A Manual of Elementary Geology Part 19

The distribution of the fossil species from which the results before mentioned were obtained in 1830 by M. Deshayes was as follows:--

In the formations of the Pliocene periods, older and newer 777 In the Miocene 1021 In the Eocene 1238 ---- 3036 ----

Since the year 1830 the progress of conchological science has been most rapid, and the number of living species obtained from different parts of the globe has been raised from about 5000 to more than 10,000. New fossil species have also been added to our collections in great abundance; and at the same time a more copious supply of individuals both of fossil and recent species, some of which were previously very rare, have been procured, affording more ample data for determining the specific character. Besides the reforms introduced in consequence of these new zoological facilities, other errors of a geological nature have been in many instances removed.

POST-PLIOCENE FORMATIONS.

I have adopted the term Post-Pliocene for those strata which are sometimes called post-tertiary or modern, and which are characterized by having all the imbedded fossil sh.e.l.ls identical with species now living, whereas even the Newer Pliocene, or newest of the tertiary deposits above alluded to, contain always some small proportion of sh.e.l.ls of extinct species.

These modern formations, thus defined, comprehend not only those strata which can be shown to have originated since the earth was inhabited by man, but also deposits of far greater extent and thickness, in which no signs of man or his works can be detected. In some of these, of a date long anterior to the times of history and tradition, the bones of extinct quadrupeds have been met with of species which probably never co-existed with the human race, as, for example, the mammoth, mastodon, megatherium, and others, and yet the sh.e.l.ls are the same as those now living.

That portion of the post-pliocene group which belongs to the human epoch, and which is sometimes called _Recent_, forms a very unimportant feature in the geological structure of the earth's crust. I have shown, however, in "The Principles," where the recent changes of the earth ill.u.s.trative of geology are described at length, that the deposits acc.u.mulated at the bottom of lakes and seas within the last 4000 or 5000 years can neither be insignificant in volume or extent. They lie hidden, for the most part, from our sight; but we have opportunities of examining them at certain points where newly-gained land in the deltas of rivers has been cut through during floods, or where coral reefs are growing rapidly, or where the bed of a sea or lake has been heaved up by subterranean movements and laid dry. Their age may be recognized either by our finding in them the bones of man in a fossil state, that is to say, imbedded in them by natural causes, or by their containing articles fabricated by the hands of man.

Thus at Puzzuoli, near Naples, marine strata are seen containing fragments of sculpture, pottery, and the remains of buildings, together with innumerable sh.e.l.ls retaining in part their colour, and of the same species as those now inhabiting the Bay of Baiae. The uppermost of these beds is about 20 feet above the level of the sea. Their emergence can be proved to have taken place since the beginning of the sixteenth century.[112-A] Now here, as in almost every instance where any alterations of level have been going on in historical periods, it is found that rocks containing sh.e.l.ls, all, or nearly all, of which still inhabit the neighbouring sea, may be traced for some distance into the interior, and often to a considerable elevation above the level of the sea. Thus, in the country round Naples, the post-pliocene strata, consisting of clay and horizontal beds of volcanic tuff, rise at certain points to the height of 1500 feet. Although the marine sh.e.l.ls are exclusively of living species, they are not accompanied like those on the coast at Puzzuoli by any traces of man or his works. Had any such been discovered, it would have afforded to the antiquary and geologist matter of great surprise, since it would have shown that man was an inhabitant of that part of the globe, while the materials composing the present hills and plains of Campania were still in the progress of deposition at the bottom of the sea; whereas we know that for nearly 3000 years, or from the times of the earliest Greek colonists, no material revolution in the physical geography of that part of Italy has occurred.

In Ischia, a small island near Naples, composed in like manner of marine and volcanic formations, Dr. Philippi collected in the stratified tuff and clay ninety-two species of sh.e.l.ls of existing species. In the centre of Ischia, the lofty hill called Epomeo, or San Nicola, is composed of greenish indurated tuff, of a prodigious thickness, interstratified in some parts with marl, and here and there with great beds of solid lava. Visconti ascertained by trigonometrical measurement that this mountain was 2605 feet above the level of the sea. Not far from its summit, at the height of about 2000 feet, as also near Moropano, a village only 100 feet lower, on the southern declivity of the mountain, I collected, in 1828, many sh.e.l.ls of species now inhabiting the neighbouring gulf. It is clear, therefore, that the great ma.s.s of Epomeo was not only raised to its present height, but was also _formed_ beneath the waters, within the post-pliocene period.

