A Manual of Elementary Geology Part 32

[Ill.u.s.tration: Fig. 202. _Ostrea vesicularis._ _Gryphaea globosa_, Min. Con.

Upper chalk and upper greensand.]

[Ill.u.s.tration: Fig. 203. _Pecten 5-costatus._ White chalk, upper and lower greensands.]

[Ill.u.s.tration: Fig. 204. _Ostrea carinata._ Chalk marl, upper and lower greensands.]

[Ill.u.s.tration: Fig. 205. _Crania Parisiensis_, inferior or attached valve.

Upper white chalk.]

[Ill.u.s.tration: Fig. 206. _Plagiostoma Hoperi_, Sow. Syn. _Lima Hoperi_.

White chalk and upper greensand.]

[Ill.u.s.tration: Fig. 207. _Plagiostoma spinosum_, Sow. Syn. _Spondylus spinosus_. Upper white chalk.]

Among the rest, no form marks the cretaceous era in Europe, America, and India, in a more striking manner than the extinct genus _Inoceramus_ (_Catillus_ of Lamk.), the sh.e.l.ls of which are distinguished by a fibrous texture, and are often met with in fragments, having, probably, been extremely friable.

[Ill.u.s.tration: Fig. 208. _Inoceramus Lamarckii._

Syn. _Catillus Lamarckii_.

White Chalk (Dixon's Geol. Suss.e.x, Tab. 28. fig. 29.)]

[Ill.u.s.tration: Fig. 209. _Eschara disticha._

_a._ Natural size.

_b._ Portion magnified.

White chalk.]

[2 Ill.u.s.trations: Fig. 210. Fig. 211. A branching sponge in a flint, from the white chalk. From the collection of Mr. Bowerbank.]

With these mollusca are many corals (figs. 209, 210, 211.) and sea urchins (fig. 212.), which are alike marine, and, for the most part, indicative of a deep sea. They are dispersed indifferently through the soft chalk, and hard flint, and some of the flinty nodules owe their irregular forms to inclosed zoophytes, as in the specimen represented in fig. 211., where the hollows in the exterior are caused by the branches of a sponge seen on breaking open the flint, fig. 210.

[Ill.u.s.tration: Fig. 212. _Ananchytes ovata_. White chalk, upper and lower.

_a_. Side view.

_b_. Bottom of the sh.e.l.l on which both the oral and a.n.a.l apertures are placed; the a.n.a.l being more round, and at the smaller end.]

Of the singular family called _Rudistes_, by Lamarck, hereafter to be mentioned, as extremely characteristic of the chalk of Southern Europe, a single representative only (fig. 213.) has been discovered in the white chalk of England.

[4 Ill.u.s.trations: _Hippurites Mortoni_, Mantell. Houghton, Suss.e.x. White chalk. Diameter one seventh of nat. size.

Fig. 213. Two individuals deprived of their opercula, adhering together.

Fig. 214. Same seen from above.

Fig. 215. Transverse section of part of the wall of the sh.e.l.l, magnified to show the structure.

Fig. 216. Vertical section of the same.

On the side where the sh.e.l.l is thinnest, there is one external furrow and corresponding internal ridge, a, b. figs. 213, 214.; but they are usually less prominent than in these figures. This species has been referred to _Hippurites_, but does not, I believe, fully agree in character with that genus. I have never seen the opercular piece, or _valve_, as it is called by those conchologists who regard the _Rudistes_ as bivalve mollusca. The specimen above figured was discovered by the late Mr. Dixon.]

The remains of fishes of the Upper Cretaceous formations consist chiefly of teeth of the shark family of genera, in part common to the tertiary, and partly distinct. But we meet with no bones of land animals, nor any terrestrial or fluviatile sh.e.l.ls, nor any plants, except sea weeds, and here and there a piece of drift wood. All the appearances concur in leading us to conclude that the white chalk was the product of an open sea of considerable depth.

The existence of turtles and oviparous saurians, and of a Pterodactyl or winged-lizard, found in the white chalk of Maidstone, implies, no doubt, some neighbouring land; but a few small islets in mid-ocean, like Ascension, so much frequented by migratory droves of turtles, might perhaps have afforded the required retreat where these creatures might lay their eggs in the sand, or from which the flying species may have been blown out to sea. Of the vegetation of such islands we have scarcely any indication, but it consisted partly of cycadeous plants; for a fragment of one of these was found by Capt. Ibbetson in the chalk marl of the Isle of Wight, and is referred by A. Brongniart to _Clathraria Lyellii_, Mantell, a species common to the antecedent Wealden period.

