Obrázky na stránke

Strata; the other of a class of stones distinguishable from the strata by peculiar mineral composition, by never containing pebbles or the remains of animals and plants, and by never being arranged in parallel layers, from which last character they have been denominated the Unstratified Rocks. We shall now proceed to show in what manner these two classes of rocks are associated together. It is quite evident, that the mode of formation of the two must have been totally different. While the strata, by their parallel arrangement, by the pebbles of pre-existing rocks, and by the remains of living bodies which they contain, demonstrate, that they must have been formed under water, by deposition from the surface downwards, the whole characters of the unstratified rocks equally prove, that they must have come to the surface from the interior of the earth, after the deposition of the strata ; that is, that they have been ejected among the strata from below in a melted condition, either fluid or in a soft yielding state. Geologists have come to this conclusion, from a careful examination and comparison of the unstratified rocks with the products of existing volcanoes, or those, burning mountains, that have thrown out streams of melted stone or lava, both in past ages, as recorded in history, and in our own time. By this comparison they have discovered a great similarity, often an identity, of composition, between the unstratified rocks and lava, and the closest analogy in the phenomena exhibited by the masses of both kinds, and in their relations to the stratified rocks, with which they come in contact.

In every case the unstratified rocks lie under the stratified. This order has never been reversed, except in cases, which have been afterwards discovered to be deceptive appearances, and where they have been protruded between strata. But it


be said, that this fact of inferiority of position is no proof of ejection from below, far less of posteriority of formation : for they might have been the foundation on which the strata are deposited. But their eruption from the

interior, and that that eruption took place after the strata were formed, are proved by other evidence, as we shall presently show.

A section of the crust of the earth, where the stratified and unstratified rocks have been found associated together, has often exhibited the appearance represented by the diagram.

[blocks in formation]

A and B are mountains of granite or of whinstone, with strata of limestone lying upon it. From A branches or shoots connected with the principal mass are seen to penetrate into the superincumbent strata ; and in the mountain B, the granite overlies the limestone for a considerable way near the top, as if it had flowed over at that place, and lower down it has forced its way between two strata, ending like a wedge. Now, as the penetrating substance must necessarily be of subsequent formation to the body that it penetrates, it is evident, that the granite must have been formed after the limestone, although the latter rests upon it. But if any doubt remained, it would be removed by the additional fact, that the granite veins in the mountain A, contain angular fragments of limestone, identical with the strata above; and the fractured ends are seen to fit the places of the continuous stratum, from which they have been broken off.

The posteriority of the formation of the unstratified rocks to the strata is thus made evident from their relative positions; their forcible ejection from below is equally proved by the penetration of their veins or shoots into the superincumbent strata in an upward direction, often with the most slender ramifications to a great distance, and by the portions broken from the strata and enveloped in the substance of the vein. That they were ejected in a soft melted state, produced


by the action of heat, is shown by the close resemblance, in mineral composition, of the unstratified rocks to the products of existing volcanoes, and by remarkable changes often observed to have taken place in the strata, where they come in contact with granite and whinstone. Soft chalk is converted into a hard crystalline limestone like statuary marble; clay and sandstone are changed into a substance as hard and compact as Alint, and coal is turned into coke ; all of them changes which are analagous to what takes place, when the substances are subjected to a strong artificial heat under great pressure. In the case of coal, it is very remarkable ; for when a bed of that substance, and a stratum of clay lying next to it, come in contact with whinstone, the tar of the coal is often driven into the clay, and the coal loses all property of giving fame, although, at a distance from the whin. stone, it is of a rich caking quality.

We have shown, that we are enabled to fix a chronological order of succession of the strata with a considerable degree of precision; and although we have not the same accurate means of determining the relative ages of the unstratified rocks, there are yet very decisive proofs, that certain classes of them are older than others, that different members of the same class have been ejected at distinct periods, and that the same substances have been thrown up at different times far distant from each other. Granite, in veins, has never been seen to penetrate beyond the lower strata ; but whinstone and the lavas of existing volcanoes protrude in masses, and send out veins through all the strata : veins of one sort of granite traverse masses of another kind, and whinstone and basalt veins are not only found crossing masses and other veins of similar rocks, but even of granite. Upon the principle, therefore, before stated, that the penetrating substance must necessarily have been formed subsequently to the body penetrated, the above phenomena demonstrate successive formations or eruptions of the unstratified rocks.

As the highly elevated, broken, and contorted

positions of the strata are only explicable on the supposition of a powerful force acting upon them from below, and as they are seen so elevated and contorted in the neighbourhood of the unstratified rocks, it is a very legitimate inference, that the mountain chains and other inequalities on the earth's surface have been occasioned by the horizontally deposited strata having been heaved up by the eruption of these rocks, although the latter may not always appear, but be only occasionally protruded to the surface, through the rents produced by the eruptive force. The phenomena of earthquakes are connected with the same internal action, and these have often been accompanied by permanent elevations of entire portions of a country. This theory of the elevation of mountains by a force acting from the interior of the earth is not a mere inference from appearances presented by rocks, but is supported by numerous events, which have occurred repeatedly within the period of history down to our own time. In the middle of a gulf in the island of Santorino, in the Grecian Archipelago, an island rose from the sea 144 years before the Christian era; in 1427, it was raised in height, and increased in dimensions; in 1573, another island arose in the same gulf; and in 1707, a third. These islands are composed of hard rock; and in that last formed, there are beds of limestone and of other rocks containing shells. In the year 1822, Chili was visited by a violent earthquake, which raised the whole line of coast, for the distance of above one hundred miles, to the height of three or four feet above its former level. Valparaiso is situated about the middle of the tract thus permanently elevated. A portion of Cutch, near the mouth of the Indus, underwent a similar revolution in the year 1819, when a district, nearly sixty miles in length by sixteen in breadth, was raised by an earthquake about ten feet above its original level. A volcanic eruption burst out in an adjoining part of India at Bhooi, at the exact period when the shocks of this earthquake terminated. These cases must not be con founded with the production of new mountains, such

as that of Jorullo, in Mexico, in the year 1759, which was raised to the height of 1600 feet above the table land of Malpais by eruptions of scoriæ and the outpouring of lava.

The appearance of a new island off the coast of Sicily, in the year 1831, is another phenomenon of the latter class. It rose from a part of the sea, which was known by soundings a few years before to have been 600 feet deep, to the height of 107 feet above the water, and formed a circumference of nearly two-thirds of a mile. It was composed of loose cinders, and the part that rose above the level of the sea, was washed away in the winter of the same year; but an extensive shoal remains.

It must not be supposed, that these internal movements only took place after the whole series of strata had been deposited. There must have been long intervals between the termination of the deposition of one member of the series and the commencement of that of the stratum immediately above it; and internal inovements, accompanied with disturbance of the already deposited strata, after they had come to consolidate into stone, appear to have taken place during the whole period, that the strata, from the lowest to the uppermost in the series, were deposited. The clearest evidence of this is afforded by certain appearances exhibited by the strata, in all parts of the globe, that have yet been examined. The diagram that follows represents a case of very common occurrence, and will explain our meaning. It must be borne in mind, that it is an acknowledged principle in geology, that all stratified rocks, in whatever position they are now found, must have been originally deposited horizontally.

[blocks in formation]

There are here five different series of strata, a, b, c, d, e. Now, it is evident that the series a must have been first disturbed ; that after its change of position, the series b and c were deposited, covering the ends of

« PredošláPokračovať »