The Elements of Geology; Adapted to the Use of Schools and Colleges Part 10

The occurrence of layers of different composition, as one way in which the stratification is indicated, is produced by local and frequently recurring causes. There are, however, other alternations of much greater extent; those, for example, nearly twenty in number, distinguished by striking differences in lithological character, into which the New York system of rocks is divided. These alternations have resulted from more general causes. The physical geography of a wide region must have been so different, at the different periods during which these several formations were deposited, as to change, at each period, the kind of sediment furnished to the forming currents, and modify the types of animal life.

We have seen that the same causes that determined the stratified arrangement will determine the alternations of strata of coa.r.s.e and fine materials.

It is obvious that the stratification of the marine deposits will be nearly horizontal. If the surface were very irregular upon which the deposition commenced, the irregularity would constantly diminish; for the movement of the water over this surface, however slow, would tend to remove the acc.u.mulations from the highest points, and leave them at the lowest (Fig. 75). Delta and lake deposits will, however, dip somewhat, though never at a high angle, towards the deep water. In certain situations, where a river and a tidal wave, coming in conflict, cause, in succession, eddies and currents in opposite directions, we should expect to find the stratification very irregular (Fig. 76); sometimes false stratifications (_a b_), sometimes the strata cut off abruptly, and at other times contorted or dipping in opposite directions within short distances.

[Ill.u.s.tration: Fig. 75.]

Wherever sediment is deposited, it will entomb whatever of the remains of animal or vegetable life may be mingled with it. They will be at once protected against the influence of all the ordinary decomposing agencies, and will continue for ages to retain their peculiar markings, and even their colors. They will thus const.i.tute, in all future time, a record of the present condition of the organic world. The lacustrine deposits can contain only fresh-water species of animals, marine deposits only marine animals, while deltas may contain the remains of marine life mingled with those which have been washed down by rivers.

The remains of birds, insects, and terrestrial animals, may occasionally occur, in every kind of deposit. Sediment deposited in deep water will never contain fossils in abundance, the deep parts of the ocean being almost wholly dest.i.tute of animal or vegetable life. It is only in water of a few fathoms that the greater number of species and of individuals occur. In all these particulars the deposits now forming sustain a close resemblance to the older formations.

[Ill.u.s.tration: Fig. 76.]

There are certain formations, as that of the coal, which required conditions for their formation different from those of ordinary sedimentary deposits. Coal consists of mineralized vegetable matter. Its vegetable origin is proved by the uniform occurrence of vegetable fossils almost exclusively in the coal measures. When reduced to thin slices and examined under a high magnifying power, a structure very similar to the ligneous tissue of existing coniferae is sometimes found to exist. There are probably vegetable deposits now taking place not altogether unlike those which produced the coal measures.

We know that many rivers--the Mississippi, for example--now carry into the sea great quant.i.ties of ligneous matter. Before the country was inhabited by man, the quant.i.ty was undoubtedly much greater than it now is. It floats for a time; but the ligneous tissue itself is heavier than water, and as soon as the air is excluded from the pores, and they are filled with water, it will sink. The woody and earthy matters are swept into the sea together; but, as they sink under different circ.u.mstances, they will be deposited separately. _Thus wood may continue to acc.u.mulate in particular places in the sea_ for long periods, with but little intermixture of earthy substances.

It is, however, to be expected that, in the progress of geological changes, the places which at one time receive deposits of wood will at another receive detrital matter, and thus _the wood will become deeply buried_ beneath sedimentary strata.

Wood thus situated will become converted into coal. Trees which had been covered to considerable depth with earth have been found near the Mississippi river changed to lignite, a substance resembling charcoal.

In this case, the wood had been exposed to no greater heat than is common to the crust of the earth at the depth where it was found; and yet it had undergone this change since the country has been known to Europeans, as it retained the marks of the axe when it was discovered.

It has also been found by experiment that vegetable matter, by long submersion in water, pa.s.ses into the state of lignite. This is the first step in the conversion of wood into mineral coal.

When lignite is exposed to moderate heat and great pressure, it loses the characters of lignite, and becomes mineral coal. This is shown by facts observed in Germany, Ireland and Iceland, where beds of lignite have been overspread by basalt. The upper portions of the lignite are changed to mineral coal. The lower portions, which the heat did not reach, retain the characters of lignite.

Beds of vegetable matter, with a great thickness of rock deposited above them, would therefore be subject to all the conditions necessary to convert them into coal, namely, pressure from the superinc.u.mbent ma.s.s, and the heat which the strata uniformly a.s.sume at great depths.

