The Student's Elements of Geology Part 32

The highest beds of chalk in England and France consist of a pure, white, calcareous ma.s.s, usually too soft for a building-stone, but sometimes pa.s.sing into a more solid state. It consists, almost purely, of carbonate of lime; the stratification is often obscure, except where rendered distinct by interstratified layers of flint, a few inches thick, occasionally in continuous beds, but oftener in nodules, and recurring at intervals generally from two to four feet distant from each other. This upper chalk is usually succeeded, in the descending order, by a great ma.s.s of white chalk without flints, below which comes the chalk marl, in which there is a slight admixture of argillaceous matter. The united thickness of the three divisions in the south of England equals, in some places, 1000 feet. The section in Figure 231 will show the manner in which the white chalk extends from England into France, covered by the tertiary strata described in former chapters, and reposing on lower cretaceous beds.

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, Belemnitella mucronata, and Ostrea vesicularis (Figure 251).

(Figures 232 to 236.-- Organic bodies forming the ooze of the bed of the Atlantic at great depths.

(FIGURE 232. Globigerina bulloides. Calcareous Rhizopod.)

(FIGURE 233. Actinocyclus. Siliceous Diatomaceae. )

(FIGURE 234. Pinnularia. Siliceous Diatomaceae.)

(FIGURE 235. Eunotia bidens. Siliceous Diatomaceae.)

(FIGURE 236. Spicula of sponge. Siliceous sponge.))

Great light has recently been thrown upon the origin of the unconsolidated white chalk by the deep soundings made in the North Atlantic, previous to laying down, in 1858, the electric telegraph between Ireland and Newfoundland. At depths sometimes exceeding two miles, the mud forming the floor of the ocean was found, by Professor Huxley, to be almost entirely composed (more than nineteen- twentieths of the whole) of minute Rhizopods, or foraminiferous sh.e.l.ls of the genus Globigerina, especially the species Globigerina bulloides (see Figure 232.) the organic bodies next in quant.i.ty were the siliceous sh.e.l.ls called Polycystineae, and next to them the siliceous skeletons of plants called Diatomaceae (Figures 233, 234, 235), and occasionally some siliceous spiculae of sponges (Figure 236) were intermixed. These were connected by a ma.s.s of living gelatinous matter to which he has given the name of Bathybius, and which contains abundance of very minute bodies termed Coccoliths and Coccospheres, which have also been detected fossil in chalk.

Sir Leopold MacClintock and Dr. Wallich have ascertained that 95 per cent of the mud of a large part of the North Atlantic consists of Globigerina sh.e.l.ls. But Captain Bullock, R.N., lately brought up from the enormous depth of 16,860 feet a white, viscid, chalky mud, wholly devoid of Globigerinae. This mud was perfectly h.o.m.ogeneous in composition, and contained no organic remains visible to the naked eye. Mr. Etheridge, however, has ascertained by microscopical examination that it is made up of Coccoliths, Discoliths, and other minute fossils like those of the Chalk cla.s.sed by Huxley as Bathybius, when this term is used in its widest sense. This mud, more than three miles deep, was dredged up in lat.i.tude 20 degrees 19' N., longitude 4 degrees 36' E., or about midway between Madeira and the Cape of Good Hope.

The recent deep-sea dredgings in the Atlantic conducted by Dr. Wyville Thomson, Dr. Carpenter, Mr. Gwyn Jeffreys, and others, have shown that on the same white mud there sometimes flourish Mollusca, Crustacea, and Echinoderms, besides abundance of siliceous sponges, forming, on the whole, a marine fauna bearing a striking resemblance in its general character to that of the ancient chalk.

POPULAR ERROR AS TO THE GEOLOGICAL CONTINUITY OF THE CRETACEOUS PERIOD.

