A Manual of Elementary Geology Part 33

LOWER CRETACEOUS DIVISION. (No. 6. Tab. p. 209.)

That part of the Cretaceous series which is older than the Gault has been commonly called the Lower Greensand. The greater number of its fossils are specifically distinct from those of the upper cretaceous system. Dr.

Fitton, to whom we are indebted for an excellent monograph on this formation as developed in England, gives the following as the succession of rocks seen in parts of Kent.

No. 1. Sand, white, yellowish, or ferruginous, with concretions of limestone and chert 70 feet.

2. Sand with green matter 70 to 100 feet.

3. Calcareous stone, called Kentish rag 60 to 80 feet.

In his detailed description of the fine section displayed at Atherfield, in the south of the Isle of Wight, we find the limestone wholly wanting; in fact, the variations in the mineral composition of this group, even in contiguous districts, is very great; and on comparing the Atherfield beds with corresponding strata at Hythe in Kent, distant 95 miles, the whole series has lost half its thickness, and presents a very dissimilar aspect.[219-A]

On the other hand, Professor E. Forbes has shown that when the sixty-three strata at Atherfield are severally examined, the total thickness of which he gives as 843 feet, there are some fossils which range through the whole series, others which are peculiar to particular divisions. As a proof that all belong chronologically to one system, he states that whenever similar conditions are repeated in overlying strata the same species reappear.

Changes of depth, or of the mineral nature of the bottom, the presence or absence of lime or of peroxide of iron, the occurrence of a muddy, or a sandy, or a gravelly bottom, are marked by the banishment of certain species and the predominance of others. But these differences of conditions being mineral, chemical, and local in their nature, have nothing to do with the extinction, throughout a large area, of certain animals or plants. The rule laid down by this eminent naturalist for enabling us to test the arrival of a new state of things in the animate world, is the representation by new and different species of corresponding genera of mollusca or other beings. When the forms proper to loose sand or soft clay, or a stony or calcareous bottom, or a moderate or a great depth of water, recur with all the same species, the interval of time has been, geologically speaking, small, however dense the ma.s.s of matter acc.u.mulated.

But if, the genera remaining the same, the species are changed, we have entered upon a new period; and no similarity of climate, or of geographical and local conditions, can then recall the old species which a long series of destructive causes in the animate and inanimate world has gradually annihilated. On pa.s.sing from the lower greensand to the gault, we suddenly reach one of these new epochs, scarcely any of the fossil species being common to the lower and upper cretaceous systems, a break in the chain implying no doubt many missing links in the series of geological monuments which we may some day be able to supply.

One of the largest and most abundant sh.e.l.ls in the lowest strata of the lower greensand, as displayed in the Atherfield section, is the large _Perna mulleti_ of which a reduced figure is here given (fig. 222.).

[Ill.u.s.tration: Fig. 222. _Perna mulleti._ Desh. in Leym.

_a._ Exterior.

_b._ Hinge of upper valve.]

In the south of England, during the acc.u.mulation of the lower greensand above described, the bed of the sea appears to have been continually sinking, from the commencement of the period, when the freshwater Wealden beds were submerged, to the deposition of those strata on which the gault immediately reposes.

Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains of chlorite and mica, speak plainly of the nature of the pre-existing rocks, from the wearing down of which the greensand beds were derived. The land, consisting of such rocks, was doubtless submerged before the origin of the white chalk, as corals can only multiply in the clear waters of the sea in s.p.a.ces to which no mud or sand are conveyed by currents.

HIPPURITE LIMESTONE.

_Difference between the chalk of the north and south of Europe._--By the aid of the three tests of relative age, namely, 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 the chalk and Greensand in the neighbourhood of London and Paris 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 the annexed map, fig. 223., in which the shaded part represents chalk).

[Ill.u.s.tration: Fig. 223. Map of south-western France.]

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 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 a formation which we at once recognize by its mineral character to be chalk, although there are some places where the rock becomes oolitic. The fossils are, upon the whole, very similar; especially certain species of the genera _Spatangus_, _Ananchytes_, _Cidarites_, _Nucula_, _Ostrea_, _Gryphaea_ (_Exogyra_), _Pecten_, _Plagiostoma_ (_Lima_), _Trigonia_, _Catillus_, (_Inoceramus_), and _Terebratula_.[221-A] 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.

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.

[Ill.u.s.tration: Fig. 224.

_a._ _Radiolites radiosus_, D'Orb. (_Hippurites_, Lamk.) _b._ Opercular valve of same.

White chalk of France.]

[Ill.u.s.tration: Fig. 225. _Radiolites foliaceus_, D'Orb. Syn. _Sphaerulites agariciformis_, Blainv. White chalk of France.]

[Ill.u.s.tration: Fig. 226. _Hippurites organisans_, Desmoulins. Upper chalk:--chalk marl of Pyrenees?[222-A]

_a._ Young individual; when full grown they occur in groups adhering laterally to each other.

_b._ Upper side of the opercular valve, showing a reticulated structure in those parts, _b_, where the external coating is worn off.

_c._ Upper side of the lower and cylindrical valve.

_d._ Cast of the interior of the lower conical valve.]

