at right angles one to the other. When we consider that ten or twelve tubes are packed within the compass of a cubic inch, and that some single strata of this limestone are 6 feet thick, and may be traced over a considerable area, we may form some idea of the countless number of insects and mollusca which contributed their integuments and shells to compose this singularly constructed rock. It is unnecessary to suppose that the Phryganeæ lived on the spots where their cases are now found; they may have multiplied in the shallows near the margin of the lake, or in the streams by which it was fed, and their cases may have been drifted by a current far into the deep water.

In the summer of 1837, when examining, in company with Dr. Beck, a small lake near Copenhagen, I had an opportunity of witnessing a beautiful exemplification of the manner in which the tubular cases of Auvergne were probably accumulated. This lake, called the Fuure-Soe, occurring in the interior of Seeland, is about twenty English miles in circumference, and in some parts 200 feet in depth. Round the shallow borders an abundant crop of reeds and rushes may be observed, covered with the indusiæ of the Phryganea grandis and other species, to which shells are attached. The plants which support them are the bullrush, Scirpus lacustris, and common reed, Arundo phragmitis, but chiefly the former. In summer, especially in the month of June, a violent gust of wind sometimes causes a current by which these plants are torn up by the roots, washed away, and floated off in long bands, more than a mile in length, into deep water. The Cypris swarms in the same lake; and calcareous springs alone are wanting to form extensive beds of indusial limestone, like those of Auvergne.

4. Gypseous marls. More than 50 feet of thinly laminated gypseous marls, exactly resembling those in the hill of Montmartre, at Paris, are worked for gypsum at St. Romain, on the right bank of the Allier. They rest on a series of green cypriferous marls which alternate with grit, the united thickness of this inferior group being seen, in a vertical section on the banks of the river, to exceed 250 feet.

General arrangement, origin, and age of the freshwater formations of Auvergne.-The relations of the different groups above described cannot be learnt by the study of any one section; and the geologist who sets out with the expectation of finding a fixed order of succession may perhaps complain that the different parts of the basin give contradictory results. The arenaceous division, the marls, and the limestone, may all be seen in some places to alternate with each other; yet it can, by no means, be affirmed that there is no order of arrangement. The sands, sandstone, and conglomerate, constitute in general a littoral group; the foliated white and green marls, a contemporaneous central deposit; and the limestone is for the most part subordinate to the newer portions of both. The uppermost marls and sands are more calcareous than the lower; and we never meet with calcareous rocks covered by a considerable thickness of quartzose sand or green marl. From the resemblance of the limestones to the

Italian travertins, we may conclude that they were derived from the waters of mineral springs, such springs as even now exist in Auvergne, and which may be seen rising up through the granite, and precipitating travertin. They are sometimes thermal, but this character is by no means constant.

It seems that, when the ancient lake of the Lemagne first began to be filled with sediment, no volcanic action had yet produced lava and scoriæ on any part of the surface of Auvergne. No pebbles, therefore, of lava were transported into the lake,—no fragments of volcanic rocks embedded in the conglomerate. But at a later period, when a considerable thickness of sandstone and marl had accumulated, eruptions broke out, and lava and tuff were deposited, at some spots, alternately with the lacustrine strata. It is not improbable that cold and thermal springs, holding different mineral ingredients in solution, became more numerous during the successive convulsions attending this development of volcanic agency, and thus deposits of carbonate and sulphate of lime, silex, and other minerals were produced. Hence these minerals predominate in the uppermost strata. The subterranean movements may then have continued until they altered the relative levels of the country, and caused the waters of the lakes to be drained off, and the farther accumulation of regular freshwater strata to cease. We may easily conceive a similar series of events to give rise to analogous results in any modern basin, such as that of Lake Superior, for example, where numerous rivers and torrents are carrying down the detritus of a chain of mountains into the lake. The transported materials must be arranged according to their size and weight, the coarser near the shore, the finer at a greater distance from land; but in the gravelly and sandy beds of Lake Superior no pebbles of modern volcanic rocks can be included, since there are none of these at present in the district. If igneous action should break out in that country, and produce lava, scoriæ, and thermal springs, the deposition of gravel, sand, and marl might still continue as before; but, in addition, there would then be an intermixture of volcanic gravel and tuff, and of rocks precipitated from the waters of mineral springs.

