The Student's Elements of Geology Part 2

(FIGURE 6. Cliff between mismer and Dunwich.)

There is also another phenomenon of frequent occurrence. We find a series of larger strata, each of which is composed of a number of minor layers placed obliquely to the general planes of stratification. To this diagonal arrangement the name of "false or cross bedding" has been given. Thus in the section (Figure 3) we see seven or eight large beds of loose sand, yellow and brown, and the lines a, b, c mark some of the princ.i.p.al planes of stratification, which are nearly horizontal. But the greater part of the subordinate laminae do not conform to these planes, but have often a steep slope, the inclination being sometimes towards opposite points of the compa.s.s. When the sand is loose and incoherent, as in the case here represented, the deviation from parallelism of the slanting laminae can not possibly be accounted for by any rearrangement of the particles acquired during the consolidation of the rock. In what manner, then, can such irregularities be due to original deposition? We must suppose that at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots, instead of being spread out uniformly over a wide area. Sometimes, when banks are thus formed, currents may cut pa.s.sages through them, just as a river forms its bed. Suppose the bank A (Figure 4) to be thus formed with a steep sloping side, and, the water being in a tranquil state, the layer of sediment No. 1 is thrown down upon it, conforming nearly to its surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession, so that the bank B C D is formed. If the current then increases in velocity, it may cut away the upper portion of this ma.s.s down to the dotted line e, and deposit the materials thus removed farther on, so as to form the layers 5, 6, 7, 8. We have now the bank B, C, D, E (Figure 5), of which the surface is almost level, and on which the nearly horizontal layers, 9, 10, 11, may then acc.u.mulate. It was shown in Figure 3 that the diagonal layers of successive strata may sometimes have an opposite slope. This is well seen in some cliffs of loose sand on the Suffolk coast. A portion of one of these is represented in Figure 6, where the layers, of which there are about six in the thickness of an inch, are composed of quartzose grains. This arrangement may have been due to the altered direction of the tides and currents in the same place.

(FIGURE 7. Section from Monte Calvo to the sea by the valley of the Magnan, near Nice.

A. Dolomite and sandstone. (Green-sand formation?) a, b, d. Beds of gravel and sand.

c. Fine marl and sand of Ste. Madeleine, with marine (Pliocene) sh.e.l.ls.)

The description above given of the slanting position of the minor layers const.i.tuting a single stratum is in certain cases applicable on a much grander scale to ma.s.ses several hundred feet thick, and many miles in extent. A fine example may be seen at the base of the Maritime Alps near Nice. The mountains here terminate abruptly in the sea, so that a depth of one hundred fathoms is often found within a stone's throw of the beach, and sometimes a depth of 3000 feet within half a mile. But at certain points, strata of sand, marl, or conglomerate intervene between the sh.o.r.e and the mountains, as in the section (Figure 7), where a vast succession of slanting beds of gravel and sand may be traced from the sea to Monte Calvo, a distance of no less than nine miles in a straight line. The dip of these beds is remarkably uniform, being always southward or towards the Mediterranean, at an angle of about 25 degrees. They are exposed to view in nearly vertical precipices, varying from 200 to 600 feet in height, which bound the valley through which the river Magnan flows.

Although, in a general view, the strata appear to be parallel and uniform, they are nevertheless found, when examined closely, to be wedge-shaped, and to thin out when followed for a few hundred feet or yards, so that we may suppose them to have been thrown down originally upon the side of a steep bank where a river or Alpine torrent discharged itself into a deep and tranquil sea, and formed a delta, which advanced gradually from the base of Monte Calvo to a distance of nine miles from the original sh.o.r.e. If subsequently this part of the Alps and bed of the sea were raised 700 feet, the delta may have emerged, a deep channel may then have been cut through it by the river, and the coast may at the same time have acquired its present configuration.

(FIGURE 8. Slab of ripple-marked (New Red) sandstone from Ches.h.i.+re.)