It is not improbable, therefore, that coal-beds are now forming, and that they have been formed at every geological period since an abundant terrestrial vegetation commenced. Accordingly, there occurs in Virginia an extensive coal-field in the oolite formation. Coal-fields also occur in England, of less extent, in the same formation. In France, and other parts of Europe, there are extensive beds of lignite in the tertiary formation.

We have therefore no difficulty in accounting, in a general way, for the formations of the carboniferous period. The vegetables were probably less woody than those of the present time of equal size, and were therefore more easily prostrated and committed to the waters. They grew rapidly in moist ground, and perhaps in shoal-water, and required an atmosphere charged with moisture and of a high temperature. Thus much is inferred from the conditions most favorable for the growth of recent species a.n.a.logous to the coal-plants. These recent species are tropical plants, and grow in moist insular situations, conditions which would have existed at the carboniferous period, if the present coal-fields were then an archipelago dotted with low islands.

Such being regarded as the origin of the coal-beds, the _alternations_ of the earthy and carbonaceous strata may be referred, provisionally, to those great changes in physical geography upon which the other alternations of strata on a large scale depend. But the regularity with which the coal-seams and sandstone succeed each other presents some difficulties which, in the present state of knowledge, we cannot satisfactorily account for.

_Beds of salt_ occur, interstratified with other rocks, in nearly all countries. Still, it is not a sedimentary deposit, and its formation must depend upon peculiar circ.u.mstances. In New York, saline, together with earthy matter, const.i.tutes the Onondaga limestone, one of the formations of the New York system. In Kentucky, the strata of rock-salt are in the coal formation; in England, they are in the new red sandstone; in Spain, they are in the greensand, and in Poland they are in tertiary strata. The conditions of its formation have therefore existed in connection with the deposition of every fossiliferous rock.

It has been shown that the ocean is the princ.i.p.al reservoir of the saline matters which are taken up whenever water percolates through rocks. It must happen not unfrequently, in the course of submarine elevations, that a basin of sea-water will be cut off from its communication with the sea; and from this basin the evaporation might be more rapid than the supply of water. The great salt-lake of Utah is undoubtedly a basin of this kind. The Mediterranean Sea is another such basin, not yet wholly separated from the ocean. The evaporation exceeds the supply of water from the rivers, and a powerful stream is therefore continually thrown in from the ocean, through the Strait of Gibraltar.

The waters of the Mediterranean are already more highly charged with salt than ordinary sea-water. This sea may ultimately become a saturated solution, and begin to deposit salt. But whether it does, or not, it indicates the way in which salt-beds may be formed.

V. _Solidification of Aqueous Deposits._

Sediment is generally deposited as a soft mud, but in nearly all the older formations it has become solidified. When rocks are deposited from a chemical solution, they take at once the solid form. Such is the case with rock-salt and with limestone, when the material has been held in solution. Solidification takes place in nearly the same way when water which holds carbonate of lime or oxide of iron in solution filters through beds of sand or gravel. The substance held in solution is deposited in the interstices till they become filled, and the whole is changed to solid rock.

Some rocks are composed of such materials that they _set_, like hydraulic cement, when they are deposited. Other rocks become solid simply by drying. Thus a deposit now forming in Lake Superior becomes, by drying, nearly as hard as granite. Such a deposit will therefore become solid whenever it shall be elevated above the water.

The pressure to which all but the upper layers are subjected is probably sufficient to reduce most rocks to the solid state. Dry and pulverized clay is reduced by artificial pressure, for a moment, almost to stone.

The pressure upon the deep-seated rocks is constant, and greater than any artificial pressure can be.

In addition to these causes, all the older rocks have been subjected to a high temperature, some of them nearly to that of fusion. By this means the solidification of every kind of rock would be promoted, and probably some may have been reduced by it to the solid state, which would otherwise have remained as an incoherent ma.s.s.

SECTION V.--AQUEO-GLACIAL ACTION.

1. _Glaciers._--A glacier is a ma.s.s of ice occupying the bed of a mountain valley, having a slow progressive motion, and reaching somewhat lower in the valley than the line of constant snow. (Fig. 77.) The Glacier des Bois, which may be regarded as a specimen of the Alpine glaciers, covers an area of about seventeen square miles. In its lowest portion, when all its branches have become united into one stream, it has an average width of half a mile, and is five miles long.

It is estimated that the glaciers of the Alps cover an area of fourteen hundred square miles. These have been the most carefully studied, though glaciers are found in the valleys of various other ranges of mountains.

