페이지 이미지
PDF
ePub

short time the whole mountain appeared like a mass of liquid fire, which gradually extended in every direction. As the eruption continued, a darkness supervened, s0 profound as to obscure even the light of the flames; showers of stones and ashes fell continuously over the whole island; the sea rose twelve feet higher than it had ever been known to do before; and finally a whirlwind ensued, which tore up the largest trees, and carried them into the air, together with men, horses, cattle, and whatever else came within its influence. Of 12,000 inhabitants in the vicinity only six are believed to have es. caped, and of some entire villages not even a vestige remained.

In 1772, the Papandayang, one of the loftiest volcanic mountains in Java, after a short but severe eruption, suddenly fell in and disappeared in the earth, carrying with it about ninety square miles of territory. Forty villages were engulfed, or covered with ejected matter, at the same time, and nearly 3,000 persons perished.- Weils' Illustrated Geology.

DESCRIPTIONS OF AN EARTHQUAKE.

"A powerful eathquake," says Mr. Darwin, "at once destroys the oldest associations; tho world, the very emblem of all that is solid, has moved beneath our feet like a crust over a fluid; one second of time has conveyed to the mind a strange idea of insecurity, which hours of reflection would never have created."

“To man,” says Humbolt, “the earthquake conveys an idos of some universal and unlimited danger. We may flee from the crater of a volcano in active eruption, or from a locality threatened by the approach of a lava stream; but in an earthquake, direct our flight whithersoever we will, we still feel as though we trod upon the very focus of destruction. Every sound-the faintest motion in the air-arrests our attention, and we no longer trust the ground on which we stand. Ani. mals, especially dogs and swine, participate in the same anxious disquietude; and even crocodiles, in the rivers of South Amer. ica, which at other times are dumb, have been observed to quit the water and run, with loud cries, into the adjacent forests."

AQUEOUS AND ATMOSPHERIC AGENCIES.

The aqueous and atmospheric agencies most prominently concerned in producing geological changes, are rains, and the

gasses and moisture of the atmosphere, winds, ice, and snow, springs, rivers, waves, tides, and oceanic currents.

The operation of water, acting mechanically, is, under all circumstances, to wear down the higher portions of the earth's crust, and transport the materials to lower localities-an action which obviously tends to reduce the whole surface to a smooth and uniform level. On the other hand, the operations of igneous agents-volcanoes, earthquakes, etc.—by breaking op and elevating the crust of the earth, tend to counteract the equalizing action of water and to produce that diversity ofsurface which is indispensable to variety in both the vegetable and animal kingdoms. These two forces, therefore-the aqueous and the igneous-may be considered as antagonistic to each other, and to them may be ascribed the principal modifications which have taken place, and are still taking place, in the crust of the globe.- Well's Ilustrated Geology.

CORAL REEFS.

"The ocean,” says Mr. Darwin, throwing its breakers on the outer shore, appears an invincible enemy, yet we see it resisted, and even conquered, by means which at first seem weak and inefficient. No periods of repose are granted, and the heavy swell caused by the steady action of the trade wind never ceases. The breakers exceed in violence those of our temperate regions; and it is impossible to behold them without feeling a conviction that rocks of granite or quartz would ultimately be demolished by such irresistable forces. Yet these low coral islands stand and are victorious, for hero another power, antagonistic to the former, takes part in the contest. The organic forces separate the atoms of carbonate of lime, one by one, from the foaming breakere, and unite them into a symmetrical structure; myriads of architects are at work day and night, month after month, and we see their soft and gelatinous bodies, through the agency of the vital laws, conquering the great mechanical power of the waves of the ocean, which neither the art of man nor the mechanical works of nature could successfully resist." The animals which produce coral are very simple, and resemble plants both in their figures and colors.

THE FIRST FORMED STRATIFIED ROCKS.

The adoption of the theory, that our earth was once in a state of entire molten fluidity, involves the existence of a subsequent period, when its primeval crust had sufficiently cooled down to allow of the condensation of watery vapor and of the existence of a sea upon its surface. Whenever this happened, the eroding and destructive action of water must have immediately manifested itself, while the particles of the consolidated igneous crust, worn off by the action of waves, tides, and currents, and deposited as sediments, would naturally produce stratified formations.

The internal heat of the earth at that period, however, must have continued to act with great intensity near the surface, and the strata first deposited, consequently, were, in all probability, soon greatly metamorphosed, 2. e., remelted dowr to form igneous rocks, or converted into hard crystalline semiigneous rocks, that retained, in part, their original lines of stratification.

Whether any of these first formed stratified rocks are in existence, and open to our inspection, it is impossible to affirm. Some geologists incline to the opinion that they were entirely remelted, and are now represented by the older or fundamental granites, which, in some instances, appear to have an obscurely stratified structure.