It is well known that the torrents and streams which now descend from the Alpine declivities to the sh.o.r.e, bring down annually, when the snow melts, vast quant.i.ties of s.h.i.+ngle and sand, and then, as they subside, fine mud, while in summer they are nearly or entirely dry; so that it may be safely a.s.sumed that deposits like those of the valley of the Magnan, consisting of coa.r.s.e gravel alternating with fine sediment, are still in progress at many points, as, for instance, at the mouth of the Var. They must advance upon the Mediterranean in the form of great shoals terminating in a steep talus; such being the original mode of acc.u.mulation of all coa.r.s.e materials conveyed into deep water, especially where they are composed in great part of pebbles, which can not be transported to indefinite distances by currents of moderate velocity. By inattention to facts and inferences of this kind, a very exaggerated estimate has sometimes been made of the supposed depth of the ancient ocean. There can be no doubt, for example, that the strata a, Figure 7, or those nearest to Monte Calvo, are older than those indicated by b, and these again were formed before c; but the vertical depth of gravel and sand in any one place can not be proved to amount even to 1000 feet, although it may perhaps be much greater, yet probably never exceeding at any point 3000 or 4000 feet. But were we to a.s.sume that all the strata were once horizontal, and that their present dip or inclination was due to subsequent movements, we should then be forced to conclude that a sea several miles deep had been filled up with alternate layers of mud and pebbles thrown down one upon another.

In the locality now under consideration, situated a few miles to the west of Nice, there are many geological data, the details of which can not be given in this place, all leading to the opinion that, when the deposit of the Magnan was formed, the shape and outline of the Alpine declivities and the sh.o.r.e greatly resembled what we now behold at many points in the neighbourhood. That the beds a, b, c, d are of comparatively modern date is proved by this fact, that in seams of loamy marl intervening between the pebbly beds are fossil sh.e.l.ls, half of which belong to species now living in the Mediterranean.

RIPPLE-MARK.

The ripple-mark, so common on the surface of sandstones of all ages (see Figure 8), and which is so often seen on the sea-sh.o.r.e at low tide, seems to originate in the drifting of materials along the bottom of the water, in a manner very similar to that which may explain the inclined layers above described. This ripple is not entirely confined to the beach between high and low water mark, but is also produced on sands which are constantly covered by water. Similar undulating ridges and furrows may also be sometimes seen on the surface of drift snow and blown sand.

The ripple-mark is usually an indication of a sea-beach, or of water from six to ten feet deep, for the agitation caused by waves even during storms extends to a very slight depth. To this rule, however, there are some exceptions, and recent ripple-marks have been observed at the depth of 60 or 70 feet. It has also been ascertained that currents or large bodies of water in motion may disturb mud and sand at the depth of 300 or even 450 feet. (Darwin Volcanic Islands page 134.) Beach ripple, however, may usually be distinguished from current ripple by frequent changes in its direction. In a slab of sandstone, not more than an inch thick, the furrows or ridges of an ancient ripple may often be seen in several successive laminae to run towards different points of the compa.s.s.

CHAPTER III.

ARRANGEMENT OF FOSSILS IN STRATA.-- FRESH-WATER AND MARINE FOSSILS.

Successive Deposition indicated by Fossils.

Limestones formed of Corals and Sh.e.l.ls.

Proofs of gradual Increase of Strata derived from Fossils.

Serpula attached to Spatangus.

Wood bored by Teredina.

Tripoli formed of Infusoria.

Chalk derived princ.i.p.ally from Organic Bodies.

Distinction of Fresh-water from Marine Formations.

Genera of Fresh-water and Land Sh.e.l.ls.

Rules for recognising Marine Testacea.

Gyrogonite and Chara.

Fresh-water Fishes.

Alternation of Marine and Fresh-water Deposits.

Lym-Fiord.

Having in the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic remains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if particular kinds of fossils did not occur here and there at certain depths in the ma.s.s. At one level, for example, univalve sh.e.l.ls of some one or more species predominate; at another, bivalve sh.e.l.ls; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.