In the higher valleys, the snow, which falls at all seasons of the year, acc.u.mulates in immense quant.i.ties, and the steep mountain sides contribute, by frequent avalanches, to this acc.u.mulation. The snow, when thus increased, does not become a compact, adhesive ma.s.s; but, changing into particles of solid ice, it resembles sand rather than snow. It is this _neve_ which const.i.tutes the upper part of every _glacier_, and which, in a modified form, const.i.tutes the lower part.

The valleys descend rapidly towards the base of the mountains; and this snow-ice, having no cohesion between its particles, _moves slowly down the slope of the valley_, like a very imperfect liquid. After descending below the line of perpetual snow, the surface will melt during the day; and the water, sinking into the porous ma.s.s, becomes frozen, and converts the whole into more or less compact ice, yet never into a rigid ma.s.s. Influenced by its own weight, and by the pressure of the snow-ice behind, it still continues its motion, and conforms itself to the shape and curves of the valley through which it pa.s.ses. The average movement per annum may be stated at about five hundred feet.

The temperature of the rocky bed of the valley will be a little, and but a little, higher than thirty-two degrees. There will therefore be but little melting at the bed of this river of ice. As it receives continual accessions from the atmosphere, it will therefore increase in volume till it descends to the level of perpetual snow. Below this line _the waste exceeds the addition_; and as it approaches the lower and cultivated portions of the valley, it rapidly diminishes, till it finally loses the solid form, and becomes a rivulet. The terminus of the glacier is determined princ.i.p.ally by the general climate of the country.

Any considerable variation of climate will cause it to recede, or descend lower down the valley. The terminus varies, however, somewhat with the seasons, being lower in winter than in summer, though the motion is much less in the cold season than in the warm; and it descends many rods further some seasons than it does others.

The glacier consists princ.i.p.ally of snow, more or less modified in structure; but it also contains whatever else may have been thrown upon its surface, or into the snows by which it is fed. Tributary glaciers extend up through all the gorges into which the irregular surface of the mountain-top is divided. On these rough peaks there are always fragments of rock, varying in size from fine sand to ma.s.ses weighing many tons; some of them loosened when the mountain was upheaved, some by subsequent earthquake vibrations, and others still by tempests, lightnings, and changes of temperature. When the snow has acc.u.mulated to a certain extent on the steep slopes, it falls in avalanches into the valleys, carrying with it loosened ma.s.ses of rock, and often breaking off large fragments from the rocky escarpments against which it strikes. These avalanches are almost constantly descending, and hence a glacier always contains considerable _earthy matter distributed through it_.

[Ill.u.s.tration: Fig. 77.]

The friction of the glacier, at its edges and along its bed, separates more or less of the rock over which it moves; and hence there is always a layer of mud and pebbles under the glacier, and a line of loose fragments, called a _lateral moraine_, at the sides. When two glaciers unite, the two lateral moraines, thus brought together, come to the surface, forming a medial moraine, and show the line of junction sometimes for miles.

The friction of the glacier on the bed of rock, a.s.sisted by the layer of pebbles, will wear down the prominent portions, and everywhere polish the surface. Fragments of rocks may be frozen into the glacier at all depths. Those which lie near the lower surface of the glacier would, by slight melting of that surface, project downward so as to act as a graver's tool on the rock over which it pa.s.ses. Hence, when the extremity of the glacier has receded beyond its ordinary limit, the surface of rock exposed is found, upon examination, to be _polished_, _striated_, and _occasionally grooved_ an inch or two deep.

Since the waste is almost wholly superficial, earthy matter, which was at first concealed in the ma.s.s of the glacier, is continually coming to view, as the surface melts and runs off. Thus, none of the freight of the glacier is left along its course, but all is carried to its terminus and discharged there. Hence, at the lower extremity of the glacier there is always an embankment of earth, pebbles, and boulders. If the glacier recedes a few yards at one season of the year, and leaves its earthy fragments scattered over this surface, they will be pushed forward into a ridge, as the glacier again advances. This ridge is called a _terminal moraine_, and consists wholly of substances which have been separated from the mountain ma.s.s, often at the highest beginnings of the glacier.

At the terminus of all the Alpine glaciers, there is a series of these moraines (_a a a_, Fig. 77) marking the successive limits of the glacier in former times.

There is a ridge of boulders on the north side of the Swiss valley, near the base of the Jura Mountains, resembling a terminal moraine. These boulders consist of several groups, distinguished by peculiarities of structure and composition; and each group lies opposite to the particular Alpine valley which now furnishes the same kind of fragments. It has been thought that, at a former period of more severe climate, the Swiss valley was filled in part with ice, and that the present glaciers extended across it to the Jura Mountains.