_Geographical extent and origin of the While Chalk._--The area over which the white chalk preserves a nearly h.o.m.ogeneous aspect is so vast, that the earlier geologists despaired of discovering any a.n.a.logous deposits of recent date. Pure chalk, of nearly uniform aspect and composition, is met with in a north-west and south-east direction, from the north of Ireland to the Crimea, a distance of about 1140 geographical miles; and in an opposite direction it extends from the south of Sweden to the south of Bordeaux, a distance of about 840 geographical miles. In Southern Russia, according to Sir R. Murchison, it is sometimes 600 feet thick, and retains the same mineral character as in France and England, with the same fossils, including _Inoceramus Cuvieri_, _Belemnites mucronatus_, and _Ostrea vesicularis_.

But it would be an error to imagine, that the chalk was ever spread out continuously over the whole of the s.p.a.ce comprised within these limits, although it prevailed in greater or less thickness over large portions of that area. On turning to those regions of the Pacific where coral reefs abound, we find some archipelagoes of lagoon islands, such as that of the Dangerous Archipelago, for instance, and that of Radack, with several adjoining groups, which are from 1100 to 1200 miles in length, and 300 or 400 miles broad; and the s.p.a.ce to which Flinders proposed to give the name of the Corralline Sea is still larger; for it is bounded on the east by the Australian barrier--all formed of coral rock,--on the west by New Caledonia, and on the north by the reefs of Louisiade. Although the islands in these areas may be thinly sown, the mud of the decomposing zoophytes may be scattered far and wide by oceanic currents. That this mud would resemble chalk I have already hinted when speaking of the Faxoe limestone, p. 211.; and it was also remarked in an early part of this volume, that some even of that chalk which appears to an ordinary observer quite dest.i.tute of organic remains, is nevertheless, when seen under the microscope, full of fragments of corals and sponges; together with the valves of entomostraca, the sh.e.l.ls of foraminifera, and still more minute infusoria.[215-A] (See p. 26.)

Now it had been often suspected, before these discoveries, that white chalk might be of animal origin, even where every trace of organic structure has vanished. This bold idea was partly founded on the fact, that the chalk consisted of pure carbonate of lime, such as would result from the decomposition of testacea, echini, and corals; and partly on the pa.s.sage observable between these fossils when half decomposed and chalk. But this conjecture seemed to many naturalists quite vague and visionary, until its probability was strengthened by new evidence brought to light by modern geologists.

We learn from Lieutenant Nelson, that, in the Bermuda Islands, there are several basins or lagoons almost surrounded and enclosed by reefs of coral. At the bottom of these lagoons a soft white calcareous mud is formed by the decomposition of _Eschara_, _Fl.u.s.tra_, _Cellepora_, and other corallines. This mud, when dried, is undistinguishable from common white earthy chalk; and some portions of it, presented to the Museum of the Geological Society of London, might, after full examination, be mistaken for ancient chalk, but for the labels attached to them. About the same time Mr. C. Darwin observed similar facts in the coral islands of the Pacific; and came also to the opinion, that much of the soft white mud found at the bottom of the sea near coral reefs has pa.s.sed through the bodies of worms, by which the stony ma.s.ses of coral are everywhere bored; and other portions through the intestines of fishes; for certain gregarious fishes of the genus _Sparus_ are visible through the clear water, browsing quietly, in great numbers, on living corals, like grazing herds of graminivorous quadrupeds. On opening their bodies, Mr. Darwin found their intestines filled with impure chalk. This circ.u.mstance is the more in point, when we recollect how the fossilist was formerly puzzled by meeting, in chalk, with certain bodies, called cones of the larch, which were afterwards recognized by Dr. Buckland to be the excrement of fish.[216-A] These spiral coprolites (see figures), like the scales and bones of fossil fish in the chalk, are composed chiefly of phosphate of lime.

[2 Ill.u.s.trations: Fig. 217. Fig. 218. Coprolites of fish called _Iulo-eido-copri_, from the chalk.]

Mr. Dana, when describing the elevated coral reef of Oahu, in the Sandwich Islands, says, that some varieties of the rock consist of aggregated sh.e.l.ls, imbedded in a compact calcareous base as firm in texture as any secondary limestone; while others are like chalk, having its colour, its earthy fracture, its soft h.o.m.ogeneous texture, and being an equally good writing material. The same author describes, in many growing coral reefs, a similar formation of modern chalk, undistinguishable from the ancient.[216-B] The extension over a wide submarine area of the calcareous matrix of the chalk, as well as of the imbedded fossils, would take place the more readily, in consequence of the low specific gravity of the sh.e.l.ls of mollusca and zoophytes, when compared with ordinary sand and mineral matter. The mud also derived from their decomposition would be much lighter than argillaceous and other inorganic mud, and very easily transported by currents, especially in salt water.