It is a fact, however, of no small interest, that the fossil sh.e.l.ls from these modern tuffs of the volcanic region surrounding the Bay of Baiae, although none of them extinct, indicate a slight want of correspondence between the ancient fauna and that now inhabiting the Mediterranean.

Philippi informs us that when he and M. Scacchi had collected ninety-nine species of them, he found that only one, _Pecten medius_, now living in the Red Sea, was absent from the Mediterranean. Notwithstanding this, he adds, "the condition of the sea when the tufaceous beds were deposited must have been considerably different from its present state; for _Tellina striata_ was then common, and is now rare; _Lucina spinosa_ was both more abundant and grew to a larger size; _Lucina fragilis_, now rare, and hardly measuring 6 lines, then attained the enormous dimensions of 14 lines, and was extremely abundant; and _Ostrea lamellosa_, Broc., no longer met with near Naples, existed at that time, and attained a size so large that one lower valve has been known to measure 5 inches 9 lines in length, 4 inches in breadth, 1-1/2 inch in thickness, and weighed 26-1/2 ounces."[113-A]

There are other parts of Europe where no volcanic action manifests itself at the surface, as at Naples, whether by the eruption of lava or by earthquakes, and yet where the land and bed of the adjoining sea are undergoing upheaval. The motion is so gradual as to be insensible to the inhabitants, being only ascertainable by careful scientific measurements compared after long intervals. Such an upward movement has been proved to be in progress in Norway and Sweden throughout an area about 1000 miles N.

and S., and for an unknown distance E. and W., the amount of elevation always increasing as we proceed towards the North Cape, where it may equal 5 feet in a century. If we could a.s.sume that there had been an average rise of 2-1/2 feet in each hundred years for the last fifty centuries, this would give an elevation of 125 feet in that period. In other words, it would follow that the sh.o.r.es, and a considerable area of the former bed of the Baltic and North Sea, had been uplifted vertically to that amount, and converted into land in the course of the last 5000 years. Accordingly, we find near Stockholm, in Sweden, horizontal beds of sand, loam, and marl containing the same peculiar a.s.semblage of testacea which now live in the brackish waters of the Baltic. Mingled with these, at different depths, have been detected various works of art implying a rude state of civilization, and some vessels built before the introduction of iron, the whole marine formation having been upraised, so that the upper beds are now 60 feet higher than the surface of the Baltic. In the neighbourhood of these recent strata, both to the north-west and south of Stockholm, other deposits similar in mineral composition occur, which ascend to greater heights, in which precisely the same a.s.semblage of fossil sh.e.l.ls is met with, but without any intermixture of human bones or fabricated articles.

On the opposite or western coast of Sweden, at Uddevalla, post-pliocene strata, containing recent sh.e.l.ls, not of that brackish water character peculiar to the Baltic, but such as now live in the northern ocean, ascend to the height of 200 feet; and beds of clay and sand of the same age attain elevations of 300 and even 700 feet in Norway, where they have been usually described as "raised beaches." They are, however, thick deposits of submarine origin, spreading far and wide, and filling valleys in the granite and gneiss, just as the tertiary formations, in different parts of Europe, cover or fill depressions in the older rocks.

It is worthy of remark, that although the fossil fauna characterizing these upraised sands and clays consists exclusively of existing northern species of testacea, yet, according to Loven (an able living naturalist of Norway), the species do not const.i.tute such an a.s.semblage as now inhabits corresponding lat.i.tudes in the German Ocean. On the contrary, they decidedly represent a more arctic fauna.[114-A] In order to find the same species flouris.h.i.+ng in equal abundance, or in many cases to find them at all, we must go northwards to higher lat.i.tudes than Uddevalla in Sweden, or even nearer the pole than Central Norway.