It is found that the polished and striated surfaces of the rocks in the Alpine valleys are precisely like the surface of the rock, which has not been exposed to atmospheric influences, in the north of Europe and America. It has been proposed to extend the glacier theory, and account for these phenomena by supposing that the north polar regions were, at the ice period, capped with a glacier-ma.s.s, extending as far south as the drift phenomena appear.

It is not to be doubted that the phenomena of polished surfaces and transported materials in the immediate vicinity of the Alps, and near other high mountains, are correctly referred to glacial action. This theory has therefore solved, in part, one of the most difficult problems in geology; but there is great difficulty in extending it so as to account for the drift phenomena in general. If the motion depends upon gravitation only, the origin must have a much greater elevation than the terminus, which would not be the case in the great glacier supposed to extend southward from the Arctic regions. Elevation of temperature, it has been thought, might account for the movement of the ma.s.s southward.

2. _Icebergs._--In very high lat.i.tudes, the ice, which makes out from the land into the sea during the cold season, suffers but little waste at any time. This sheet of ice continues to increase in breadth and thickness, by congelation, from year to year. The spray and the snows of each succeeding year will also add to the ma.s.s. It thus acc.u.mulates to the height of several hundred yards. It will also reach down a good many feet below the surface of the sea, and will extend back on the land, or lie heaped up against a precipitous escarpment, and firmly frozen to it.

After a certain amount of extension over the sea, the acc.u.mulated weight of the ice and snow would tend to depress it, and break it loose from the sh.o.r.e. The waves would tend to the same result, and would act at greater mechanical advantage, as its extension from the sh.o.r.e becomes greater. _Hence, it would ultimately become separated from the sh.o.r.e_, and float in the water.

At its commencement, the earth, pebbles and rocks, which may lie along the sh.o.r.e, and as far down into the sea as the congelation extends, are frozen into it. In many situations its ma.s.s would be increased by avalanches while it remained attached to the land, and these would supply also ma.s.ses of earth and rocks, as they do to glaciers. When it becomes loosened from the sh.o.r.e, it will break off, and carry with it some of the earthy portions of the coast, or the less firmly fixed ma.s.ses of rock from the escarpment against which it formed. Thus every iceberg becomes _freighted_, more or less, _with earth and rocks_. This has almost uniformly been found to be the case, when they have been landed upon by s.h.i.+ps' crews and examined.

We have seen that the general tendency of the waters of the ocean, and of the lower stratum of the atmosphere, is to a motion from the poles towards the equator. However irregular, therefore, the course of an iceberg may be, its general _movement_, influenced both by the prevailing winds and by ocean currents, will be _towards the equator_.

[Ill.u.s.tration: Fig. 78.]

These floating ice-mountains (Fig. 78) are formed in _great numbers, and of vast size_. The relative specific gravity of ice and water are such that nine cubic feet of ice, below the surface of water, will support one cubic foot above it. As icebergs are often one or two hundred feet high, their vertical depth must be a thousand feet at least; and their area is equal to a square mile, and sometimes it is much greater. In 1840, the United States Exploring Expedition, in the extreme southern ocean, coasted for eighty miles along a single iceberg. They are never absent from the polar seas; and at certain seasons they are so abundant along the usual course of vessels from New York to Liverpool, as greatly to obstruct and endanger navigation.

An iceberg may continue for some time to increase in size, while floating in the polar seas, but will at length reach a lat.i.tude where the waste will exceed the additions, in consequence of the temperature both of the air and of the water. It will, therefore, drop gradually the earthy matters which it contains, upon the bed of the ocean.

It is not improbable that icebergs may often reach down so far as to strike the highest points of the bed of the sea. The ice would be lifted, and glide over the elevation, without suffering any perceptible deviation from its general course. It would thus affect the surface of rocks exactly like a glacier. If, however, the iceberg becomes permanently stranded, and melts in one place, its earthy matters will be thrown down upon the elevation which first arrested it.

If the bed of the sea, between the fortieth and sixtieth degrees of lat.i.tude, could be exposed for examination, the rocky surface would be found to be polished and striated by the icebergs which have pa.s.sed over it, and the whole surface would be strewed with boulders and drifted materials brought from Arctic and Antarctic lands. Sometimes it would be acc.u.mulated in heaps, and sometimes spread nearly over the surface.

We have seen that very recently, probably about the close of the tertiary period, the portion of Europe and America over which the northern drift is found, has been depressed several hundred feet. It may be presumed that at that time icebergs floated over it, polished the surface of the rocks, and distributed the boulders and other drift which is now found upon it.