collected upon its surface large oysters that had not ceased to adhere to the rock. The island was seen sensibly to increase in size.
“From the 23d May until the 13th or 14th June, the island gradually increased in extent and elevation, without agitation and without noise. On the 13th June it might be about half a mile in circuit, and from 20 to 25 feet in height. Neither fame nor smoke had issued from it.
“ From the first appearance of the island, the water near its shores bad been troubled ; on the 15th June it became almost boiling.
« On the 16th, seventeen or eighteen black rocks rose from the sea between the new island and the little Rameni.
“ On the 17th they had considerably increased in height.
“ On the 18th smoke arose from them, and great subterranean noises were heard for the first time.
“On the 19th all the black rocks had united and formed a continuous island, totally distinct from the first; flames, columns of ashes, and red-hot stones arose from it.
“ The volcanic phenomena still continued on the 23d May 1708. The black island, a year after its appearance, was five miles in circuit, a mile in breadth, and more than 200 feet in height.
“On the 19th November 1822, at a quarter past ten in the evening, the cities of Valparaiso, Melipilla, Quillota, and Casa Blanca, in Chili, were destroyed by a terrible earthquake that lasted three minutes. The following day several observers discovered that the coast, for an extent of thirty leagues, had been visibly elevated, for upon a coast where the tide never rises higher than five or six feet, any rise in the land is easily detected.
“ At Valparaiso, near the mouth of the Coucon, and to the north of Quintero, rocks were seen in the sea, near the bank, that no person had before perceived. A vessel that had been stranded on the coast, and whose wreck had been visited by the curious, in boats, at low water, was left, after the earthquake, perfectly dry. In traversing the shore of the sea, for a considerable distance near Quintero, Lord Cochran, and Mrs. Maria Graham, found that the water, even at bigh tide, did not reach rocks, on which oysters, muscles, and shells
still adhered, the animals inhabiting which, recently dead, were in a state of putrefaction. Finally the whole banks of the lake of Quintero, which communicates with the sea, had evidently mounted considerably above the level of the water, and in this locality the fact could not escape the least attentive ob
"At Valparaiso the country appeared to be raised about three feet, near Quintero about four. It has been pretended, that at a distance of a mile inland, the rise had been more than six feet; but I do not know the particulars of the measures that led to this last inference.
“In this case there was no volcanic eruption, no lava poured forth, no stones or ashes projected into the atmosphere, and unless it be maintained that the level of the ocean bave fallen, it must be admitted that the earthquake of 19th November 1822, has raised the whole of Chili. Now the last consequence is inevitable, for a change of level in the ocean would have manifested itself equally along the whole extent of the coast of America, while nothing of the kind was observed in the ports of Peru, such as Paytu and Callao.
“If this discussion had not already carried us so far, the preceding observations, from which it results, that in a few hours, and by the effect of a few shocks of an earthquake, an immense extent of country rose above its former level, might have been compared with those which show, that there exists in Europe, a great country (Sweden and Norway) whose level is also rising, but in a gradual manner, and by a cause that acts unceasingly, but which cause is unknown.”
Thus, then, to whatever portion of the earth's surface we turn our eyes, we find the proofs of igneous action ; our existing volcanoes, protruding themselves through the newer stratified formations, and even the diluvium, being in some cases more recent in their origin than the last great catastrophe to which the earth has been subjected; those of more ancient date forcing their way through the upper and lower secondary and transition formations, which are also cut and intersected by dykes of trap, while granite from the size of mountain masses down to their veins, has upheaved and penetrated the oldest stratified rocks. We also find great extents of country rising, sometimes gradually, sometimes suddenly, above their former level.
Mountains, then, are not the nucleus on which our continents and islands have been deposited, but are of subsequent origin, and have in their rise elevated the land to such a height as to be no longer accessible to the waters of the ocean. We may, even by examining through what strata the mountains have been raised, or those which compose their sides and crests when the elevating agent has not pierced through to the surface, infer the geological age which
them birth. A research of this sort has been recently attempted and conducted with great ability by M. E. De Beaumont.
We shall quote an abstract of his reasoning from the “ Annuaire,” for 1830, in the words of Arago, which will also serve to illustrate various other points upon which we have touched.
“ Among the formations of so many different kinds that form the crust of our globe, there is a class which has been called sedimentary (terrains de sediment). Those formations to which this name is properly applied, are composed wholly, or in part, of detritus, carried by water like the mud of our rivers, or the sands
of the beaches of the sea. These sands, in a state of greater or less division, and agglutinated by siliceous or calcareous cements, form the rocks called sandstones.
“ Certain calcareous formations may also be reckoned in the same class, even when they are wholly soluble, as is however rare, in nitric acid ; for the fragments of shells which they contain, show, in another and perhaps better manner, that their formation has also taken place in the bosom of the waters.
“Sedimentary formations are always composed successive layers, that are very distinctly marked. The more recent of them may be arranged into four great divisions, which, in the order of their antiquity, are
“The colitic series or limestone of Jura;
“Although all these formations have been deposited by water, and although they may all be found in the same locality lying upon each other, the passage from the one to the other is never made by insensible gradations. A sudden and marked change is always to be perceived in the physical nature of the deposit, and in that of the organized beings whose remains are found in it. Thus it is evident, that between the epoch at which the limestone of Jura was deposited, and that of the precipitation of the system of greensand and chalk which covers it, there has been upon the surface of the globe a complete change in the state of things. The same may be said of the epoch that separates the precipitation of the chalk from that of the tertiary formations; as it is also evident that in every place the state or nature of the liquid, whence the earths were precipitated, must bave changed completely between the time of the formation of the tertiary strata, and that of the diluvium.