It may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become full of fossils from top to bottom; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the acc.u.mulation of sediment. He must never lose sight of the fact that, during the process of deposition, each separate layer was once the uppermost, and immediately in contact with the water in which aquatic animals lived. Each stratum, in fact, however far it may now lie beneath the surface, was once in the state of s.h.i.+ngle, or loose sand or soft mud at the bottom of the sea, in which sh.e.l.ls and other bodies easily became enveloped.

RATE OF DEPOSITION INDICATED BY FOSSILS.

By attending to the nature of these remains, we are often enabled to determine whether the deposition was slow or rapid, whether it took place in a deep or shallow sea, near the sh.o.r.e or far from land, and whether the water was salt, brackish, or fresh. Some limestones consist almost exclusively of corals, and in many cases it is evident that the present position of each fossil zoophyte has been determined by the manner in which it grew originally. The axis of the coral, for example, if its natural growth is erect, still remains at right angles to the plane of stratification. If the stratum be now horizontal, the round spherical heads of certain species continue uppermost, and their points of attachment are directed downward. This arrangement is sometimes repeated throughout a great succession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and some of the fossils must have flourished for ages like forest-trees, before they attained so large a size. During these ages, the water must have been clear and transparent, for such corals can not live in turbid water.

(FIGURE 9. Fossil Gryphaea, covered both on the outside and inside with fossil Serpulae.)

In like manner, when we see thousands of full-grown sh.e.l.ls dispersed everywhere throughout a long series of strata, we can not doubt that time was required for the multiplication of successive generations; and the evidence of slow acc.u.mulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with Serpulae, or barnacles (acorn-sh.e.l.ls), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the creatures whose remains now adhere to it grew from an embryonic to a mature state. Attached sh.e.l.ls which are merely external, like some of the Serpulae (a) in Figure 9, may often have grown upon an oyster or other sh.e.l.l while the animal within was still living; but if they are found on the inside, it could only happen after the death of the inhabitant of the sh.e.l.l which affords the support. Thus, in Figure 9, it will be seen that two Serpulae have grown on the interior, one of them exactly on the place where the adductor muscle of the Gryphaea (a kind of oyster) was fixed.

(FIGURE 10. Serpula attached to a fossil Micraster from the Chalk.)

(FIGURE 11. Recent Spatangus with the spines removed from one side.

b. Spine and tubercles, natural size.

a. The same magnified.)

Some fossil sh.e.l.ls, even if simply attached to the OUTSIDE of others, bear full testimony to the conclusion above alluded to, namely, that an interval elapsed between the death of the creature to whose sh.e.l.l they adhere, and the burial of the same in mud or sand. The sea-urchins, or Echini, so abundant in white chalk, afford a good ill.u.s.tration. It is well known that these animals, when living, are invariably covered with spines supported by rows of tubercles. These last are only seen after the death of the sea-urchin, when the spines have dropped off. In Figure 11 a living species of Spatangus, common on our coast, is represented with one half of its sh.e.l.l stripped of the spines. In Figure 10 a fossil of a similar and allied genus from the white chalk of England shows the naked surface which the individuals of this family exhibit when denuded of their bristles. The full-grown Serpula, therefore, which now adheres externally, could not have begun to grow till the Micraster had died, and the spines became detached.

(FIGURE 12.

a. Ananchytes from the chalk with lower valve of Crania attached.

b. Upper valve of Crania detached.)

Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin (Ananchytes) in the chalk (see Figure 12) which has fixed to it the lower valve of a Crania, a genus of bivalve mollusca. The upper valve (b, Figure 12) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away.

Then the young Crania adhered to the bared sh.e.l.l, grew and perished in its turn; after which the upper valve was separated from the lower before the Ananchytes became enveloped in chalky mud.

(FIGURES 13 AND 14. Fossil and recent wood drilled by perforating Mollusca.

