The Student's Elements of Geology Part 34

The break between the Upper and Lower Cretaceous formations will be appreciated when it is stated that, although the Neocomian contains 31 species of Ammonite, and the Gault, as we have seen, 21, there are only three of those common to both divisions. Nevertheless, we may expect the discovery in England, and still more when we extend our survey to the Continent, of beds of pa.s.sage intermediate between the Upper and Lower Cretaceous. Even now the Blackdown beds in Devonshire, which rest immediately on Tria.s.sic strata, and which evidently belong to some part of the Cretaceous series, have been referred by some geologists to the Upper group, by others to the Lower or Neocomian. They resemble the Folkestone beds of the latter series in mineral character, and 59 out of 156 of their fossil mollusca are common to them; but they have also 16 species common to the Gault, and 20 to the overlying Chloritic series; and what is very important, out of seven Ammonites six are found also in the Gault and Chloritic series, only one being peculiar to the Blackdown beds.

Professor Ramsay has remarked that there is a stratigraphical break; for in Kent, Surrey, and Suss.e.x, at those few points where there are exposures of junctions of the Gault and Neocomian, the surface of the latter has been much eroded or denuded, while to the westward of the great chalk escarpment the unconformability of the two groups is equally striking. At Blackdown this unconformability is still more marked, for though distant only 100 miles from Kent and Surrey, no formation intervenes between these beds and the Trias; all intermediate groups, such as the Lower Neocomian and Oolite, having either not been deposited or destroyed by denudation.

FLORA OF THE UPPER CRETACEOUS PERIOD.

As the Upper Cretaceous rocks of Europe are, for the most part, of purely marine origin, and formed in deep water usually far from the nearest sh.o.r.e, land-plants of this period, as we might naturally have antic.i.p.ated, are very rarely met with. In the neighbourhood of Aix-la-Chapelle, however, an important exception occurs, for there certain white sands and laminated clays, 400 feet in thickness, contain the remains of terrestrial plants in a beautiful state of preservation. These beds are the equivalents of the white chalk and chalk marl of England, or Senonien of d'Orbigny, although the white siliceous sands of the lower beds, and the green grains in the upper part of the formation, cause it to differ in mineral character from our white chalk.

Beds of fine clay, with fossil plants, and with seams of lignite, and even perfect coal, are intercalated. Floating wood, containing perforating sh.e.l.ls, such as Pholas and Gastrochoena, occur. There are likewise a few beds of a yellowish-brown limestone, with marine sh.e.l.ls, which enable us to prove that the lowest and highest plant-beds belong to one group. Among these sh.e.l.ls are Pecten quadricostatus, and several others which are common to the upper and lower part of the series, and Trigonia limbata, d'Orbigny, a sh.e.l.l of the white chalk. On the whole, the organic remains and the geological position of the strata prove distinctly that in the neighbourhood of Aix-la-Chapelle a gulf of the ancient Cretaceous sea was bounded by land composed of Devonian rocks. These rocks consisted of quartzose and schistose beds, the first of which supplied white sand and the other argillaceous mud to a river which entered the sea at this point, carrying down in its turbid waters much drift-wood and the leaves of plants. Occasionally, when the force of the river abated, marine sh.e.l.ls of the genera Trigonia, Turritella, Pecten, etc., established themselves in the same area, and plants allied to Zostera and Fucus grew on the bottom.

The fossil plants of this member of the upper chalk at Aix have been diligently collected and studied by Dr. Debey, and as they afford the only example yet known of a terrestrial flora older than the Eocene, in which the great divisions of the vegetable kingdom are represented in nearly the same proportions as in our own times, they deserve particular attention. Dr. Debey estimates the number of species as amounting to more than two hundred, of which sixty-seven are cryptogamous, chiefly ferns, twenty species of which can be well determined, most of them being in fructification. The scars on the bark of one or two are supposed to indicate tree-ferns. Of thirteen genera three are still existing, namely, Gleichenia, now inhabiting the Cape of Good Hope, and New Holland; LyG.o.dium, now spread extensively through tropical regions, but having some species which live in j.a.pan and North America; and Asplenium, a cosmopolite form. Among the phaenogamous plants, the Conifers are abundant, the most common belonging to a genus called Cycadopteris by Debey, and hardly separable from Sequoia (or Wellingtonia), of which both the cones and branches are preserved.

When I visited Aix, I found the silicified wood of this plant very plentifully dispersed through the white sands in the pits near that city. In one silicified trunk 200 rings of annual growth could be counted. Species of Araucaria like those of Australia are also found. Cycads are extremely rare, and of Monocotyledons there are but few. No palms have been recognised with certainty, but the genus Panda.n.u.s, or screw pine, has been distinctly made out. The number of the Dicotyledonous Angiosperms is the most striking feature in so ancient a flora.

