A Manual of Elementary Geology Part 8

A. Dolomite and sandstone. (Greensand formation?) _a_, _b_, _d_. Beds of gravel and sand.

_c._ Fine marl and sand of St. Madeleine, with marine 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 many 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 annexed fig. 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 9 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. 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 9 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 coast would acquire its present configuration, the delta would emerge, and a deep channel might then be cut through it by a river.

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 shingle 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 cannot 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_, fig. 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 cannot 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 9 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 cannot 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.

[Ill.u.s.tration: Fig. 8. Slab of ripple-marked (new red) sandstone from Cheshire.]

_Ripple mark._--The ripple mark, so common on the surface of sandstones of all ages (see fig. 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 following is the manner in which I once observed the motion of the air to produce this effect on a large extent of level beach, exposed at low tide near Calais. Clouds of fine white sand were blown from the neighbouring dunes, so as to cover the sh.o.r.e, and whiten a dark level surface of sandy mud, and this fresh covering of sand was beautifully rippled. On levelling all the small ridges and furrows of this ripple over an area of several yards square, I saw them perfectly restored in about ten minutes, the general direction of the ridges being always at right angles to that of the wind. The restoration began by the appearance here and there of small detached heaps of sand, which soon lengthened and joined together, so as to form long sinuous ridges with intervening furrows. Each ridge had one side slightly inclined, and the other steep; the lee-side being always steep, as _b, c,--d, e_; the windward-side a gentle slope, as _a, b,--c, d_, fig. 9.

When a gust of wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the furrows had occupied. The mode of advance was by the continual drifting of grains of sand up the slopes _a b_ and _c d_, many of which grains, when they arrived at _b_ and _d_, fell over the scarps _b c_ and _d e_, and were under shelter from the wind; so that they remained stationary, resting, according to their shape and momentum, on different parts of the descent, and a few only rolling to the bottom. In this manner each ridge was distinctly seen to move slowly on as often as the force of the wind augmented. Occasionally part of a ridge, advancing more rapidly than the rest, overtook the ridge immediately before it, and became confounded with it, thus causing those bifurcations and branches which are so common, and two of which are seen in the slab, fig. 8. We may observe this configuration in sandstones of all ages, and in them also, as now on the sea-coast, we may often detect two systems of ripples interfering with each other; one more ancient and half effaced, and a newer one, in which the grooves and ridges are more distinct, and in a different direction.

This crossing of two sets of ripples arises from a change of wind, and the new direction in which the waves are thrown on the sh.o.r.e.

[Ill.u.s.tration: Fig. 9. Sketch of ripples.]

The ripple mark is usually an indication of a sea-beach, or of water from 6 to 10 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.[21-A]

FOOTNOTES:

[11-A] The kaolin of China consists of 7115 parts of silex, 1586 of alumine, 192 of lime, and 673 of water (W. Phillips, Mineralogy, p. 33.); but other porcelain clays differ materially, that of Cornwall being composed, according to Boase of nearly equal parts of silica and alumine, with 1 per cent. of magnesia. (Phil. Mag. vol. x. 1837.)

[11-B] See W. Phillips's Mineralogy, "Alumine."

[14-A] Consult Index to Principles of Geology, "Stratification,"

"Currents," "Deltas," "Water," &c.

[21-A] Siau. Edin. New Phil. Journ. vol. x.x.xi.; and Darwin, Volc.

Islands, p. 134.

CHAPTER III.

ARRANGEMENT OF FOSSILS IN STRATA--FRESHWATER AND MARINE.

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 and semi-opal formed of infusoria--Chalk derived princ.i.p.ally from organic bodies--Distinction of freshwater from marine formations--Genera of freshwater and land sh.e.l.ls--Rules for recognizing marine testacea--Gyrogonite and chara--Freshwater fishes--Alternation of marine and freshwater 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 filled with 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 covered immediately by 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 shingle, or loose sand or soft mud at the bottom of the sea, in which sh.e.l.ls and other bodies easily became enveloped.

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 downwards. 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 remained clear and transparent, for such corals cannot live in turbid water.

[Ill.u.s.tration: Fig. 10. 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 every where throughout a long series of strata, we cannot 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 testacea, now adhering to it, grew from an embryo state to full maturity. Attached sh.e.l.ls which are merely external, like some of the serpulae (_a_) in the annexed figure (fig. 10.), 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 fig. 10., 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.