We must be careful, however, not to overrate the points of resemblance which the deep-sea investigations have placed in a strong light. They have been supposed by some naturalists to warrant a conclusion expressed in these words: "We are still living in the Cretaceous epoch;" a doctrine which has led to much popular delusion as to the bearing of the new facts on geological reasoning and cla.s.sification. The reader should be reminded that in geology we have been in the habit of founding our great chronological divisions, not on foraminifera and sponges, nor even on echinoderms and corals, but on the remains of the most highly organised beings available to us, such as the mollusca; these being met with, as explained in Chapter 9, in stratified rocks of almost every age. In dealing with the mollusca, it is those of the highest or most specialised organisation, which afford us the best characters in proportion as their vertical range is the most limited. Thus the Cephalopoda are the most valuable, as having a more restricted range in time than the Gasteropoda; and these, again, are more characteristic of the particular stratigraphical subdivisions than are the Lamellibranchiate Bivalves, while these last, again, are more serviceable in cla.s.sification than the Brachiopoda, a still lower cla.s.s of sh.e.l.l-fish, which are the most enduring of all.

When told that the new dredgings prove that "we are still living in the Chalk Period," we naturally ask whether some cuttle-fish has been found with a Belemnite forming part of its internal framework; or have Ammonites, Baculites, Hamites, Turrilites, with four or five other Cephalopodous genera characteristic of the chalk and unknown as tertiary, been met with in the abysses of the ocean?

Or, in the absence of these long-extinct forms, has a single spiral univalve, or species of Cretaceous Gasteropod, been found living? Or, to descend still lower in the scale, has some characteristic Cretaceous genus of Lamellibranchiate Bivalve, such as the Inoceramus, or Hippurite, foreign to the Tertiary seas, been proved to have survived down to our time? Or, of the numerous genera of lamellibranchiates common to the Cretaceous and Recent seas, has one species been found living? The answer to all these questions is-- not one has been found. Even of the humblest sh.e.l.l-fish, the Brachiopods, no new species common to the Cretaceous and recent seas has yet been met with. It has been very generally admitted by conchologists that out of a hundred species of this tribe occurring fossil in the Upper Chalk-- one, and one only, Terebratulina striata, is still living, being thought to be identical with Terebratula caput-serpentis.

Although this ident.i.ty is still questioned by some naturalists of authority, it would certainly not surprise us if another lamp-sh.e.l.l of equal antiquity should be met with in the deep sea.

Had it been declared that we are living in the Eocene epoch, the idea would not be so extravagant, for the great reptiles of the Upper Chalk, the Mosasaurus, Pliosaurus, and Pterodactyle, and many others, as well as so many genera of chambered univalves, had already disappeared from the earth, and the marine fauna had made a greater approach to our own by nearly the entire difference which separates it from the fauna of the Cretaceous seas. The Eocene nummulitic limestone of Egypt is a rock mainly composed, like the more ancient white chalk, of globigerine mud; and if the reader will refer to what we have said of the extent to which the nummulitic marine strata, formed originally at the bottom of the sea, now enter into the framework of mountain chains of the princ.i.p.al continents, he will at once perceive that the present Atlantic, Pacific, and Indian Oceans are geographical terms, which must be wholly without meaning when applied to the Eocene, and still more to the Cretaceous Period; so that to talk of the chalk having been uninterruptedly forming in the Atlantic from the Cretaceous Period to our own, is as inadmissible in a geographical as in a geological sense.

CHALK-FLINTS.

The origin of the layers of flint, whether in the form of nodules, or continuous sheets, or in veins or cracks not parallel to the stratification, has always been more difficult to explain than that of the white chalk. But here, again, the late deep-sea soundings have suggested a possible source of such mineral matter. During the cruise of the "Bulldog," already alluded to, it was ascertained that while the calcareous Globigerinae had almost exclusive possession of certain tracts of the sea-bottom, they were wholly wanting in others, as between Greenland and Labrador. According to Dr. Wallich, they may flourish in those s.p.a.ces where they derive nutriment from organic and other matter, brought from the south by the warm waters of the Gulf Stream, and they may be absent where the effects of that great current are not felt. Now, in several of the s.p.a.ces where the calcareous Rhizopods are wanting, certain microscopic plants, called Diatomaceae, above-mentioned (Figures 233-235), the solid parts of which are siliceous, monopolise the ground at a depth of nearly 400 fathoms, or 2400 feet.