_Single pebbles in chalk._--The general absence of sand and pebbles in the white chalk has been already mentioned; but the occurrence here and there, in the south-east of England, of a few isolated pebbles of quartz and green schist, some of them 2 or 3 inches in diameter, has justly excited much wonder. If these had been carried to the spots where we now find them by waves or currents from the lands once bordering the cretaceous sea, how happened it that no sand or mud were transported thither at the same time? We cannot conceive such rounded stones to have been drifted like erratic blocks by ice[217-A], for that would imply a cold climate in the Cretaceous period; a supposition inconsistent with the luxuriant growth of large chambered univalves, numerous corals, and many fish, and other fossils of tropical forms.

Now in Keeling Island, one of those detached ma.s.ses of coral which rise up in the wide Pacific, Captain Ross found a single fragment of greenstone, where every other particle of matter was calcareous; and Mr. Darwin concludes that it must have come there entangled in the roots of a large tree. He reminds us that Chamisso, the distinguished naturalist who accompanied Kotzebue, affirms, that the inhabitants of the Radack archipelago, a group of lagoon islands, in the midst of the Pacific, obtained stones for sharpening their instruments by searching the roots of trees which are cast up on the beach.[217-B]

It may perhaps be objected, that a similar mode of transport cannot have happened in the cretaceous sea, because fossil wood is very rare in the chalk. Nevertheless wood is sometimes met with, and in the same parts of the chalk where the pebbles are found, both in soft stone and in a silicified state in flints. In these cases it has often every appearance of having been floated from a distance, being usually perforated by boring-sh.e.l.ls, such as the _Teredo_ and _Fistulana_.[217-C]

The only other mode of transport which suggests itself is sea-weed. Dr.

Beck informs me, that in the Lym-Fiord, in Jutland, the _Fucus vesiculosus_, often called kelp, sometimes grows to the height of 10 feet, and the branches rising from a single root form a cl.u.s.ter several feet in diameter. When the bladders are distended, the plant becomes so buoyant as to float up loose stones several inches in diameter, and these are often thrown by the waves high up on the beach. The _Fucus giganteus_ of Solander, so common in Terra del Fuego, is said by Captain Cook to attain the length of 360 feet, although the stem is not much thicker than a man's thumb. It is often met with floating at sea, with sh.e.l.ls attached, several hundred miles from the spots where it grew.

Some of these plants, says Mr. Darwin, were found adhering to large loose stones in the inland channels of Terra del Fuego, during the voyage of the Beagle in 1834; and that so firmly, that the stones were drawn up from the bottom into the boat, although so heavy that they could scarcely be lifted in by one person. Some fossil sea-weeds have been found in the Cretaceous formation, but none, as yet, of large size.

But we must not imagine that because pebbles are so rare in the white chalk of England and France there are no proofs of sand, s.h.i.+ngle, and clay having been acc.u.mulated contemporaneously even in the European seas. The siliceous sandstone, called "upper quader" by the Germans, overlies white argillaceous chalk, or "planer-kalk," a deposit resembling in composition and organic remains the chalk marl of the English series. This sandstone contains as many fossil sh.e.l.ls common to our white chalk as could be expected in a sea-bottom formed of such different materials. It sometimes attains a thickness of 600 feet, and by its jointed structure and vertical precipices, plays a conspicuous part in the picturesque scenery of Saxon Switzerland, near Dresden.

_Upper greensand_ (4. Tab. p. 209.).--The lower chalk without flints pa.s.ses gradually downwards, in the south of England, into an argillaceous limestone, "the chalk marl," already alluded to, in which ammonites and other cephalopoda, so rare in the higher parts of the series, appear. This marly deposit pa.s.ses in its turn into beds containing green particles of a chloritic mineral, called the upper greensand. In parts of Surrey calcareous matter is largely intermixed, forming a stone called _firestone_. In the cliffs of the southern coast of the Isle of Wight, this upper greensand is 100 feet thick, and contains bands of siliceous limestone and calcareous sandstone with nodules of chert.

[2 Ill.u.s.trations: Fossils of the Upper Greensand.

Fig. 219.

_a._ _Terebratula lyra._ } Upper greensand.

_b._ Same, seen in profile. } France.

Fig. 220. _Ammonites Rhotomagensis._ Upper greensand.]

[Ill.u.s.tration: Fig. 221. _Hamites spiniger_ (Fitton); near Folkstone. Gault.]

_Gault._--The lowest member of the upper Cretaceous group, usually about 100 feet thick in the S.E. of England, is provincially termed Gault. It consists of a dark blue marl, sometimes intermixed with greensand. Many peculiar forms of cephalopoda, such as the _Hamite_ (fig. 221.) and _Scaphite_, with other fossils, characterize this formation, which, small as is its thickness, can be traced by its organic remains to distant parts of Europe, as, for example, to the Alps.

The phosphate of lime, found lately near Farnham, in Surrey, in such abundance as to be used largely by the agriculturist for fertilizing soils, occurs exclusively, according to Mr. R. A. C. Austen, in the upper greensand and gault. It is doubtless of animal origin, and partly coprolitic, probably derived from the excrement of fish.