Be this as it may, it is, however, a matter of fact, that the oldest rocks of which wə have any knowledge, which exhibit evidence of a sedimentary origin, appear to have been formed under conditions analogous to those above supposed. Thus, they are all more or less crystalline and indurated; their lines of stratification are indistinct, and often altogether oblitera. ted; and their whole aspect is very different from what is usually ascribed to rocks deposited in water.- Wells' Illustrated Geology.

FORMATION OF COAL. It is now universally admitted by geologists, that coal is a mass of compressed, altered, and mineralized vegetation, just as sandstone is consolidated sand, and the slate and shale con. solidated clay or mud.

The evidence upon which the belief is founded may be briefly stated, as follows:

1st. The enormous profusion of fossil plante, in the form of

impressions of leaves, trunks, branches, and barks of trees, found in immediate connection with coal seams. 2d. Coal is composed of carbon, hydrogen, and oxygen, the same elements (though differing in proportion) which enter into the composition of plants. 3d. The substance of coal, when examined under the microscope, affordz unmistakable evidence of a vegetable (cellular) structure. 4ih. All the stages of gradation between perfect wood and perfect coal may be traced with the greatest certainty.

But granting the vegetable origin of coal, the question immediately suggests itself: Under what circumstances could 80 great an amount of vegetable matter have ever accumulated ?-the magnitude of which may be realized in a degree, from the asserted fact that all the forests of the United States, if gathered into one heap, would fail to furnish the materials of a single coal seam equal to that of Pittsburg, Penn."

Furthermore, coal is found stratified, laminated, and extended, in horizontal beds, which often cover very large areas, with a nearly constant thickness--the great Pittsburg coal seam, above referred to for example, having a nearly uniform thickness of from eight to twelve feet, and is estimated to have once covered a surface of 90,000 square miles. Coal, moreover, is ordinarily encased between beds of shale or sandstone, which bear evident proof of having been slowly deposited in quiet waters. In some coal fields, as many as seventy seams of coal, varying in thickness from a few inches to four, six, eight, ten, twelve, and twenty feet, occur thus interstratified with shales and sandstones; and yet, notwithstanding these frequent alternations of material, the purity of the coal is such, that it rarely contains any considerable ad. mixture of mud, sand, or other foreign mineral substances.

In explanation of these phenomena, various hypotheses have been suggested, but the general opinion of the best geologists of the present day is, that the vegetable matter constituting coal, must, in the main, have grown and accumulated in immense jangles and peat mosses for many years; that the land must have then sunk, and become the basin of a lake or estuary, into which rivers carried mud and sand; these, covering the vegetable matter, gradually consolidated into shales and sandstones, while the vegetable matter itself underwent the process of mineralization, and was converted into

[ocr errors]

coal. This being done, it is supposed that the area of deposit was again elevated, so as to become once more the scene of luxuriant vegetation; then again submerged, and overlaid by new depr jf sandstone and shale; then once more elevated and cor

with plants, and again submerged; and these alternai of submergence and elevations are presumed to have un place as often as there are beds of coal in any particular coal field.- Well's Klustrated Geology.

CLIMATIC CONDITIONS OF THE CARBONIFEROUS ERA.

There is one circumstance in connection with the formation of coal which has given rise to a vast amount of ingenious speculation and hypotheses, viz: tho apparent sameness of external conditions over such extensive areas of the earth as are now occupied by our known coal fields. Thus, the same gigantic ferns and club-mosses are found alike in the coal fields of America, Europe, Melville Island, Greenland, and Australia-regions widely separated, and at once tropical, temperate, and frigid. To account for this luxuriance and homogeneity of vegetable growth various causes have been buggested, as the earth’s central heat, a change in the earth's axis, a larger percentage of carbonic acid in the atmosphere, the planetary system moving through warmer regions of space, and the like; but thus far geologists have arrived at no definite conclusions on the subject.

Deposits of carbonaceous matter have occurred at almost every period of the earth's history, as is evidenced by the fact that thin seams of coal are found in almost all the geological systems; but the coal beds which admit of economical working are almost exclusively confined to the carboniferous bygtem. The only exceptions are a few coal fields belonging to the Dolitic or Jurassic system, which, in Virginia and some other localities, admit of profitable mining. It seems, therefore, certain, that whatever may have been the conditions which allowed of so abandant a terrestrial vegetation at this particular epoch of the earth's history, those conditions ceased about the time when the era of the Carboniferous system terminated. A high temperature was evidently not one of these conditions, for there are evidences of it afterwards; and some authorities incline to the belief that the superabundance of carbonic acid gas, which is supposed to bavo existed during

« 이전계속 »