The Student's Elements of Geology Part 21

(FIGURE 125. Trophon antiquum, Muller. (Fusus contrarius) half natural size.)

The reversed whelk (Figure 125) is common at Walton, where the dextral form of that sh.e.l.l is unknown. Here also we find most frequently specimens of lamellibranchiate molluscs, with both the valves united, showing that they belonged to this sea of the Upper Crag, and were not washed in from an older bed, such as the Coralline, in which case the ligament would not have held together the valves in strata so often showing signs of the boisterous action of the waves. No less than forty species of lamellibranchiate molluscs, with double valves, have been collected by Mr. Bell from the various localities of the Red Crag.

At and near the base of the Red Crag is a loose bed of brown nodules, first noticed by Professor Henslow as containing a large percentage of earthy phosphates. This bed of coprolites (as it is called, because they were originally supposed to be the faeces of animals) does not always occur at one level, but is generally in largest quant.i.ty at the junction of the Crag and the underlying formation. In thickness it usually varies from six to eighteen inches, and in some rare cases amounts to many feet. It has been much used in agriculture for manure, as not only the nodules, but many of the separate bones a.s.sociated with them, are largely impregnated with phosphate of lime, of which there is sometimes as much as sixty per cent. They are not unfrequently covered with barnacles, showing that they were not formed as concretions in the stratum where they now lie buried, but had been previously consolidated. The phosphatic nodules often collect fossil crabs and fishes from the London Clay, together with the teeth of gigantic sharks. In the same bed have been found many ear- bones of whales, and the teeth of Mastodon arvernensis, Rhinoceros Schleiermacheri, Tapirus priscus, and Hipparion (a quadruped of the horse family), and antlers of a stag, Cervus anoceros. Organic remains also of the older chalk and Lias are met with, showing how great was the denudation of previous formations during the Pliocene period. As the older White Crag, presently to be mentioned, contains similar phosphatic nodules near its base, those of the Red Crag may be partly derived from this source.

WHITE OR CORALLINE CRAG.

The lower or Coralline Crag is of very limited extent, ranging over an area about twenty miles in length, and three or four in breadth, between the rivers Stour and Alde, in Suffolk. It is generally calcareous and marly-- often a ma.s.s of comminuted sh.e.l.ls, and the remains of bryozoa (or polyzoa), pa.s.sing occasionally into a soft building-stone. (Ehrenberg proposed in 1831 the term Bryozoum, or "Moss-animal," for the molluscous or ascidian form of polyp, characterised by having two openings to the digestive sack, as in Eschara, Fl.u.s.tra, Retepora, and other zoophytes popularly included in the corals, but now cla.s.sed by naturalists as mollusca. The term Polyzoum, synonymous with Bryozoum, was, it seems, proposed in 1830, or the year before, by Mr. J.O. Thompson.) At Sudbourn and Gedgrave, near Orford, this building-stone has been largely quarried. At some places in the neighbourhood the softer ma.s.s is divided by thin flags of hard limestone, and bryozoa placed in the upright position in which they grew. From the abundance of these coralloid mollusca the lowest or White Crag obtained its popular name, but true corals, as now defined, or zoantharia, are very rare in this formation.

The Coralline Crag rarely, if ever, attains a thickness of thirty feet in any one section. Mr. Prestwich imagines that if the beds found at different localities were united in the probable order of their succession, they might exceed eighty feet in thickness, but Mr. Searles Wood does not believe in the possibility of establis.h.i.+ng such a chronological succession by aid of the organic remains, and questions whether proof could be obtained of more than forty feet. I was unable to come to any satisfactory opinion on the subject, although at Orford, especially at Gedgrave, in the neighbourhood of that place, I saw many sections in pits, where this crag is cut through. These pits are so unconnected, and of such limited extent, that no continuous section of any length can be obtained, so that speculations as to the thickness of the whole deposit must be very vague. At the base of the formation at Sutton a bed of phosphatic nodules, very similar to that before alluded to in the Red Crag, with remains of mammalia, has been met with.

(FIGURE 126. Section near Woodbridge, in Suffolk.

Through Sutton (left), Shottisham Creek, Ramsholt (right) and R. Deben.

a. Red Crag.

b. Coralline Crag.

c. London Clay.)