The species called _Hippurites organisans_ (fig. 226.) is more abundant than any other in the south of Europe; and the geologist should make himself well acquainted with the cast _d_, which is far more common in many compact marbles of the upper cretaceous period than the sh.e.l.l itself, which has often wholly disappeared. The flutings, or smooth, rounded, longitudinal ribs, representing the form of the interior, are wholly unlike the hippurite itself, and in some individuals, which attain a great size and length, are very conspicuous.

Between the region of chalk last mentioned in which Perigueux is situated, and the Pyrenees, the s.p.a.ce B intervenes. (See Map, p. 221.) Here the tertiary strata cover, and for the most part conceal, the cretaceous rocks, except in some spots where they have been laid open by the denudation of newer formations. In these places they are seen still preserving the form of a white chalky rock, which is charged in part with grains of green sand.

Even as far south as Tercis, on the Adour, near Dax, where I examined them in 1828, the cretaceous rocks retain this character. In that region M.

Grateloup has found in them _Ananchytes ovata_ (fig. 212.), and other fossils of the English chalk, together with _Hippurites_.

FLORA OF THE CRETACEOUS PERIOD.

Although the fossil plants of the Cretaceous era at present known are few in number, the rocks being princ.i.p.ally marine, they suffice, according to M. Ad. Brongniart, to show a transition character between the vegetation of the secondary and that of the tertiary formations. The tertiary strata, when compared to the older rocks, are marked by the predominance of _Exogens_, which now const.i.tute three-fourths of the living plants of the globe.[223-A]

These exogens are wanting in the secondary strata generally, but in the Cretaceous period they equal in number the _Gymnogens_ (_Coniferae_ and _Cycadeae_) which abounded so much in the preceding Oolitic period, and disappeared before the Eocene rocks were formed.[223-B] The discovery of a tree-fern in the ferruginous sands of the Lower Cretaceous group of the department of Ardennes in France is one of many signs of the contrast of the flora, and doubtless of the climate, of this era with that of the Pliocene and Modern periods.

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 our Upper Cretaceous system; which we can, nevertheless, recognize as referable, paleontologically, to the same division.

That they were about the same age generally as the European chalk and greensand, was the conclusion to which Dr. Morton and Mr. Conrad came after their investigation of the fossils in 1834. The strata consist chiefly of greensand 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_, _Belemnites mucronatus_. 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 are different, the generic forms, such as the Baculite and certain sections of Ammonites, as also the Inoceramus (see above, fig. 208.) 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.[224-A]

Fish of the genera _Lamna_, _Galeus_, and _Carcharias_ are common to New Jersey and the European cretaceous rocks. So also is the genus _Mosasaurus_ among reptiles, and _Pliosaurus_ (Owen), another saurian likewise obtained from the English chalk. From New Jersey the cretaceous formation extends southwards to North Carolina, Georgia, and Alabama, cropping out at intervals from beneath the tertiary strata, between the Appalachian Mountains and the Atlantic. They then sweep round the southern extremity of that chain, and stretch northwards again to Tennessee and Kentucky. They have also been traced far up the valley of the Missouri 275 English miles above its mouth, to the neighbourhood of Fort Leavenworth; and southwards to Texas, according to the observations of Ferdinand Romer; so that already the area which they are ascertained to occupy in North America may perhaps equal their extent in Europe. So little do they resemble mineralogically the European white chalk, that limestone in North America is, upon the whole, an exception to the rule; and, even in Alabama, where I saw a calcareous member of this group, the marlstones are much more like the English and French Lias than any other secondary deposit of the Old World.

At the base of the system in Alabama I found dense ma.s.ses of s.h.i.+ngle, perfectly loose and unconsolidated, derived from the waste of paleozoic (or carboniferous) rocks, a ma.s.s in no way distinguishable, except by its position, from ordinary alluvium, but covered with marls abounding in Inocerami.

In Texas, according to F. Romer, the chalk a.s.sumes a new lithological type, a large portion of it consisting of hard siliceous limestone, but the organic remains leaving no doubt in regard to its age.

In South America the cretaceous strata have been discovered in Columbia, as at Bogota and elsewhere, containing Ammonites, Hamites, Inocerami, and other characteristic sh.e.l.ls.[225-A]

In the South of India, also, at Pondicherry, Verdach.e.l.lum, and Trinconopoly, Messrs. Kaye and Egerton have collected fossils belonging to the cretaceous system. Taken in connection with those from the United States they prove, says Prof. E. Forbes, that those powerful causes which stamped a peculiar character on the forms of marine animal life at this period, exerted their full intensity through the Indian, European, and American seas.[225-B] Here, as in North and South America, the cretaceous character can be recognized even where there is no specific ident.i.ty in the fossils; and the same may be said of the organic type of those rocks in Europe and India which succeed next in the ascending and descending order, the Eocene and the Oolitic.

FOOTNOTES:

[209-A] M. Alcide d'Orbigny, in his valuable work ent.i.tled Paleontologie Francaise, has adopted new terms for the French subdivisions of the Cretaceous Series, which, so far as they can be made to tally with English equivalents, seem explicable thus:

Danien. Maestricht beds.

Senonien. Upper and lower white chalk, and chalk marl.

Turonien. Part of the chalk marl and the upper greensand, the latter being in his last work (Cours Elementaire) termed Cenomanien.

Albien. Gault.

Aptien. Upper part of lower greensand.

Neocomien. Lower part of same.

[211-A] See paper by the author, Trans. of Geol. Soc., vol. v.

p. 246., 1840.