[Ill.u.s.tration: Fig. 212.--Toothed Birds (_Odontornithes_) of the Cretaceous Rocks of America. a. Left lower jaw of _Ichthyornis dispar_, slightly enlarged; b, Left lower jaw of _Hesperornis regalis_, reduced to nearly one-fourth of the natural size; c.

Cervical vertebra of _Ichthyornis dispar_, front view, twice the natural size; c', Side view of the same; d, Tooth of _Hesperornis regalis_, enlarged to twice the natural size. (After Marsh.)]

No remains of _Mammals_, finally, have as yet been detected in any sedimentary acc.u.mulations of Cretaceous age.

LITERATURE.

The following list comprises some of the more important works and memoirs which may be consulted with reference to the Cretaceous strata and their fossil contents:--

(1) 'Memoirs of the Geological Survey of Great Britain.'

(2) 'Geology of England and Wales.' Conybeare and Phillips.

(3) 'Geology of Yorkshire,' vol. ii. Phillips.

(4) 'Geology of Oxford and the Thames Valley.' Phillips.

(5) 'Geological Excursions through the Isle of Wight.' Mantell.

(6) 'Geology of Suss.e.x.' Mantell.

(7) 'Report on Londonderry,' &c. Portlock.

(8) 'Recherches sur le Terrain Cretace Superieur de l'Angleterre et de l'Irlande.' Barrois.

(9) "Geological Survey of Canada"--'Report of Progress, 1872-73.'

(10) 'Geological Survey of California.' Whitney.

(11) 'Geological Survey of Montana, Idaho, Wyoming, and Utah.'

Hayden and Meek.

(12) 'Report on Geology,' &c. (British North American Boundary Commission). G. M. Dawson.

(13) 'Manual of Geology.' Dana.

(14) 'Lethaea Rossica.' Eichwald.

(15) 'Petrefacta Germaniae.' Goldfuss.

(16) 'Fossils of the South Downs.' Mantell.

(17) 'Medals of Creation.' Mantell.

(18) 'Mineral Conchology.' Sowerby.

(19) 'Lethaea Geognostica.' Bronn.

(20) 'Malacostracous Crustacea of the British Cretaceous Formation'

(Palaeontographical Society). Bell.

(21) 'Brachiopoda of the Cretaceous Formation' (Palaeontographical Society). Davidson.

(22) 'Corals of the Cretaceous Formation' (Palaeontographical Society). Milne-Edwards and Haime.

(23) 'Supplement to the Fossil Corals' (Palaeontographical Society).

Martin Duncan.

(24) 'Echinodermata or the Cretaceous Formation' (Palaeontographical Society). Wright.

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

Phillips.

(26) 'Monograph of the Trigoniae' (Palaeontographical Society).

Lycett.

(27) 'Fossil Cirripedes' (Palaeontographical Society). Darwin.

(28) 'Fossil Mollusca of the Chalk of Britain' (Palaeontographical Society). Sharpe.

(29) 'Entomostraca of the Cretaceous Formation' (Palaeontographical Society). Rupert Jones.

(30) 'Monograph of the Fossil Reptiles of the Cretaceous Formation'

(Palaeontographical Society). Owen.

(31) 'Manual of Palaeontology.' Owen.

(32) 'Synopsis of Extinct Batrachia and Reptilia.' Cope.

(33) "Structure of the Skull and Limbs in Mosasauroid Reptiles"--'American Journ. Sci. and Arts, 1872.' Marsh.

(34) "On Odontornithes"--'American Journ. Sci. and Arts, 1875.'

Marsh.

(35) 'Oss.e.m.e.ns Fossiles.' Cuvier.

(36) 'Catalogue of Ornithosauria.' Seeley.

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

(38) 'Synopsis des Echinides fossiles.' Desor.

(39) 'Cat. Raisonne des Echinides.' Aga.s.siz and Desor.

(40) "Echinoids"--'Decades of the Geol. Survey of Britain.'

E. Forbes.

(41) 'Paleontologie Francaise.' Cotteau.

(42) 'Versteinerungen der Bohmischen Kreide-formation.' Reuss.

(43) "Cephalopoda, Gasteropoda, Pelecypoda, Brachiopoda; &c., of the Cretaceous Rocks of India"--'Palaeontologica Indica,' ser. i., iii., v., vi., viii. Stoliczka.

(44) "Cretaceous Reptiles of the United States"--'Smithsonian Contributions to Knowledge,' vol. xiv. Leidy.

(45) 'Invertebrate Cretaceous, and Tertiary Fossils of the Upper Missouri Country,' 1876. Meek.

CHAPTER XVIII.

THE EOCENE PERIOD.

Before commencing the study of the subdivisions of the Kainozoic series, there are some general considerations to be noted. In the first place, there is in the Old World a complete and entire physical break between the rocks of the Mesozoic and Kainozoic periods. In no instance in Europe are Tertiary strata to be found resting conformably upon any Secondary rock. The Chalk has invariably suffered much erosion and denudation before the lowest Tertiary strata were deposited upon it. This is shown by the fact that the actually eroded surface of the Chalk can often be seen; or, failing this, that we can point to the presence of the chalk-flints in the Tertiary strata. This last, of course, affords unquestionable proof that the Chalk must have been subjected to enormous denudation prior to the formation of the Tertiary beds, all the chalk itself having been removed, and nothing left but the flints, while these are all rolled and rounded. In the continent of North America, on the other hand, the lowest Tertiary strata have been shown to graduate downwards conformably with the highest Cretaceous beds, it being a matter of difficulty to draw a precise line of demarcation between the two formations.