Whenever the Red and Coralline Crag occur in the same district, the Red Crag lies uppermost; and in some cases, as in the section represented in Figure 126, which I had an opportunity of seeing exposed to view in 1839, it is clear that the older deposit, or Coralline Crag, b, had suffered denudation, before the newer formation, a, was thrown down upon it. At D there was not only seen a distinct cliff, eight or ten feet high, of Coralline Crag, running in a direction N.E. and S.W., against which the Red Crag abuts with its horizontal layers, but this cliff occasionally overhangs. The rock composing it is drilled everywhere by Pholades, the holes which they perforated having been afterwards filled with sand, and covered over when the newer beds were thrown down. The older formation is shown by its fossils to have acc.u.mulated in a deeper sea, and contains none of those littoral forms such as the limpet, Patella, found in the Red Crag. So great an amount of denudation could scarcely take place, in such incoherent materials, without some of the fossils of the inferior beds becoming mixed up with the overlying crag, so that considerable difficulty must be occasionally experienced by the palaeontologist in deciding which species belong severally to each group.

(FIGURE 127. Fascicularia aurantium, Milne Edwards. Family, Tubuliporidae, of same author. Bryozoan of extinct genus, from the inferior or Coralline Crag, Suffolk.

a. Exterior.

b. Vertical section of interior.

c. Portion of exterior magnified.

d. Portion of interior magnified, showing that it is made up of long, thin, straight tubes, united in conical bundles.)

(FIGURE 128. Astarte omalii, laj.; species common to Upper and lower crag.)

Mr. Searles Wood estimates the total number of marine testaceous mollusca of the Coralline Crag at 350, of which 110 are not known as living, being in the proportion of thirty-one per cent extinct. No less than 130 species of bryozoa have been found in the Coralline Crag, and some belong to genera unknown in the living creation, and of a very peculiar structure; as, for example, that represented in Figure 127, which is one of several species having a globular form. Among the testacea the genus Astarte (see Figure 128) is largely represented, no less than fourteen species being known, and many of these being rich in individuals. There is an absence of genera peculiar to hot climates, such as Conus, Oliva, Fasciolaria, Cra.s.satella, and others. The absence also of large cowries (Cyprea), those found belonging exclusively to the section Trivia, is remarkable. The large volute, called Voluta Lamberti (Figure 123), may seem an exception; but it differs in form from the volutes of the torrid zone, and, like the living Voluta Magellanica, must have been fitted for an extra-tropical climate.

(FIGURE 129. Lingula Dumortieri, Nyst; Suffolk and Antwerp Crag.)

(FIGURE 130. Pyrula reticulata, Lam.; Coralline Crag, Ramsholt.)

(FIGURE 131. Temnechinus excavatus, Forbes; Temnopleurus excavatus, Wood; Coralline Crag, Ramsholt.)

The occurrence of a species of Lingula at Sutton (see Figure 129) is worthy of remark, as these Brachiopoda seem now confined to more equatorial lat.i.tudes; and the same may be said still more decidedly of a species of Pyrula, supposed by Mr. Wood to be identical with P. reticulata (Figure 130), now living in the Indian Ocean. A genus also of echinoderms, called by Professor Forbes Temnechinus (Figure 131), occurs in the Red and Coralline Crag of Suffolk, and until lately was unknown in a living state, but it has been brought to light as an existing form by the deep-sea dredgings, both of the United States survey, off Florida, at a depth of from 180 to 480 feet, and more recently (1869), in the British seas, during the explorations of the "Porcupine."

CLIMATE OF THE CRAG DEPOSITS.

One of the most interesting conclusions deduced from a careful comparison of the sh.e.l.ls of the British Pliocene strata and the fauna of our present seas has been pointed out by Professor E. Forbes. It appears that, during the Glacial period, a period intermediate, as we have seen, between that of the Crag and our own time, many sh.e.l.ls, previously established in the temperate zone, retreated southward to avoid an uncongenial climate, and they have been found fossil in the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian Archipelago, where they may have enjoyed, during the era of floating icebergs, a climate resembling that now prevailing in higher European lat.i.tudes. (E. Forbes Mem. Geological Survey of Great Britain volume 1 page 386.) The Professor gave a list of fifty sh.e.l.ls which inhabited the British seas while the Coralline and Red Crag were forming, and which, though now living in our seas, were wanting, as far as was then known, in the glacial deposits. Some few of these species have subsequently been found in the glacial drift, but the general conclusion of Forbes remains unshaken.

The transport of blocks by ice, when the Red Crag was being deposited, appears to me evident from the large size of some huge, irregular, quite unrounded chalk flints, retaining their white coating, and 2 feet long by 18 inches broad, in beds worked for phosphatic nodules at Foxhall, four miles south-east of Ipswich.

These must have been tranquilly drifted to the spot by floating ice. Mr.

