The Ancient Life History of the Earth Part 19

(10) 'Das Flotzgebirge Wurttembergs.' Quenstedt.

(11) 'Jura Formation.' Oppel.

(12) 'Paleontologie du Departement de la Moselle.' Terquem.

(13) 'Cours elementaire de Paleontologie.' D'Orbigny.

(14) 'Paleontologie Francaise.' D'Orbigny.

(15) 'Fossil Echinodermata of the Oolitic Formation'

(Palaeontographical Society). Wright.

(16) 'Brachiopoda of the Oolitic Formation' (Palaeontographical Society). Davidson.

(17) 'Mollusca of the Great Oolite' (Palaeontographical Society).

Morris and Lycett.

(18) 'Monograph of the Fossil Trigoniae' (Palaeontographical Society).

Lycett.

(19) 'Corals of the Oolitic Formation' (Palaeontographical Society).

Edwards and Haime.

(20) 'Supplement to the Corals of the Oolitic Formation'

(Palaeontographical Society). Martin Duncan.

(21) 'Monograph of the Belemnitidae' (Palaeontographical Society).

Phillips.

(22) 'Structure of the Belemnitidae' (Mem. Geol. Survey). Huxley.

(23) 'Sur les Belemnites.' Blainville.

(24) 'Cephalopoden.' Quenstedt.

(25) 'Mineral Conchology.' Sowerby.

(26) 'Jura.s.sic Cephalopoda' (Palaeontologica Indica). Waagen.

(27) 'Manual of the Mollusca.' Woodward.

(28) 'Petrefaktenkunde.' Schlotheim.

(29) 'Bridgewater Treatise.' Buckland.

(30) 'Versteinerungen des Oolithengebirges.' Roemer.

(31) 'Catalogue of British Fossils.' Morris.

(32) 'Catalogue of Fossils in the Museum of Practical Geology.'

Etheridge.

(33) 'Beitrage zur Petrefaktenkunde.' Munster.

(34) 'Petrefacta Germaniae.' Goldfuss.

(35) 'Lethaea Rossica.' Eichwald.

(36) 'Fossil Fishes' (Decades of the Geol. Survey). Sir Philip Egerton.

(37) 'Manual of Palaeontology.' Owen.

(38) 'British Fossil Mammals and Birds.' Owen.

(39) 'Monographs of the Fossil Reptiles of the Oolitic Formation'

(Palaeontographical Society). Owen.

(40) 'Fossil Mammals of the Mesozoic Formations' (Palaeontographical Society). Owen.

(41) 'Catalogue of Ornithosauria.' Seeley.

(42) "Cla.s.sification of the Deinosauria"--'Quart. Journ. Geol. Soc.,'

vol. xxvi., 1870. Huxley.

CHAPTER XVII.

THE CRETACEOUS PERIOD.

The next series of rocks in ascending order is the great and important series of the Cretaceous Rocks, so called from the general occurrence in the system of chalk (Lat. _creta_, chalk).

As developed in Britain and Europe generally, the following leading subdivisions may be recognised in the Cretaceous series:--

1. Wealden, _ Lower Cretaceous.

2. Lower Greensand or Neocomian, / 3. Gault, 4. Upper Greensand, _ Upper Cretaceous.

5. Chalk, 6. Maestricht beds, /

I. _Wealden_.--The _Wealden_ formation, though of considerable importance, is a local group, and is confined to the southeast of England, France, and some other parts of Europe. Its name is derived from the _Weald_, a district comprising parts of Surrey, Suss.e.x, and Kent, where it is largely developed. Its lower portion, for a thickness of from 500 to 1000 feet, is arenaceous, and is known as the Hastings Sands. Its Upper portion, for a thickness of 150 to nearly 300 feet, is chiefly argillaceous, consisting of clays with sandy layers, and occasionally courses of limestone.