Judging by the uniformity of climate now prevailing from century to century, and the insensible rate of variation in the organic world in our own times, we may presume that an extremely lengthened period was required even for so slight a modification of the molluscous fauna, as that of which the evidence is here brought to light. On the other hand, we have every reason for inferring on independent grounds (namely, the rate of upheaval of land in modern times) that the antiquity of the deposits in question must be very great. For if we a.s.sume, as before suggested, that the mean rate of continuous vertical elevation has amounted to 2-1/2 feet in a century (and this is probably a high average), it would require 27,500 years for the sea-coast to attain the height of 700 feet, without making allowance for any pauses such as are now experienced in a large part of Norway, or for any oscillations of level.

In England, buried s.h.i.+ps have been found in the ancient and now deserted channels of the Rother in Suss.e.x, of the Mersey in Kent, and the Thames near London. Canoes and stone hatchets have been dug up, in almost all parts of the kingdom, from peat and sh.e.l.l-marl; but there is no evidence, as in Sweden, Italy, and many other parts of the world, of the bed of the sea, and the adjoining coast, having been uplifted bodily to considerable heights within the human period. Recent strata have been traced along the coasts of Peru and Chili, inclosing sh.e.l.ls in abundance, all agreeing specifically with those now swarming in the Pacific. In one bed of this kind, in the island of San Lorenzo, near Lima, Mr. Darwin found, at the alt.i.tude of 85 feet above the sea, pieces of cotton-thread, plaited rush, and the head of a stalk of Indian corn, the whole of which had evidently been imbedded with the sh.e.l.ls. At the same height on the neighbouring mainland, he found other signs corroborating the opinion that the ancient bed of the sea had there also been uplifted 85 feet, since the region was first peopled by the Peruvian race.[115-A] But similar sh.e.l.ly ma.s.ses are also met with at much higher elevations, at innumerable points between the Chilian and Peruvian Andes and the sea-coast, in which no human remains were ever, or in all probability ever will be, discovered.

In the West Indies, also, in the island of Guadaloupe, a solid limestone occurs, at the level of the sea-beach, enveloping human skeletons. The stone is extremely hard, and chiefly composed of comminuted sh.e.l.l and coral, with here and there some entire corals and sh.e.l.ls, of species now living in the adjacent ocean. With them are included arrow-heads, fragments of pottery, and other articles of human workmans.h.i.+p. A limestone with similar contents has been formed, and is still forming, in St. Domingo. But there are also more ancient rocks in the West Indian Archipelago, as in Cuba, near the Havanna, and in other islands, in which are sh.e.l.ls identical with those now living in corresponding lat.i.tudes; some well-preserved, others in the state of casts, all referable to the post-pliocene period.

I have already described in the seventh chapter, p. 84., what would be the effects of oscillations and changes of level in any region drained by a great river and its tributaries, supposing the area to be first depressed several hundred feet, and then re-elevated. I believe that such changes in the relative level of land and sea have actually occurred in the post-pliocene era in the hydrographical basin of the Mississippi and in that of the Rhine. The acc.u.mulation of fluviatile matter in a delta during a slow subsidence may raise the newly gained land superficially at the same rate at which its foundations sink, so that these may go down hundreds or thousands of feet perpendicularly, and yet the sea bordering the delta may always be excluded, the whole deposit continuing to be terrestrial or freshwater in character. This appears to have happened in the deltas both of the Po and Ganges, for recent artesian borings, penetrating to the depth of 400 feet, have there shown that fluviatile strata, with sh.e.l.ls of recent species, together with ancient surfaces of land supporting turf and forests, are depressed hundreds of feet below the sea level.[116-A] Should these countries be once more slowly upraised, the rivers would carve out valleys through the horizontal and unconsolidated strata as they rose, sweeping away the greater portion of them, and leaving mere fragments in the shape of terraces skirting newly-formed alluvial plains, as monuments of the former levels at which the rivers ran. Of this nature are "the bluffs," or river cliffs, now bounding the valley of the Mississippi throughout a large portion of its course. Thus let _a b_, fig. 106., represent the alluvial plain of the Mississippi, a plain which, at the point alluded to, is more than 30 miles broad, and is truly a prolongation of the modern delta of that river. It is bounded by bluffs, the upper portions of which consist, both on the east and west side, of sh.e.l.ly loam, No. 2. rising from 100 to 200 feet above the level of the plain, and containing land and freshwater sh.e.l.ls of the genera _Helix_, _Pupa_, _Succinea_, and _Lymnea_, of the same species as those now inhabiting the neighbouring forests and swamps. In the same loam also, No. 2., are found the bones of the Mastodon, Elephant, Megalonyx, and other extinct quadrupeds.