The large quant.i.ties of silex in solution required for the formation of these plants may probably arise from the disintegration of feldspathic rocks, which are universally distributed. As more than half of their bulk is formed of siliceous earth, they may afford an endless supply of silica to all the great rivers which flow into the ocean. We may imagine that, after a lapse of many years or centuries, changes took place in the direction of the marine currents, favouring at one time a supply in the same area of siliceous, and at another of calcareous matter in excess, giving rise in the one case to a preponderance of Globigerinae, and in the other of Diatomaceae. These last, and certain sponges, may by their decomposition have furnished the silex, which, separating from the chalky mud, collected round organic bodies, or formed nodules, or filled shrinkage cracks.

POT-STONES.

(FIGURE 237. View of a chalk-pit at Horstead, near Norwich, showing the position of the pot-stones. From a drawing by Mrs. Gunn.)

A more difficult enigma is presented by the occurrence of certain huge flints, or pot-stones, as they are called in Norfolk, occurring singly, or arranged in nearly continuous columns at right angles to the ordinary and horizontal layers of small flints. I visited in the year 1825 an extensive range of quarries then open on the river Bure, near Horstead, about six miles from Norwich, which afforded a continuous section, a quarter of a mile in length, of white chalk, exposed to the depth of about twenty-six feet, and covered by a bed of gravel.

The pot-stones, many of them pear-shaped, were usually about three feet in height and one foot in their transverse diameter, placed in vertical rows, like pillars, at irregular distances from each other, but usually from twenty to thirty feet apart, though sometimes nearer together, as in Figure 237. These rows did not terminate downward in any instance which I could examine, nor upward, except at the point where they were cut off abruptly by the bed of gravel. On breaking open the pot-stones, I found an internal cylindrical nucleus of pure chalk, much harder than the ordinary surrounding chalk, and not crumbling to pieces like it, when exposed to the winter's frost. At the distance of half a mile, the vertical piles of pot-stones were much farther apart from each other. Dr. Buckland has described very similar phenomena as characterising the white chalk on the north coast of Antrim, in Ireland. (Geological Transactions 1st Series volume 4 page 413.)

VITREOUS SPONGES OF THE CHALK.

These pear-shaped ma.s.ses of flint often resemble in shape and size the large sponges called Neptune's Cups (Spongia patera, Hardw.), which grow in the seas of Sumatra; and if we could suppose a series of such gigantic sponges to be separated from each other, like trees in a forest, and the individuals of each successive generation to grow on the exact spot where the parent sponge died and was enveloped in calcareous mud, so that they should become piled one above the other in a vertical column, their growth keeping pace with the acc.u.mulation of the enveloping calcareous mud, a counterpart of the phenomena of the Horstead pot-stones might be obtained.

(FIGURE 238. Ventriculites radiatus, Mantell. Syn. Ocellaria radiata. D'Orbigny.

White chalk.)

Professor Wyville Thomson, describing the modern soundings in 1869 off the north coast of Scotland, speaks of the ooze or chalk mud brought from a depth of about 3000 feet, and states that at one haul they obtained forty specimens of vitreous sponges buried in the mud. He suggests that the Ventriculites of the chalk were nearly allied to these sponges, and that when the silica of their spicules was removed, and was dissolved out of the calcareous matrix, it set into flint.

BOULDERS AND GROUPS OF PEBBLES IN CHALK.