The geological importance of the Wealden formation is very great, as it is undoubtedly the delta of an ancient river, being composed almost wholly of fresh-water beds, with a few brackish-water and even marine strata, intercalated in the lower portion. Its geographical extent, though uncertain, owing to the enormous denudation to which it has been subjected, is nevertheless great, since it extends from Dorsetshire to France, and occurs also in North Germany. Still, even if it were continuous between all these points, it would not be larger than the delta of such a modern river as the Ganges. The river which produced the Wealden series must have flowed from an ancient continent occupying what is now the Atlantic Ocean; and the time occupied in the formation of the Wealden must have been very great, though we have, of course, no data by which we can accurately calculate its duration.

The fossils of the Wealden series are, naturally, mostly the remains of such animals as we know at the present day as inhabiting rivers. We have, namely, fresh-water Mussels (_Unio_), River-snails (_Paludina_), and other fresh-water sh.e.l.ls, with numerous little bivalved Crustaceans, and some fishes.

II. _Lower Greensand_ (_Neocomien_ of D'Orbigny).--The Wealden beds pa.s.s upward, often by insensible gradations, into the Lower Greensand. The name Lower Greensand is not an appropriate one, for green sands only occur sparingly and occasionally, and are found in other formations. For this reason it has been proposed to subst.i.tute for Lower Greensand the name _Neocomian_, derived from the town of Neufchatel--anciently called _Neocomum_--in Switzerland. If this name were adopted, as it ought to be, the Wealden beds would be called the Lower Neocomian.

The Lower Greensand or Neocomian of Britain has a thickness of about 850 feet, and consists of alternations of sands, sandstones, and clays, with occasional calcareous bands. The general colour of the series is dark brown, sometimes red; and the sands are occasionally green, from the presence of silicate of iron.

The fossils of the Lower Greensand are purely marine, and among the most characteristic are the sh.e.l.ls of _Cephalopods_.

The most remarkable point, however, about the fossils of the Lower Cretaceous series, is their marked divergence from the fossils of the Upper Cretaceous rocks. Of 280 species of fossils in the Lower Cretaceous series, only 51, or about 18 per cent, pa.s.s on into the Upper Cretaceous. This break in the life of the two periods is accompanied by a decided physical break as well; for the Gault is often, if not always, unconformably superimposed on the Lower Greensand. At the same time, the Lower and Upper Cretaceous groups form a closely-connected and inseparable series, as shown by a comparison of their fossils with those of the underlying Jura.s.sic rocks and the overlying Tertiary beds. Thus, in Britain no marine fossil is known to be common to the marine beds of the Upper Oolites and the Lower Greensand; and of more than 500 species of fossils in the Upper Cretaceous rocks, almost everyone died out before the formation of the lowest Tertiary strata, the only survivors being one Brachiopod and a few _Foraminifera_.

III. _Gault_ (_Aptien_ of D'Orbigny).--The lowest member of the Upper Cretaceous series is a stiff, dark-grey, blue, or brown clay, often worked for brick-making, and known as the _Gault_, from a provincial English term. It occurs chiefly in the south-east of England, but can be traced through France to the flanks of the Alps and Bavaria. It never exceeds 100 feet in thickness; but it contains many fossils, usually in a state of beautiful preservation.

IV. _Upper Greensand_ (_Albien_ of D'Orbigny; _Unterquader_ and _Lower Planerkalk_ of Germany).--The Gault is succeeded upward by the _Upper Greensand_, which varies in thickness from 3 up to 100 feet, and which derives its name from the occasional occurrence in it of green sands. These, however, are local and sometimes wanting, and the name "Upper Greensand" is to be regarded as a _name_ and not a description. The group consists, in Britain, of sands and clays, sometimes with bands of calcareous grit or siliceous limestone, and occasionally containing concretions of phosphate of lime, which are largely worked for agricultural purposes.

