Part 33 (1/2)

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.