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 numerous spines, which serve as organs of motion, and are supported by rows of tubercles, which last are only seen after the death of the sea-urchin, when the spines have dropped off. In fig. 12. 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 fig. 11. a fossil of the same 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 _Spatangus_ had died, and the spines were detached.

[Ill.u.s.tration: Fig. 11. _Serpula_ attached to a fossil _Spatangus_ from the chalk.]

[Ill.u.s.tration: Fig. 12. Recent _Spatangus_ with the spines removed from one side.

_b._ Spine and tubercles, nat. size.

_a._ The same magnified.]

[Ill.u.s.tration: Fig. 13.

_a._ _Echinus_ from the chalk, with lower valve of the _Crania_ attached.

_b._ Upper valve of the _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 in the chalk (see fig. 13.), which has fixed to it the lower valve of a _Crania_, a genus of bivalve mollusca. The upper valve (_b_, fig. 13.) 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 _Echinus_ became enveloped in chalky mud.

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 ship-worms at various depths in the clay on which London is built. Entire branches and stems of trees, several feet in length, are sometimes dug out, 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 fig. 15. _e_, a representation is given of a piece of recent wood pierced by the _Teredo navalis_, or common ship-worm, which destroys wooden piles and ships. When the cylindrical tube _d_ has been extracted from the wood, a sh.e.l.l is seen at the larger extremity, composed of two pieces, as shown at _c_. In like manner, a piece of fossil wood (_a_, fig. 14.) has been perforated by an animal of a kindred but extinct genus, called _Teredina_ by 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 cannot 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 it, perforating it in all directions. Again, before the infant colony settled upon the drift wood, the branch 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.

[2 Ill.u.s.trations: Fossil and recent wood drilled by perforating Mollusca.

Fig. 14. _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.

Fig. 15. _e_. Recent wood bored by _Teredo_.

_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 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 stony 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 by the recent 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 the Prussian naturalist refers to microscopic Infusoria, but which most others now believe to be plants.

They abound in freshwater lakes and ponds in England and other countries, and are termed Diatomaceae by those naturalists who believe in their vegetable origin. The substance alluded to has long been well known in the arts, being used in the form of powder for polishing stones and metals. It has been procured, among other places, from Bilin, in Bohemia, where a single stratum, extending over a wide area, is no less than 14 feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous plates or frustules of the above-mentioned 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 fig. 17.) 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 polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.

[3 Ill.u.s.trations: These figures are magnified nearly 300 times, except the lower figure of _G. ferruginea_ (fig. 18. _a_), which is magnified 2000 times.

Fig. 16. _Bacillaria vulgaris?_

Fig. 17. _Gaillonella distans._

Fig. 18. _Gaillonella ferruginea._]

[2 Ill.u.s.trations: Fragment of semi-opal from the great bed of Tripoli, Bilin.

Fig. 19. Natural size.

Fig. 20. The same magnified, showing circular articulations of a species of _Gaillonella_, and spiculae of _Spongilla_.]

The remains of these Diatomaceae are of pure silex, and their forms are various, but very marked and constant in particular genera and species.

Thus, in the family _Bacillaria_ (see fig. 16.), the fossils preserved in tripoli are seen to exhibit the same divisions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spiculae or internal supports of the freshwater sponge, or _Spongilla_ of Lamarck, are sometimes intermingled (see the needle-shaped bodies in fig. 20.). These flinty cases and spiculae, although hard, are very fragile, breaking like gla.s.s, and are therefore admirably adapted, when rubbed, for wearing down into a fine powder fit for polishing the surface of metals.

Besides the tripoli, formed exclusively of the fossils above described, there occurs in the upper part of the great stratum at Bilin another heavier and more compact stone, a kind of semi-opal, in which innumerable parts of Diatomaceae and spiculae of the _Spongilla_ are filled with, and cemented together by, siliceous matter. It is supposed that the siliceous remains of the most delicate Diatomaceae have been dissolved by water, and have thus given rise to this opal in which the more durable fossils are preserved like insects in amber. This opinion is confirmed by the fact that the organic bodies decrease in number and sharpness of outline in proportion as the opaline cement increases in quant.i.ty.