V. _White Chalk_.--The top of the Upper Greensand becomes argillaceous, and pa.s.ses up gradually into the base of the great formation known as the true _Chalk_, divided into the three subdivisions of the chalk-marl, white chalk without flints, and white chalk with flints. The first of these is simply argillaceous chalk, and pa.s.ses up into a great ma.s.s of obscurely-stratified white chalk in which there are no flints (_Turonien_ of D'Orbigny; _Mittelquader_ of Germany). This, in turn, pa.s.ses up into a great ma.s.s of white chalk, in which the stratification is marked by nodules of black flint arranged in layers (_Senonien_ of D'Orbigny; _Oberquader_ of Germany). The thickness of these three subdivisions taken together is sometimes over 1000 feet, and their geographical extent is very great. White Chalk, with its characteristic appearance, may be traced from the north of Ireland to the Crimea, a distance of about 1140 geographical miles; and, in an opposite direction, from the south of Sweden to Bordeaux, a distance of about 840 geographical miles.

VI. In Britain there occur no beds containing Chalk fossils, or in any way referable to the Cretaceous period, above the true White Chalk with flints. On the banks of the Maes, however, near Maestricht in Holland, there occurs a series of yellowish limestones, of about 100 feet in thickness, and undoubtedly superior to the White Chalk. These _Maestricht beds_ (_Danien_ of D'Orbigny) contain a remarkable series of fossils, the characters of which are partly Cretaceous and partly Tertiary. Thus, with the characteristic Chalk fossils, _Belemnites, Baculites_, Sea-Urchins, &c., are numerous Univalve Molluscs, such as Cowries and Volutes, which are otherwise exclusively Tertiary or Recent.

Holding a similar position to the Maestricht beds, and showing a similar intermixture of Cretaceous forms with later types, are certain beds which occur in the island of Seeland, in Denmark, and which are known as the _Faxoe Limestone_.

Of a somewhat later date than the Maestricht beds is the _Pisolitic Limestone_ of France, which rests unconformably on the White Chalk, and contains a large number of Tertiary fossils along with some characteristic Cretaceous types.

The subjoined sketch-section exhibits the general succession of the Cretaceous deposits in Britain:--

[Ill.u.s.tration: Fig. 185. GENERALIZED SECTION OF THE CRETACEOUS SERIES OF BRITAIN.]

In North America, strata of Lower Cretaceous age are well represented in Missouri, Wyoming, Utah, and in some other areas; but the greater portion of the American deposits of this period are referable to the Upper Cretaceous. The rocks of this series are mostly sands, clays, and limestones--_Chalk_ itself being unknown except in Western Arkansas. Amongst the sandy acc.u.mulations, one of the most important is the so-called "marl" of New Jersey, which is truly a "Greensand," and contains a large proportion of glauconite (silicate of iron and potash). It also contains a little phosphate of lime, and is largely worked for agricultural purposes. The greatest thickness attained by the Cretaceous rocks of North America is about 9000 feet, as in Wyoming, Utah, and Colorado.

According to Dana, the Cretaceous rocks of the Rocky Mountain territories pa.s.s upwards "without interruption into a coal-bearing formation, several thousand feet thick, on which the following Tertiary strata lie unconformably." The lower portion of this "Lignitic formation" appears to be Cretaceous, and contains one or more beds of Coal; but the upper part of it perhaps belongs to the Lower Tertiary. In America, therefore, the lowest Tertiary strata appear to rest conformably upon the highest Cretaceous; whereas in Europe, the succession at this point is invariably an unconformable one. Owing, however, to the fact that the American "Lignitic formation" is a shallow-water formation, it can hardly be expected to yield much material whereby to bridge over the great palaeontological gap between the White Chalk and Eocene in the Old World.

Owing to the fact that so large a portion of the Cretaceous formation has been deposited in the sea, much of it in deep water, the _plants_ of the period have for the most part been found special members of the series, such as the Wealden beds, the Aix-la-Chapelle sands, and the Lignitic beds of North America. Even the purely marine strata, however, have yielded plant-remains, and some of these are peculiar and proper to the deep-sea deposits of the series. Thus the little calcareous discs termed "coccoliths," which are known to be of the nature of calcareous sea-weeds (_Algoe_) have been detected in the White Chalk; and the flints of the same formation commonly contain the spore-cases of the microscopic _Desmids_ (the so-called Xanthidia), along with the siliceous cases of the equally diminutive _Diatoms_.