[Ill.u.s.tration: Fig. 106. Valley of the Mississippi.

1. Alluvium.

2. Loess.

3. _f_. Eocene.

4. Cretaceous.]

I have endeavoured to show that the deposits forming the delta and alluvial plain of the Mississippi consist of sedimentary matter, extending over an area of 30,000 square miles, and known in some parts to be several hundred feet deep. Although we cannot estimate correctly how many years it may have required for the river to bring down from the upper country so large a quant.i.ty of earthy matter--the data for such a computation being as yet incomplete--we may still approximate to a minimum of the time which such an operation must have taken, by ascertaining experimentally the annual discharge of water by the Mississippi, and the mean annual amount of solid matter contained in its waters. The lowest estimate of the time required would lead us to a.s.sign a high antiquity, amounting to many tens of thousands of years to the existing delta, the origin of which is nevertheless an event of yesterday when contrasted with those terraces, _c_, and _d e_, fig. 106., formed of the loam No. 2. above mentioned. These materials of the bluffs _a_ and _d_ were produced, the reader will observe, during the first part of that great oscillation of level which depressed to a depth of 200 feet a larger area than the modern delta and plain of the Mississippi, and then restored the region to its former position.[117-A]

_Loess of the Valley of the Rhine._--A similar succession of geographical changes, attended by the production of a fluviatile formation, singularly resembling that which bounds the great plain of the Mississippi, seems to have occurred in the hydrographical basin of the Rhine, since the time when that basin had already acquired its present outline of hill and valley. I allude to the deposit provincially termed _loess_ in part of Germany, or _lehm_ in Alsace, filled with land and freshwater sh.e.l.ls of existing species. It is a finely comminuted sand or pulverulent loam of a yellowish grey colour, consisting chiefly of argillaceous matter combined with a sixth part of carbonate of lime, and a sixth of quartzose and micaceous sand. It often contains calcareous sandy concretions or nodules, rarely exceeding the size of a man's head. Its entire thickness amounts, in some places, to between 200 and 300 feet; yet there are often no signs of stratification in the ma.s.s, except here and there at the bottom, where there is occasionally a slight intermixture of drifted materials derived from subjacent rocks. Unsolidified as it is, and of so perishable a nature, that every streamlet flowing over it cuts out for itself a deep gully, it usually terminates in a vertical cliff, from the surface of which land sh.e.l.ls are seen here and there to project in relief. In all these features it presents a precise counterpart to the loess of the Mississippi. It is so h.o.m.ogeneous as generally to exhibit no signs of stratification, owing, probably, to its materials having been derived from a common source, and having been acc.u.mulated by a uniform action. Yet it displays in some few places decided marks of successive deposition, where coa.r.s.er and finer materials alternate, especially near the bottom. Calcareous concretions, also enclosing land-sh.e.l.ls, are sometimes arranged in horizontal layers. It is a remarkable deposit, from its position, wide extent, and thickness, its h.o.m.ogeneous mineral composition, and freshwater origin. Its distribution clearly shows that after the great valley of the Rhine, from Schaffhausen to Bonn, had acquired its present form, having its bottom strewed over with coa.r.s.e gravel, a period arrived when it became filled up from side to side with fine mud, which was also thrown down in the valleys of the princ.i.p.al tributaries of the Rhine.