In the second place, there is a marked break in the _life_ of the Mesozoic and Kainozoic periods. With the exception of a few _Foraminifera_, and one _Brachiopod_ (the latter doubtful), no Cretaceous species is known to have survived the Cretaceous period; while several characteristic _families_, such as the _Ammonitidoe, Belemnitidoe_, and _Hippuritidoe_, died out entirely with the close of the Cretaceous rocks. In the Tertiary rocks, on the other hand, not only are all the animals and plants more or less like existing types, but we meet with a constantly-increasing number of _living species_ as we pa.s.s from the bottom of the Kainozoic series to the top. Upon this last fact is founded the modern cla.s.sification of the Kainozoic rocks, propounded by Sil Charles Lyell.

The absence in strata of Tertiary age of the chambered Cephalopods, the Belemnites, the _Hippurites_, the _Inocerami_, and the diversified types of Reptiles which form such conspicuous features in the Cretaceous fauna, render the palaeontological break between the Chalk and the Eocene one far too serious to be overlooked. At the same time, it is to be remembered that the evidence afforded by the explorations carried out of late years as to the animal life of the deep sea, renders it certain that the extinction of marine forms of life at the close of the Cretaceous period was far less extensive than had been previously a.s.sumed. It is tolerably certain, in fact, that we may look upon some of the inhabitants of the depths of our existing oceans as the direct, if modified, descendants of animals which were in existence when the Chalk was deposited.

It follows from the general want of conformity between the Cretaceous and Tertiary rocks, and still more from the great difference in life, that the Cretaceous and Tertiary periods are separated, in the Old World at any rate, by an enormous lapse of unrepresented time. How long this interval may have been, we have no means of judging exactly, but it very possibly was as long as the whole Kainozoic epoch itself. Some day we shall doubtless find, at some part of the earth's surface, marine strata which were deposited during this period, and which will contain fossils intermediate in character between the organic remains which respectively characterise the Secondary and Tertiary periods. At present, we have only slight traces of such deposits--as, for instance, the Maestricht beds, the Faxoe Limestone, and the Pisolitic Limestone of France.

CLa.s.sIFICATION OF THE TERTIARY ROCKS.--The cla.s.sification of the Tertiary rocks is a matter of unusual difficulty, in consequence of their occurring in disconnected basins, forming a series of detached areas, which hold no relations of superposition to one another. The order, therefore, of the Tertiaries in point of time, can only be determined by an appeal to fossils; and in such determination Sir Charles Lyell proposed to take as the basis of cla.s.sification the _proportion of living or existing species of Mollusca which occurs in each stratum or group of strata_. Acting upon this principle, Sir Charles Lyell divides the Tertiary series into four groups:--

I. The _Eocene_ formation (Gr. _eos_, dawn; _kainos_, new), containing the smallest proportion of existing species, and being, therefore, the oldest division. In this cla.s.sification, only the _Mollusca_ are taken into account; and it was found that of these about three and a half per cent were identical with existing species.

II. The _Miocene_ formation (Gr. _meion_, less; _kainos_, new), with more recent species than the Eocene, but _less_ than the succeeding formation, and less than one-half the total number in the formation. As before, only the _Mollusca_ are taken into account, and about 17 per cent of these agree with existing species.

III. The _Pliocene_ formation (Gr. _pleion_, more; _kainos_, new), with generally _more_ than half the species of sh.e.l.ls identical with existing species--the proportion of these varying from 35 to 50 per cent in the lower beds of this division, up to 90 or 95 per cent in its higher portion.

IV. The _Post-Tertiary Formations_, in which all _the sh.e.l.ls belong to existing species_. This, in turn, is divided into two minor groups--the _Post-Pliocene_ and _Recent Formations_. In the _Post-Pliocene_ formations, while all the _Mollusca_ belong to existing species, most of the _Mammals_ belong to extinct species. In the Recent period, the quadrupeds, as well as the sh.e.l.ls, belong to living species.

The above, with some modifications, was the original cla.s.sification proposed by Sir Charles Lyell for the Tertiary rocks, and now universally accepted. More recent researches, it is true, have somewhat altered the proportions of existing species to extinct, as stated above. The general principle, however, of an increase in the number of living species, still holds good; and this is as yet the only satisfactory basis upon which it has been proposed to arrange the Tertiary deposits.

EOCENE FORMATION.

The Eocene rocks are the lowest of the Tertiary series, and comprise all those Tertiary deposits in which there is only a small proportion of existing _Mollusca_--from three and a half to five per cent.

The Eocene rocks occur in several basins in Britain, France, the Netherlands, and other parts of Europe, and in the United States. The subdivisions which have been established are extremely numerous, and it is often impossible to parallel those of one basin with those of another. It will be sufficient, therefore, to accept the division of the Eocene formation into three great groups--Lower, Middle, and Upper Eocene--and to consider some of the more important beds comprised under these heads in Europe and in North America.

I. EOCENE OF BRITAIN. (1.) LOWER EOCENE.--The base of the Eocene series in Britain is const.i.tuted by about 90 feet of light-coloured, sometimes argillaceous sands (_Thanet Sands_), which are of marine origin. Above these, or forming the base of the formation where these are wanting, come mottled clays and sands with lignite (_Woolwich and Reading series_), which are estuarine or fluvio-marine in origin. The highest member of the Lower Eocene of Britain is the "London Clay," consisting of a great ma.s.s of dark-brown or blue clay, sometimes with sandy beds, or with layers of "septaria,"

the whole attaining a thickness of from 200 to as much as 500 feet. The London Clay is a purely marine deposit, containing many marine fossils, with the remains of terrestrial animals and plants; all of which indicate a high temperature of the sea and tropical or sub-tropical conditions of the land.