Prestwich also mentions the occurrence of a large block of porphyry in the base of the Coralline Crag at Sutton, which would imply that the ice-action had begun in our seas even in this older period. The cold seems to have gone on increasing from the time of the Coralline to that of the Norwich Crag, and became more and more severe, not perhaps without some oscillations of temperature, until it reached its maximum in what has been called the Glacial period, or at the close of the Newer Pliocene, and in the Post-pliocene periods.

RELATION OF THE FAUNA OF THE CRAG TO THAT OF THE RECENT SEAS.

By far the greater number of the recent marine species occurring in the several Crag formations are still inhabitants of the British seas; but even these differ considerably in their relative abundance, some of the commonest of the Crag sh.e.l.ls being now extremely scarce-- as, for example, Buccinum Dalei-- while others, rarely met with in a fossil state, are now very common, as Murex erinaceus and Cardium echinatum. Some of the species also, the ident.i.ty of which with the living would not be disputed by any conchologist, are nevertheless distinguishable as varieties, whether by slight deviations in form or a difference in average dimensions. Since Mr. Searles Wood first described the marine testacea of the Crags, the additions made to that fossil fauna have not been considerable, whereas we have made in the same period immense progress in our knowledge of the living testacea of the British and arctic seas, and of the Mediterranean. By this means the naturalist has been enabled to identify with existing species many forms previously supposed to be extinct.

In the forthcoming supplement to the invaluable monograph communicated by Mr.

Wood to the Palaeontographical Society, in which he has completed his figures and descriptions of the British crag sh.e.l.ls of every age, list will be found of all the fossil sh.e.l.ls, of which a summary is given in the table below.

TABLE OF NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN THE CRAG.

COLUMN 1: KNOWN SPECIES.

COLUMN 2: TOTAL NUMBER OF KNOWN SPECIES.

COLUMN 3: NUMBER OF SPECIES NOT KNOWN AS LIVING.

CHILLESFORD AND ALDEBY BEDS: Bivalves: 61 : 4.

Univalves: 33 : 5.

Brachiopods: 0 : 0.

PERCENTAGE OF Sh.e.l.lS NOT KNOWN AS LIVING : 9.5.

NORWICH OR FLUVIO-MARINE CRAG: Bivalves: 61 : 10.

Univalves: 64 : 12.

Brachiopods: 1 : 0.

PERCENTAGE OF Sh.e.l.lS NOT KNOWN AS LIVING : 17.5.

RED CRAG (Exclusive of many derivative sh.e.l.ls): Bivalves: 128 : 31.

Univalves: 127 : 33.

Brachiopods: 1 : 1.

PERCENTAGE OF Sh.e.l.lS NOT KNOWN AS LIVING : 25.0.

CORALLINE CRAG: Bivalves: 161 : 47.

Univalves: 184 : 60 Brachiopods: 5 : 3 PERCENTAGE OF Sh.e.l.lS NOT KNOWN AS LIVING : 31.5

To begin with the uppermost or Chillesford beds, it will be seen that about 9 per cent only are extinct, or not known as living, whereas in the Norwich, which succeeds in the descending order, seventeen in a hundred are extinct. Formerly, when the Norwich or Fluvio-marine Crag was spoken of, both these formations were included under the same head, for both at Bramerton and Thorpe, the chief localities where the Norwich Crag was studied, an overlying deposit occurs referable to the Chillesford age. If now the two were fused together as of old, their sh.e.l.ls would, according to Mr. Wood, yield a percentage of fifteen in a hundred of species extinct or not known as living.

To come next to the Red Crag, the reader will observe that a percentage of 25 is given of sh.e.l.ls unknown as living, and this increases to 31 in the antecedent Coralline Crag. But the gap between these two stages of our Pliocene deposits is really wider than these numbers would indicate, for several reasons. In the first place, the Coralline Crag is more strictly the product of a single period, the Red Crag, as we have seen, consisting of separate and independent patches, slightly varying in age, of which the newest is probably not much anterior to the Norwich Crag. Secondly, there was a great change of conditions, both as to the depth of the sea and climate, between the periods of the Coralline and Red Crag, causing the fauna in each to differ far more widely than would appear from the above numerical results.

The value of the a.n.a.lysis given in the above table of the sh.e.l.ls of the Red and Coralline Crags is in no small degree enhanced by the fact that they were all either collected by Mr. Wood himself, or obtained by him direct from their discoverers, so that he was enabled in each case to test their authenticity, and as far as possible to avoid those errors which arise from confounding together sh.e.l.ls belonging to the sea of a newer deposit, and those washed into it from a formation of older date. The danger of this confusion may be conceived when we remember that the number of species rejected from the Red Crag as derivative by Mr. Wood is no less than 25. Some geologists have held that on the same grounds it is necessary to exclude as spurious some of the species found in the Norwich Crag proper; but Mr. Wood does not entertain this view, believing that the spurious sh.e.l.ls which have sometimes found their way into the lists of this crag have been introduced by want of care from strata of Red Crag.