Thus, for example, it may be traced far into Wurtemberg, up the valley of the Neckar, and from Frankfort, up the valley of the Main, to above Dettelbach. I have also seen it spreading over the country of Mayence, Eppelsheim, and Worms, on the left bank of the Rhine, and on the opposite side on the table-land above the Bergstra.s.se, between Wiesloch and Bruchsal, where it attains a thickness of 200 feet. Near Strasburg, large ma.s.ses of it appear at the foot of the Vosges on the left bank, and at the base of the mountains of the Black Forest on the right bank. The Kaiserstuhl, a volcanic mountain which stands in the middle of the plain of the Rhine near Freiburg, has been covered almost everywhere with this loam, as have the extinct volcanos between Coblentz and Bonn. Near Andernach, in the Kirchweg, the loess containing the usual sh.e.l.ls alternates with volcanic matter; and over the whole are strewed layers of pumice, lapilli, and volcanic sand, from 10 to 15 feet thick, very much resembling the ejections under which Pompeii lies buried. There is no pa.s.sage at this upper junction from the loess into the pumiceous superstratum; and this last follows the slope of the hill, just as it would have done had it fallen in showers from the air on a declivity partly formed of loess.

But, in general, the loess overlies all the volcanic products, even those between Neuwied and Bonn, which have the most modern aspect; and it has filled up in part the crater of the Roderberg, an extinct volcano near Bonn. In 1833 a well was sunk at the bottom of this crater, through 70 feet of loess, in part of which were the usual calcareous concretions.

The interstratification above alluded to, of loess with layers of pumice and volcanic ashes, has led to the opinion that both during and since its deposition some of the last volcanic eruptions of the Lower Eifel have taken place. Should such a conclusion be adopted, we should be called upon to a.s.sign a very modern date to these eruptions. This curious point, therefore, deserves to be reconsidered; since it may possibly have happened that the waters of the Rhine, swollen by the melting of snow and ice, and flowing at a great height through a valley choked up with loess, may have swept away the loose superficial scoriae and pumice of the Eifel volcanos, and spread them out occasionally over the yellow loam. Sometimes, also, the melting of snow on the slope of small volcanic cones may have given rise to local floods, capable of sweeping down light pumice into the adjacent low grounds.

The first idea which has occurred to most geologists, after examining the loess between Mayence and Basle, is to imagine that a great lake once extended throughout the valley of the Rhine between those two places. Such a lake may have sent off large branches up the course of the Main, Neckar, and other tributary valleys, in all of which large patches of loess are now seen. The barrier of the lake might be placed somewhere in the narrow and picturesque gorge of the Rhine between Bingen and Bonn. But this theory fails altogether to explain the phenomena; when we discover that that gorge itself has once been filled with loess, which must have been tranquilly deposited in it, as also in the lateral valley of the Lahn, communicating with the gorge. The loess has also overspread the high adjoining platform near the village of Plaidt above Andernach. Nay, on proceeding farther down to the north, we discover that the hills which skirt the great valley between Bonn and Cologne have loess on their flanks, which also covers here and there the gravel of the plain as far as Cologne, and the nearest rising grounds.

Besides these objections to the lake theory, the loess is met with near Basle, capping hills more than 1200 feet above the sea; so that a barrier of land capable of separating the supposed lake from the ocean would require to be, at least, as high as the mountains called the Siebengebirge, near Bonn, the loftiest summit of which, the Oehlberg, is 1209 feet above the Rhine and 1369 feet above the sea. It would be necessary, moreover, to place this lofty barrier somewhere below Cologne, or precisely where the level of the land is now lowest.

Instead, therefore, of supposing one continuous lake of sufficient extent and depth to allow of the simultaneous acc.u.mulation of the loess, at various heights, throughout the whole area where it now occurs, I formerly suggested that, subsequently to the period when the countries now drained by the Rhine and its tributaries had nearly acquired their actual form and geographical features, they were again depressed gradually by a movement like that now in progress on the west coast of Greenland.[119-A] In proportion as the whole district was lowered, the general fall of the waters between the Alps and the ocean was lessened; and both the main and lateral valleys, becoming more subject to river inundations, were partially filled up with fluviatile silt, containing land and freshwater sh.e.l.ls. When a thickness of many hundred feet of loess had been thrown down slowly by this operation, the whole region was once more upheaved gradually. During this upward movement most of the fine loam would be carried off by the denuding power of rains and rivers; and thus the original valleys might have been re-excavated, and the country almost restored to its pristine state, with the exception of some ma.s.ses and patches of loess such as still remain, and which, by their frequency and remarkable h.o.m.ogeneousness of composition and fossils, attest the ancient continuity and common origin of the whole. By imagining these oscillations of level, we dispense with the necessity of erecting and afterwards removing a mountain barrier sufficiently high to exclude the ocean from the valley of the Rhine during the period of the acc.u.mulation of the loess.

