Part 19 (2/2)

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.

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