There can be no doubt, on the other hand, that conchologists have occasionally rejected from the Red and Norwich Crags, as derivative, sh.e.l.ls which really belonged to the seas of those periods, because they were extinct or unknown as living, which in their eyes afforded sufficient ground for suspecting them to be intruders. The derivative origin of a species may sometimes be indicated by the extreme scarcity of the individuals, their colour, and worn condition; whereas an opposite conclusion may be arrived at by the integrity of the sh.e.l.ls, especially when they are of delicate and tender structure, or their abundance, and, in the case of the lamellibranchiata, by their being held together by the ligament, which often happens when the sh.e.l.ls have been so broken that little more than the hinges of the two valves are preserved. As to the univalves, I have seen from a pit of Red Crag, near Woodbridge, a large individual of the extinct Voluta Lamberti, seven inches in length, of which the lip, then perfect, had in former stages of its growth been frequently broken, and as often repaired. It had evidently lived in the sea of the Red Crag, where it had been exposed to rough usage, and sustained injuries like those which the reversed whelk, Trophon antiquum, so characteristic of the same formation, often exhibits. Additional proofs, however, have lately been obtained by Mr. Searles Wood that this sh.e.l.l had not died out in the era of the Red Crag by the discovery of the same fossil near Southwold, in beds of the later Norwich Crag.

ANTWERP CRAG.

Strata of the same age as the Red and Coralline Crag of Suffolk have been long known in the country round Antwerp, and on the banks of the Scheldt, below that city; and the lowest division, or Black Crag, there found, is shown by the sh.e.l.ls to be somewhat more ancient than any of our British series, and probably forms the first links of a downward pa.s.sage from the strata of the Pliocene to those of the Upper Miocene period.

NEWER PLIOCENE STRATA OF SICILY.

(FIGURE 132. Murex v.a.g.i.n.atus, Phil.)

At several points north of Catania, on the eastern sea-coast of Sicily-- as at Aci-Castello, for example, Trezza, and Nizzeti-- marine strata, a.s.sociated with volcanic tuffs and basaltic lavas, are seen, which belong to a period when the first igneous eruptions of Mount Etna were taking place in a shallow bay of the Mediterranean. They contain numerous fossil sh.e.l.ls, and out of 142 species that have been collected all but eleven are identical with species now living. Some few of these eleven sh.e.l.ls may possibly still linger in the depths of the Mediterranean, like Murex v.a.g.i.n.atus, see Figure 132. The last-mentioned sh.e.l.l had already become rare when the a.s.sociated marine and volcanic strata above alluded to were formed. On the whole, the modern character of the testaceous fauna under consideration is expressed not only by the small proportion of extinct species, but by the relative number of individuals by which most of the other species are represented, for the proportion agrees with that observed in the present fauna of the Mediterranean. The rarity of individuals in the extinct species is such as to imply that they were already on the point of dying out, having flourished chiefly in the earlier Pliocene times, when the Subapennine strata were in progress.

Yet since the acc.u.mulation of these Newer Pliocene sands and clays, the whole cone of Etna, 11,000 feet in height and about 90 miles in circ.u.mference at its base, has been slowly built up; an operation requiring many tens of thousands of years for its accomplishment, and to estimate the magnitude of which it is necessary to study in detail the internal structure of the mountain, and to see the proofs of its double axis, or the evidence of the lavas of the present great centre of eruption having gradually overwhelmed and enveloped a more ancient cone, situated 3 1/2 miles to the east of the present one. (See a Memoir on the Lavas and Mode of Origin of Mount Etna by the Author in Philosophical Transactions 1858.)

It appears that while Etna was increasing in bulk by a series of eruptions, its whole ma.s.s, comprising the foundations of subaqueous origin above alluded to, was undergoing a slow upheaval, by which those marine strata were raised to the height of 1200 feet above the sea, as seen at Catera, and perhaps to greater heights, for we can not trace their extension westward, owing to the dense and continuous covering of modern lava under which they are buried. During the gradual rise of these Newer Pliocene formations (consisting of clays, sands, and basalts) other strata of Post-pliocene date, marine as well as fluviatile, acc.u.mulated round the base of the mountain, and these, in their turn, partook of the upward movement, so that several inland cliffs and terraces at low levels, due partly to the action of the sea and partly to the river Simeto, originated in succession. Fossil remains of the elephant, and other extinct quadrupeds, have been found in these Post-Pliocene strata, a.s.sociated with recent sh.e.l.ls.