“These considerable variations, sudden, and not gradual, in the nature of the successive deposits formed by the waters, are considered by geologists as the effects of what they call. The Revolutions of the Globe.' And even although it is very difficult to say exactly in what these revolutions consisted, their occurrence is not the less certain on that account.
“I have spoken of the chronological order in which these different sedimentary strata have been deposited : I must therefore state that this order has been determined by following, without interruption, each different formation, to those regions in which it could be ascertained beyond question, and over a great horizontal space, that some particular layer was above some other. Natural excavations, such as the cliffs that border the sea, common wells, and Artesian foun. tains, with the excavation of canals, have furnished powerful aid in this inquiry.
“I have already remarked, that all these sedimentary formations are stratified. In level countries, as might be expected, the disposition of the layers is nearly horizontal. In approaching mountainous countries, this horizontality, generally speaking, ceases ; finally, on the sides of mountains, some of these layers are very much inclined; they even sometimes attain a vertical direction.
“ May not the inclined deposits that we see upon the slopes of mountains, have been deposited in inclined or vertical positions ? Or is it not more natural to suppose, that they originally formed horizontal beds, like the contemporaneous beds of the same nature with which the plains are covered, and that they have been lifted up and assumed new directions at the moment of the elevation of the mountains on whose sides they rest?
“As a general principle, it does not appear impossible that the crests of moun. tains may have been incrusted in place, and in their actual position, by sedimentary deposits, since we daily see the vertical sides of vessels, in which waters charged with sulphate of lime evaporate, covered with a saline crust, whose thickness is continually augmented; but the question before us does not present this general aspect, for it is merely required to determine whether the known sedimentary formations can have been thus deposited. To this question we must reply in the negative, as can be shown by two species of considerations, wholly different from each other.
“Incontestable geological observations have shown, that the calcareous layers which constitute the summits of Buet in Savoy, and Mount Perden in the
Pyrenees, elevated 11,000 or 12,000 feet above the level of the sea, have been formed at the same time with the chalk of the cliffs that border the British channel. If the mass of water whence these strata were precipitated had risen 11,000 or 12,000 feet, the wbole of France would have been covered, and analogous deposits must have existed upon all heights not exceeding 9,000 or 10,000 feet; now, it is found, on the contrary, that in the north of France, where these deposits appear to have undergone little change, the chalk never reaches a height of more than 600 feet above the level of the present sea. They present precisely the disposition of a deposit formed in a basin filled with a liquid whose level has never reached any points that are at the present day elevated more than 600 feet.
I pass to the second proof, borrowed from Saussure, and which appears even more convincing.
“Sedimentary formations often contain pebbles rounded by attrition, and of a figure more or less elliptical. In the places where the stratification is horizontal, the longer axes of these pebbles are all horizontal, for the same reason that an egg cannot stand upon its point. But where the strata are inclined at an angle of 45°, the greater axes of many of these pebbles form this same angle with the horizon; and when the layers become vertical, the greater axes of many of the pebbles become vertical also.
"This observation, in respect to the position of the axes of the pebbles, demonstrates, that the sedimentary formations have not been deposited in the position they now occupy; they have been raised in a greater or less degree, when the mountains, whose sides they cover, have arisen from the bosom of the earth.
“This being proved, it is evident that these sedimentary formations, whose strata present themselves upon the slopes of mountains, in inclined or vertical directions, existed before these mountains arose. The forinations of the same class that are prolonged horizontally, until they meet the same slopes, must be on the contrary of a date posterior to the formation of the mountain ; for it cannot be conceived, that, in rising from the mass of the earth, it should not have elevated at the same time all previously existing strata.
“Let us introduce proper names into the general and simple theory which we have developed, and the discovery of M. de Beaumont will be announced.
“Of the four species of sedimentary formations that we have distinguished, three, and these are the uppermost, the nearest to the surface of the globe, or the most modern, extend in horizontal layers, from the Cote d'Or and from Fo. rez, to the mountains of Saxony ; and only one, which is the oolite or limestone of Jura, shows itself elevated within this district.
“ Therefore the Hartz, the Cote d'Or, and Mount Pilus of Forez, have risen from the globe since the formation of the Jura oolite, and before the deposit of the three other formations.
“ On the slopes of the Pyrenees and Appennines, two of the formations are raised up, namely, the oolite and the greensand and chalk; the tertiary formations, and the diluvium that covers them, bave preserved their primitive horizontality. The Pyrenees and Appennines are, therefore, more modern than the limestone of Jura, and the greensand which they have raised, and more ancient than the tertiary strata and the diluvium.
“The western Alps, and among them Mount Rlanc, have, like the Pyrenees, raised the limestone of Jura, and the greensand, but, in addition, they have also raised the tertiary formations; the diluvium is alone horizontal in the vicinity of these mountains.
“The date of the elevation of Mount Blanc must, therefore, inevitably be placed between the epoch of the formation of the tertiary strata and the diluvium.
“Finally, upon the sides of the central Alps, (Mount St. Gothard,) and of the mountains of Ventorix and Liberon, near Avignon, no one of the sedimentary formations is horizontal ; all the four have been raised up. When these mountains arose, the diluvium itself must have already been deposited.”