(In this and subsequent remarks on fossil plants I shall often use Dr. Lindley's terms, as most familiar in this country; but as those of M. A. Brongniart are much cited, it may be useful to geologists to give a table explaining the corresponding names of groups so much spoken of in palaeontology.

COLUMN 1. BRONGNIART.

COLUMN 2. LINDLEY.

COLUMN 3. EXAMPLES.

CRYPTOGAMIC.

1. Cryptogamous amphigens, or cellular cryptogamic: Thallogens: Lichens, sea- weeds, fungi.

2. Cryptogamous acrogens: Acrogens: Mosses, equisetums, ferns, lycopodiums-- Lepidodendra.

PHAENEROGAMIC.

3. Dicotyledonous gymnosperms: Gymnogens: Conifers and Cycads.

4. Dicotyledonous Angiosperms: Exogens: Compositae, leguminosae, umbelliferae, cruciferae, heaths, etc. All native European trees except conifers.

5. Monocotyledons: Endogens. Palms, lilies, aloes, rushes, gra.s.ses, etc.)

Among them we find the familiar forms of the Oak, Fig, and Walnut (Quercus, Ficus, and Juglans), of the last both the nuts and leaves; also several genera of the Myrtaceae. But the predominant order is the Proteaceae, of which there are between sixty and seventy supposed species, many of extinct genera, but some referred to the following living forms-- Dryandra, Grevillea, Hakea, Banksia, Persoonia-- all now belonging to Australia, and Leucospermum, species of which form small bushes at the Cape.

The epidermis of the leaves of many of these Aix plants, especially of the Proteaceae, is so perfectly preserved in an envelope of fine clay, that under the microscope the stomata, or polygonal cellules, can be detected, and their peculiar arrangement is identical with that known to characterise some living Proteaceae (Grevillea, for example). Although this peculiarity of the structure of stomata is also found in plants of widely distant orders, it is, on the whole, but rarely met with, and being thus observed to characterise a foliage previously suspected to be proteaceous, it adds to the probability that the botanical evidence had been correctly interpreted.

An occasional admixture at Aix-la-Chapelle of Fucoids and Zosterites attests, like the sh.e.l.ls, the presence of salt-water. Of insects, Dr. Debey has obtained about ten species of the families Curculionidae and Carabidae.

The resemblance of the flora of Aix-la-Chapelle to the tertiary and living floras in the proportional number of dicotyledonous angiosperms as compared to the gymnogens, is a subject of no small theoretical interest, because we can now affirm that these Aix plants flourished before the rich reptilian fauna of the secondary rocks had ceased to exist. The Ichthyosaurus, Pterodactyl, and Mosasaurus were of coeval date with the oak, the walnut, and the fig.

Speculations have often been hazarded respecting a connection between the rarity of Exogens in the older rocks and a peculiar state of the atmosphere. A denser air, it was suggested, had in earlier times been alike adverse to the well-being of the higher order of flowering plants, and of the quick-breathing animals, such as mammalia and birds, while it was favourable to a cryptogamic and gymnospermous flora, and to a predominance of reptile life. But we now learn that there is no incompatibility in the co-existence of a vegetation like that of the present globe, and some of the most remarkable forms of the extinct reptiles of the age of gymnosperms.

If the pa.s.sage seem at present to be somewhat sudden from the flora of the Lower or Neocomian to that of the Upper Cretaceous period, the abruptness of the change will probably disappear when we are better acquainted with the fossil vegetation of the uppermost beds of the Neocomian and that of the lowest strata of the Gault or true Cretaceous series.

HIPPURITE LIMESTONE.-- DIFFERENCE BETWEEN THE CHALK OF THE NORTH AND SOUTH OF EUROPE.

(FIGURE 273. Map of part of S.W. France, from the Loire river to the Pyrenees.)

By the aid of the three tests, superposition, mineral character, and fossils, the geologist has been enabled to refer to the same Cretaceous period certain rocks in the north and south of Europe, which differ greatly both in their fossil contents and in their mineral composition and structure.

If we attempt to trace the cretaceous deposits from England and France to the countries bordering the Mediterranean, we perceive, in the first place, that in the neighbourhood of London and Paris they form one great continuous ma.s.s, the Straits of Dover being a trifling interruption, a mere valley with chalk cliffs on both sides. We then observe that the main body of the chalk which surrounds Paris stretches from Tours to near Poitiers (see Figure 273, in which the shaded part represents chalk).

Between Poitiers and La Roch.e.l.le, the s.p.a.ce marked A on the map separates two regions of chalk. This s.p.a.ce is occupied by the Oolite and certain other formations older than the Chalk and Neocomian, and has been supposed by M. E. de Beaumont to have formed an island in the Cretaceous sea. South of this s.p.a.ce we again meet with rocks which we at once recognise to be cretaceous, partly from the chalky matrix and partly from the fossils being very similar to those of the white chalk of the north: especially certain species of the genera Spatangus, Ananchytes, Cidarites, Nucula, Ostrea, Gryphaea (Exogyra), Pecten, Plagiostoma (Lima), Trigonia, Catillus (Inoceramus), and Terebratula. (d'Archiac, Sur la form. Cretacee du S.-O. de la France Mem. de la Soc. Geol. de France tome 2.) But Ammonites, as M. d'Archiac observes, of which so many species are met with in the chalk of the north of France, are scarcely ever found in the southern region; while the genera Hamite, Turrilite, and Scaphite, and perhaps Belemnite, are entirely wanting.