The plant-remains of the Lower Cretaceous greatly resemble those of the Jura.s.sic period, consisting mainly of Ferns, Cycads, and Conifers. The Upper Cretaceous rocks, however, both in Europe and in North America, have yielded an abundant flora which resembles the existing vegetation of the globe in consisting mainly of Angiospermous Exogens and of Monocotyledons.[23] In Europe the plant-remains in question have been found chiefly in certain sands in the neighbourhood of Aix-la-Chapelle, and they consist of numerous Ferns, Conifers (such as _Cycadopteris_), Screw Pines (_Panda.n.u.s_), Oaks (_Quercus_), Walnut (_Juglans_), Fig (_Ficus_), and many _Proteaceoe_, some of which are referred to existing genera (_Dryandra, Banksia, Grevillea_, &c.)

[Footnote 23: The "Flowering plants" are divided into the two great groups of the Endogens and Exogens. The _Endogens_ (such as Gra.s.ses, Palms, Lilies, &c.) have no true bark, nor rings of growth, and the stem is said to be "endogenous;" the young plant also possesses but a single seed-leaf or "cotyledon." Hence these plants are often simply called "_Monocotyledons_." The _Exogens_, on the other hand, have a true bark; and the stem increases by annual additions to the outside, so that rings of growth are produced. The young plant has two seed-leaves or "cotyledons,"

and these plants are therefore called "_Dicotyledons_." Amongst the Exogens, the Pines (_Conifers_) and the Cycads have seeds which are unprotected by a seed-vessel, and they are therefore called "_Gymnosperms_." All the other Exogens, including the ordinary trees, shrubs, and flowering plants, have the seeds enclosed in a seed-vessel, and are therefore called "_Angiosperms_." The derivation of these terms will be found in the Glossary at the end of the volume.]

In North America, the Cretaceous strata of New Jersey, Alabama, Nebraska, Kansas, &c., have yielded the remains of numerous plants, many of which belong to existing genera. Amongst these may be mentioned Tulip-trees (_Liriodendron_), Sa.s.safras (fig. 186), Oaks (_Quercus_), Beeches (_f.a.gus_), Plane-trees (_Plata.n.u.s_), Alders (_Alnus_), Dog-wood (_Cornus_), Willows (_Salix_), Poplars (_Populus_), Cypresses (_Cupressus_), Bald Cypresses (_Taxodium_), Magnolias, &c. Besides these, however, there occur other forms which have now entirely disappeared from North America--as, for example, species of _Cinnamomum_ and _Araucaria_.

It follows from the above, that the Lower and Upper Cretaceous rocks are, from a botanical point of view, sharply separated from one another. The Palaeozoic period, as we have seen, is characterised by the prevalance of "Flowerless" plants (_Cryptogams_), its higher vegetation consisting almost exclusively of Conifers. The Mesozoic period, as a whole, is characterised by the prevalence of the Cryptogamic group of the Ferns, and the Gymnospermic groups of the Conifers and the Cycads. Up to the close of the Lower Cretaceous, no Angiospermous Exogens are certainly known to have existed, and Monocotyledonous plants or Endogens are very poorly represented. With the Upper Cretaceous, however, a new era of plant-life, of which our present is but the culmination, commenced, with a great and apparently sudden development of new forms. In place of the Ferns, Cycads, and Conifers of the earlier Mesozoic deposits, we have now an astonishingly large number of true Angiospermous Exogens, many of them belonging to existing types; and along with these are various Monocotyledonous plants, including the first examples of the great and important group of the Palms. It is thus a matter of interest to reflect that plants closely related to those now inhabiting the earth, were in existence at a time when the ocean was tenanted by Ammonites and Belemnites, and when land and sea and air were peopled by the extraordinary extinct Reptiles of the Mesozoic period.