Although the freshwater strata of the Limagne approach generally to a horizontal position, the proofs of local disturbance are sufficiently numerous and violent to allow us to suppose great changes of level since the lacustrine period. We are unable to assign a northern barrier to the ancient lake, although we can still trace its limits to the east, west, and south, where they were formed of bold granite eminences. Nor need we be surprised at our inability to restore entirely the physical geography of the country after so great a series of volcanic eruptions; for it is by no means improbable that one part of it, the southern, for example, may have been moved upwards bodily, while others remained at rest, or even suffered a movement of depression.

Whether all the freshwater formations of the Limagne d'Auvergne belong to one period, I cannot pretend to decide, as large masses both of the arenaceous and marly groups are often devoid of fossils.

Much light has been thrown on the mammiferous fauna by the labours of MM. Bravard and Croizet, and by those of M. Pomel. The lastmentioned naturalist has pointed out the specific distinction of all, or nearly all, the species of mammalia, from those of the gypseous series near Paris. Nevertheless, many of the forms are analogous to those of Eocene quadrupeds. The Cainotherium, for example, is not far removed from the Anoplotherium, and is, according to Waterhouse, the same as the genus Microtherium of the Germans. There are two species of marsupial animals allied to Didelphys, a genus also found in the Paris gypsum. The Amphitragulus elegans of Pomel, has been identified with a Rhenish species from Weissenau near Mayence, called by Kaup Dorcatherium nanum; and other Auvergne fossils, e. g., Microtherium Reuggeri, and a small rodent, Titanomys, are specifically the same with mammalia of the Mayence basin.

Cantal.-A freshwater formation, very analogous to that of Auvergne, is situated in the department of Haute Loire, near the town of Le Puy, in Velay, and another occurs near Aurillac, in Cantal. The leading feature of the formation last mentioned, as distinguished from those of Auvergne and Velay, is the immense abundance of silex associated with calcareous marls and limestone.

The whole series may be separated into two divisions; the lower, composed of gravel, sand, and clay, such as might have been derived from the wearing down and decomposition of the granitic schists of the surrounding country; the upper system, consisting of siliceous and calcareous marls, contains subordinately gypsum, silex, and lime

stone.

The resemblance of the freshwater limestone of the Cantal, and its accompanying flint, to the upper chalk of England, is very instructive, and well calculated to put the student upon his guard against relying too implicitly on mineral character alone as a safe criterion of relative age.

When we approach Aurillac from the west, we pass over great heathy plains, where the sterile mica-schist is barely covered with vegetation. Near Ytrac, and between La Capelle and Viscamp, the surface is strewed over with loose broken flints, some of them black in the interior, but with a white external coating; others stained with tints of yellow and red, and in appearance precisely like the flint gravel of our chalk districts. When heaps of this gravel have thus announced our approach to a new formation, we arrive at length at the escarpment of the lacustrine beds. At the bottom of the hill which rises before us, we see strata of clay and sand, resting on mica. schist; and above, in the quarries of Belbet, Leybros, and Bruel, a white limestone, in horizontal strata, the surface of which has been hollowed out into irregular furrows, since filled up with broken flint, marl, and dark vegetable mound. In these cavities we recognize an exact counterpart to those which are so numerous on the furrowed surface of our own white chalk. Advancing from these quarries along a road made of the white limestone, which reflects as glaring a light in the sun, as do our roads composed of chalk, we reach, at

length, in the neighbourhood of Aurillac, hills of limestone and calcareous marl, in horizontal strata, separated in some places by regular layers of flint in nodules, the coating of each nodule being of an opaque white colour, like the exterior of the flinty nodules of our chalk.