The occurrence here and there, in the white chalk of the south of England, of isolated pebbles of quartz and green schist has justly excited much wonder. It was at first supposed that they had been dropped from the roots of some floating tree, by which means stones are carried to some of the small coral islands of the Pacific. But the discovery in 1857 of a group of stones in the white chalk near Croydon, the largest of which was syenite and weighed about forty pounds, accompanied by pebbles and fine sand like that of a beach, has been shown by Mr.

G.o.dwin Austen to be inexplicable except by the agency of floating ice. If we consider that icebergs now reach 40 degrees north lat.i.tude in the Atlantic, and several degrees nearer the equator in the southern hemisphere, we can the more easily believe that even during the Cretaceous epoch, a.s.suming that the climate was milder, fragments of coast ice may have floated occasionally as far as the south of England.

DISTINCTNESS OF MINERAL CHARACTER IN CONTEMPORANEOUS ROCKS OF THE CRETACEOUS PERIOD.

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, shingle, and clay having been acc.u.mulated contemporaneously even in 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. It demonstrates that in the Cretaceous sea, as in our own, distinct mineral deposits were simultaneously in progress. The quartzose sandstone alluded to, derived from the detritus of the neighbouring granite, is absolutely devoid of carbonate of lime, yet it was formed at the distance only of four hundred miles from a sea-bottom now const.i.tuting part of France, where the purely calcareous white chalk was forming. In the North American continent, on the other hand, where the Upper Cretaceous formations are so widely developed, true white chalk, in the ordinary sense of that term, does not exist.

FOSSILS OF THE WHITE CHALK.

(FIGURE 239. Ananchytes ovatus, Leske. White chalk, upper and lower.

a. Side view.

b. Base 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.)

(FIGURE 240. Micraster cor-angumum, Leske. White chalk.)

(FIGURE 241. Galerites albogalerus, Lam. White chalk.)

(FIGURE 242. Marsupites Milleri. Mant. White chalk.)

Among the fossils of the white chalk, echinoderms are very numerous; and some of the genera, like Ananchytes (see Figure 239), are exclusively cretaceous. Among the Crinoidea, the Marsupites (Figure 242) is a characteristic genus. Among the mollusca, the cephalopoda are represented by Ammonites, Baculites (Figure 229), and Belemnites (Figure 226). Although there are eight or more species of Ammonites and six of them peculiar to it, this genus is much less fully represented than in each of the other subdivisions of the Upper Cretaceous group.

(FIGURE 243. Terebratulina striata, Wahlenb. Upper white chalk.)

(FIGURE 244. Rhynchonella octoplicata, Sowerby. (Var. of R. plicatilis). Upper white chalk.

(FIGURE 245. Magas pumila, Sowerby. Upper white chalk.)

(FIGURE 246. Terebratula carnea, Sowerby. Upper white chalk.)

(FIGURE 247. Terebratula biplicata, Brocch. Upper cretaceous.)

(FIGURE 248. Crania Parisiensis, Duf. Inferior or attached valve. Upper white chalk.)

(FIGURE 249. Pecten Beaveri, Sowerby. Reduced to one-third diameter. Lower white chalk and chalk marl. Maidstone.)

(FIGURE 250. Lima spinosa, Sowerby. Syn. Spondylus spinosus. Upper white chalk.)

(FIGURE 251. Ostrea vesicularis. Syn. Gryphaea convexa. Upper chalk and upper greensand.)

Among the brachiopoda in the white chalk, the Terebratulae are very abundant (see Figures 243-247). With these are a.s.sociated some forms of oyster (see Figure 251), and other bivalves (Figures 249, 250).

(FIGURE 252. Inoceramus Lamarckii. Syn. Catillus Lamarckii. White chalk (Dixon's Geology Suss.e.x Table 28 Figure 29).)

Among the bivalve mollusca, no form marks the Cretaceous era in Europe, America, and India in a more striking manner than the extinct genus Inoceramus (Catillus of Lam.; see Figure 252), 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.