(FIGURE 274. Radiolites radiosa, d'Orbigny. White chalk of France.

b. Upper valve of same.)

(FIGURE 275. Radiolites foliaceus, d'Orbigny. Syn. Sphaerulites agarici-formis, Blainv. White chalk of France.)

(FIGURE 276. Hippurites organisans, Desmoulins. Upper chalk:-- chalk marl of Pyrenees? (d'Orbigny's Palaeontologie francaise plate 533.) a. Young individual; when full grown they occur in groups adhering laterally to each other.

b. Upper side of the upper valve, showing a reticulated structure in those parts, b, where the external coating is worn off.

c. Upper end or opening of the lower and cylindrical valve.

d. Cast of the interior of the lower conical valve.)

On the other hand, certain forms are common in the south which are rare or wholly unknown in the north of France. Among these may be mentioned many Hippurites, Sphaerulites, and other members of that great family of mollusca called Rudistes by Lamarck, to which nothing a.n.a.logous has been discovered in the living creation, but which is quite characteristic of rocks of the Cretaceous era in the south of France, Spain, Sicily, Greece, and other countries bordering the Mediterranean. The species called Hippurites organisans (Figure 276) is more abundant than any other in the south of Europe; and the geologist should make himself well acquainted with the cast of the interior, d, which is often the only part preserved in many compact marbles of the Upper Cretaceous period. The flutings on the interior of the Hippurite, which are represented on the cast by smooth, rounded longitudinal ribs, and in some individuals attain a great size and length, are wholly unlike the markings on the exterior of the sh.e.l.l.

CRETACEOUS ROCKS IN THE UNITED STATES.

If we pa.s.s to the American continent, we find in the State of New Jersey a series of sandy and argillaceous beds wholly unlike in mineral character to our Upper Cretaceous system; which we can, nevertheless, recognise as referable, palaeontologically, to the same division.

That they were about the same age generally as the European chalk and Neocomian, was the conclusion to which Dr. Morton and Mr. Conrad came after their investigation of the fossils in 1834. The strata consist chiefly of green sand and green marl, with an overlying coralline limestone of a pale yellow colour, and the fossils, on the whole, agree most nearly with those of the Upper European series, from the Maestricht beds to the Gault inclusive. I collected sixty sh.e.l.ls from the New Jersey deposits in 1841, five of which were identical with European species-- Ostrea larva, O. vesicularis, Gryphaea costata, Pecten quinque-costatus, Belemnitella mucronata. As some of these have the greatest vertical range in Europe, they might be expected more than any others to recur in distant parts of the globe. Even where the species were different, the generic forms, such as the Baculite and certain sections of Ammonites, as also the Inoceramus (see above, Figure 252) and other bivalves, have a decidedly cretaceous aspect. Fifteen out of the sixty sh.e.l.ls above alluded to were regarded by Professor Forbes as good geographical representatives of well-known cretaceous fossils of Europe. The correspondence, therefore, is not small, when we reflect that the part of the United States where these strata occur is between 3000 and 4000 miles distant from the chalk of Central and Northern Europe, and that there is a difference of ten degrees in the lat.i.tude of the places compared on opposite sides of the Atlantic. Fish of the genera Lamna, Galeus, and Carcharodon are common to New Jersey and the European cretaceous rocks. So also is the genus Mosasaurus among reptiles.

It appears from the labours of Dr. Newberry and others, that the Cretaceous strata of the United States east and west of the Appalachians are characterised by a flora decidedly a.n.a.logous to that of Aix-la-Chapelle above-mentioned, and therefore having considerable resemblance to the vegetation of the Tertiary and Recent Periods.

CHAPTER XVIII.

LOWER CRETACEOUS OR NEOCOMIAN FORMATION.

Cla.s.sification of marine and fresh-water Strata.

Upper Neocomian.

Folkestone and Hythe Beds.

Atherfield Clay.

Similarity of Conditions causing Reappearance of Species after short Intervals.

Upper Speeton Clay.

Middle Neocomian.

Tealby Series.

Middle Speeton Clay.

Lower Neocomian.

Lower Speeton Clay.

Wealden Formation.

Fresh-water Character of the Wealden.

Weald Clay.

Hastings Sands.

Punfield Beds of Purbeck, Dorsetshire.

Fossil Sh.e.l.ls and Fish of the Wealden.

Area of the Wealden.

Flora of the Wealden.