It will be remembered that the siliceous stone of Bilin, called tripoli, is a freshwater deposit, and has been shown, by Ehrenberg, to be of infusorial origin (see p. 24.). What is true of the Bohemian flint and opal, where the beds attain a thickness of 14 feet, may also, perhaps, be found to hold good respecting the silex of Aurillac, which may also have been immediately derived from the minute cases of microscopic animalcules. But even if this conclusion be established, the abundant supply both of siliceous, calcareous, and gypseous matter, which the ancient lakes of France received, may have been connected with the subterranean volcanic agency of which those regions were so long the theatre, and which may have impregnated the springs with mineral matter, even before the great outbreak of lava. It is well known that the hot springs of Iceland, and many other countries, contain silex in solution; and it has been lately affirmed, that steam at a high temperature is capable of dissolving quartzose rocks without the aid of any alkaline or other flux.*

Travellers not unfrequently mention, in their accounts of India, Australia, and other distant lands, that they have seen chalk with flints, which they have assumed to be of the same age as the Cretaceous system of Europe. A hasty observation of the white limestone and flint of Aurillac might convey the same idea; but when we turn from the mineral aspect and composition to the organic remains, we find in the flints of the Cantal the seed-vessels of the freshwater Chara, instead of the marine zoophytes so abundantly imbedded in chalk flints; and in the limestone we meet with shells of Limnea, Planorbis, and other lacustrine genera, instead of the oyster, terebratula, and echinus of the Cretaceous period.

Proofs of gradual deposition.—Some sections of the foliated marls in the valley of the Cer, near Aurillac, attest, in the most unequivocal manner, the extreme slowness with which the materials of the lacustrine series were amassed. In the hill of Barrat, for example, we find an assemblage of calcareous and siliceous marls; in which, for a depth of at least 60 feet, the layers are so thin, that thirty are sometimes contained in the thickness of an inch; and when they are separated, we see preserved in every one of them the flattened stems of Chara, or other plants, or sometimes myriads of small Paludinæ and other freshwater shells. These minute foliations of the marl resemble precisely some of the recent laminated beds of the Scotch marl lakes, and may be compared to the pages of a book, each containing a history of a certain period of the past. The different layers may be grouped together in beds from a foot to a foot and a half in thickness, which are distinguished by differences of composition and colour, the tints being white, green, and brown. Occasionally there

• See Proceedings of Roy. Soc., No. 44. p. 233.

is a parting layer of pure flint, or of black carbonaceous vegetable matter, about an inch thick, or of white pulverulent marl. We find several hills in the neighbourhood of Aurillac composed of such materials, for the height of more than 200 feet from their base, the whole sometimes covered by rocky currents of trachytic or basaltic lava.*

Thus wonderfully minute are the separate parts of which some of the most massive geological monuments are made up! When we desire to classify, it is necessary to contemplate entire groups of strata in the aggregate; but if we wish to understand the mode of their formation, and to explain their origin, we must think only of the minute subdivisions of which each mass is composed. We must bear in mind how many thin leaf-like seams of matter, each containing the remains of myriads of testacea and plants, frequently enter into the composition of a single stratum, and how vast a succession of these strata unite to form a single group! We must remember, also, that piles of volcanic matter, like the Plomb du Cantal, which rises in the immediate neighbourhood of Aurillac, are themselves equally the result of successive accumulation, consisting of reiterated sheets of lava, showers of scoriæ, and ejected fragments of rock. -Lastly, we must not forget that continents and mountain-chains, colossal as are their dimensions, are nothing more than an assemblage of many such igneous and aqueous groups, formed in succession during an indefinite lapse of ages, and superimposed upon each other.

CHAPTER XVI.

EOCENE FORMATIONS— continued.

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Subdivisions of the Eocene group in the Paris basin - Gypseous series - Extinct quadrupeds Impulse given to geology by Cuvier's osteological discoveries Shelly sands called sables moyens Calcaire grossier Miliolites Calcaire siliceux -Lower Eocene in France-Lits coquilliers-Sands and plastic clayEnglish Eocene strata- Freshwater and fluvio-marine beds - Barton beds Bagshot and Brocklesham division Large ophidians and saurians - Lower Eocene and London Clay proper - Fossil plants and shells Strata of Kyson in Suffolk-Fossil monkey and opossum - Mottled clays and sands below London Clay - Nummulitic formation of Alps and Pyrenees - Its wide geographical extent- Eocene strata in the United States - Section at Claiborne, Alabama — Colossal cetacean Orbitoid limestone - Burr stone.

FROM what was said in the two preceding chapters, it has already appeared that we have in England no true chronological representative of the Miocene faluns of the Loire, and none of the Upper Eocene group

* Lyell and Murchison, sur les Dépôts Lacust. Tertiaries du Cantal, &c. Ann. des Sci. Nat. Oct. 1829.