The proportion of land sh.e.l.ls of the genera _Helix_, _Pupa_, and _Bulimus_, is very large in the loess; but in many places aquatic species of the genera _Lymnea_, _Paludina_, and _Planorbis_ are also found. These may have been carried away during floods from shallow pools and marshes bordering the river; and the great extent of marshy ground caused by the wide overflowings of rivers above supposed would favour the multiplication of amphibious mollusks, such as the _Succinea_ (fig. 107.), which is almost everywhere characteristic of this formation, and is sometimes accompanied, as near Bonn, by another species, _S. amphibia_ (fig. 34. p. 29.). Among other abundant fossils are _Helix plebeium_ and _Pupa muscorum_. (See Figures.) Both the terrestrial and aquatic sh.e.l.ls preserved in the loess are of most fragile and delicate structure, and yet they are almost invariably perfect and uninjured. They must have been broken to pieces had they been swept along by a violent inundation. Even the colour of some of the land sh.e.l.ls, as that of _Helix nemoralis_, is occasionally preserved.

[Ill.u.s.tration: Fig. 107. _Succinea elongata._]

[Ill.u.s.tration: Fig. 108. _Pupa muscorum._]

[Ill.u.s.tration: Fig. 109. _Helix plebeium._]

Bones of vertebrated animals are rare in the loess, but those of the mammoth, horse, and some other quadrupeds have been met with. At the village of Binningen, and the hills called Bruderholz, near Basle, I found the vertebrae of fish, together with the usual sh.e.l.ls. These vertebrae, according to M. Aga.s.siz, belong decidedly to the Shark family, perhaps to the genus _Lamna_. In explanation of their occurrence among land and freshwater sh.e.l.ls, it may be stated that certain fish of this family ascend the Senegal, Amazon, and other great rivers, to the distance of several hundred miles from the ocean.[120-A]

At Cannstadt, near Stuttgart, in a valley also belonging to the hydrographical basin of the Rhine, I have seen the loess pa.s.s downwards into beds of calcareous tuff and travertin. Several valleys in northern Germany, as that of the Ilm at Weimar, and that of the Tonna, north of Gotha, exhibit similar ma.s.ses of modern limestone filled with recent sh.e.l.ls of the genera _Planorbis_, _Lymnea_, _Paludina_, &c., from 50 to 80 feet thick, with a bed of loess much resembling that of the Rhine, occasionally inc.u.mbent on them. In these modern limestones used for building, the bones of _Elephas primigenius_, _Rhinoceros tichorinus_, _Ursus spelaeus_, _Hyaena spelaea_, with the horse, ox, deer, and other quadrupeds, occur; and in 1850 Mr. H. Credner and I obtained in a quarry at Tonna, at the depth of 15 feet, inclosed in the calcareous rock and surrounded with dicotyledonous leaves and petrified leaves, four eggs of a snake of the size of the largest European Coluber, which, with three others, had been found lying in a series, or string.

They are, I believe, the first reptilian remains which have been met with in strata of this age.

The agreement of the sh.e.l.ls in these cases with recent European species enables us to refer to a very modern period the filling up and re-excavation of the valleys; an operation which doubtless consumed a long period of time, since which the mammiferous fauna has undergone a considerable change.

FOOTNOTES:

[110-A] See Princ. of Geol. vol. iii. 1st ed.

[112-A] See Principles, Index, "Serapis."

[113-A] Geol. Quart. Journ. vol. ii. Memoirs, p. 15.

[114-A] Quart. Geol. Journ. 4 Mems. p. 48.

[115-A] Journal, p. 451.

[116-A] See Principles, 8th ed. pp. 260-268.

[117-A] Lyell's Second Visit to the United States, vol. ii. chap. x.x.xiv.

[119-A] Princ. of Geol. 3d edition, 1834, vol. iii. p. 414.

[120-A] Proceedings Geol. Soc. No. 43. p. 222.