(FIGURE 13.

a. Fossil wood from London Clay, bored by Teredina.

b. Sh.e.l.l and tube of Teredina personata, the right-hand figure the ventral, the left the dorsal view.)

(FIGURE 14.

e. Recent wood bored by Toredo.

d. Sh.e.l.l and tube of Teredo navalis, from the same.

c. Anterior and posterior view of the valves of same detached from the tube.))

It may be well to mention one more ill.u.s.tration of the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We meet with many fragments of wood bored by s.h.i.+p-worms at various depths in the clay on which London is built.

Entire branches and stems of trees, several feet in length, are sometimes found drilled all over by the holes of these borers, the tubes and sh.e.l.ls of the mollusk still remaining in the cylindrical hollows. In Figure 14, e, a representation is given of a piece of recent wood pierced by the Teredo navalis, or common s.h.i.+p-worm, which destroys wooden piles and s.h.i.+ps. When the cylindrical tube d has been extracted from the wood, the valves are seen at the larger or anterior extremity, as shown at c. In like manner, a piece of fossil wood (a, Figure 13) has been perforated by a kindred but extinct genus, the Teredina of Lamarck. The calcareous tube of this mollusk was united and, as it were, soldered on to the valves of the sh.e.l.l (b), which therefore can not be detached from the tube, like the valves of the recent Teredo. The wood in this fossil specimen is now converted into a stony ma.s.s, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the Teredinae lived upon, and perforated it. Again, before the infant colony settled upon the drift wood, part of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate.

STRATA OF ORGANIC ORIGIN.

(FIGURE 15. Gaillonella ferruginea, Ehb.)

(FIGURE 16. Gaillonella distans, Ehb.)

(FIGURE 17. Bacillaria paradoxa.

a. Front view.

b. Side view.)

It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such ma.s.ses may be compared to modern oyster-beds and coral-reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stone deposits in the earth's crust, now proved to have been derived from plants and animals of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created some years since by the discovery of Professor Ehrenberg, of Berlin, that a certain kind of siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which were formerly referred to microscopic Infusoria, but which are now admitted to be plants. They abound in rivulets, lakes, and ponds in England and other countries, and are termed Diatomaceae by those naturalists who believe in their vegetable origin. The subject alluded to has long been well- known in the arts, under the name of infusorial earth or mountain meal, and is used in the form of powder for polis.h.i.+ng stones and metals. It has been procured, among other places, from the mud of a lake at Dolgelly, in North Wales, and from Bilin, in Bohemia, in which latter place a single stratum, extending over a wide area, is no less than fourteen feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous plates or frustules of the above-figured Diatomaceae, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the Gaillonella distans (see Figure 16) in every cubic inch (which weighs about 220 grains), or about 187 millions in a single grain. At every stroke, therefore, that we make with this polis.h.i.+ng powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.

A well-known substance, called bog-iron ore, often met with in peat-mosses, has often been shown by Ehrenberg to consist of innumerable articulated threads, of a yellow ochre colour, composed of silica, argillaceous matter, and peroxide of iron. These threads are the cases of a minute microscopic body, called Gaillonella ferruginea (Figure 15), a.s.sociated with the siliceous frustules of other fresh-water algae. Layers of this iron ore occurring in Scotch peat bogs are often called "the pan," and are sometimes of economical value.

It is clear much time must have been required for the acc.u.mulation of strata to which countless generations of Diatomaceae have contributed their remains; and these discoveries lead us naturally to suspect that other deposits, of which the materials have been supposed to be inorganic, may in reality be composed chiefly of microscopic organic bodies. That this is the case with the white chalk, has often been imagined, and is now proved to be the fact. It has, moreover, been lately discovered that the chambers into which these Foraminifera are divided are actually often filled with thousands of well-preserved organic bodies, which abound in every minute grain of chalk, and are especially apparent in the white coating of flints, often accompanied by innumerable needle-shaped spiculae of sponges (see Chapter 17.).