Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/374

Rh 360 depths 40 feet below without altering the position of its body. It must have wandered far from land, and that many kinds of ﬁshes formed its food is shown by the teeth and scales found in the position of its stomach” (Cope). But the real rulers of the American Cretaceous waters were the pythononiorphic sanrians or sea-serpents. Some of them attained a length of 75 feet or more. They possessed a remarkable elongation of form, particularly in the tail; their heads were large, ﬂat, and conic, with eyes directed partly upwards. They swam by means of two pairs of paddles, like the ﬂippers of the whale, and the eel-like strokes of their ﬂattened tail. Like snakes they had four rows of formidable teeth on the roof of the month, which served as weapons for seizing their prey. But the most remarkable feature in these creatures was the unique arrangement for permitting them to swallow their prey entire, in the manner of snakes. Each half of the lower jaw was articulated at a point nearly midway between the ear and the chin, so as greatly to widen the space between the jaws, and the throat must, conse- quently, have been loose and baggy like a pelican’s. Nine species of birds have been obtained from the American Cretaceous rocks. Three of these belonged to the order of Natatores or swimmers, which includes our modern gulls, ducks, and geese; four were G'raIIa2 or waders ; while two belonged to a long extinct order, and united eertain ichthyic and reptilian characters with those of birds. (Sec Cope, Report of U.S. Gcol. Sure. of T c-rrilories, vol. ii., 1875 ; Marsh, American Journ. Science, 3d ser., i. to iv.; Leidy, Smith- sonian Contributions, 1865, N o. 192 ; Lesqnereux, Cretaceous Flora, Report of U.S. Geol. Surv. of Territories, vol. vi.) IV. TERTIARY OR CAINOZOIC. The close of the Secondary periods was marked in the west of Europe by great geographical changes, during which the ﬂoor of the Cretaceous sea was raised partly into land and partly into shallow marine and estuarine waters. These events must have occupied a vast period of time, so that, when sedimentation once more began in the region, the organic remains of the Secondary ages had (save in a few low forms of life) entirely disappeared and given place to others of a distinctly more modern type. In England, the interval between the Cretaceous and the next geological period represented there by sedimentary formations is marked by the abrupt line which separates the top of the Chalk from all later accumulations, and by the evidence that the Chalk seems to have been in some places extensively denuded before even the oldest of what are called the Tertiary beds were deposited upon its surface. There is evidently here a considerable gap in the geological record. We have no data for ascertaining what was the general march of events in the south of England between the eras chronicled respect- ively by the Upper Chalk and the overlying Thanet beds. Here and there on the Continent a few scraps of evidence are obtainable which help to ﬁll up this gap. Thus, on the banks of the Meuse at Maestricht, a series of shelly and polyzoan limestones with a conglomeratic base (S_1/sléme Jllaestric/ztien of Dumont, who places it above his Senonian system in the Upper Cretaceous series) contains a mingling of true Cretaceous organisms with others which are char- acteristic of the older Tertiary formations. It contains, for example, the characteristic Upper Chalk crinoid, ]}ozn~ge(i- crinus ellipticus, in great numbers; also Oslrea vesicularis, Baculites Faujrzsii, Belemnitella mucronata, and the great reptile Jlosasanrus ; but with these occur such Tertiary genera as Voluta, Fasciolaria., and others. At Faxoe, on the Danish island of Seeland, the uppermost member of the Senonian series contains in like manner a blending of well- known Upper Chalk organisms with the Tertiary genera C3/prrea, Oliva, and Jllitra. In the neighbourhood of Paris also, and in scattered patches over the north of France, a formation known as the pisolitic limestone occurs, which was formerly classed with the Tertiary formations, seeing that its fossils had more affinities with later than with older rocks. But the discovery in it of numerous distinctively Upper Cretaceous forms has led to its being placed at the top of the Senonian series, from which, however, it is marked off by a decided unconformability, for it rests on a GEOLOGY [vI. STRATIGRAPHICAL. denuded surface of__ the White Chalk. These fragmentary formations are interesting, in so far as they help to show that, though in western Europe there is a tolerably abrupt separation between Cretaceous and Tertiary deposits, there was nevertheless no real break between the two periods. The one merged insensibly into the other; but the chroni- cles of the intervening ages have been i11 great measure destroyed. In entering upon the Tertiary series of formations, we ﬁnd ourselves upon the threshold of the modern type of life- The ages of lycopods, ferns, cycads, and yew-like conifers have passed away, and that of the dicotyledonous angio- sperms—the hard-wood trees and evergreens of to-day— -now succeeds them, but not by any sudden extinction and re- creation, for, as we have seen (ante, p. 359), some of these trees had already begun to make their appearance even in Cretaceous times. The ammonites, baculites, and other ccphalopods, which had played so large a part in the mol- luscan life of the Secondary periods, now cease. The great reptiles, too, which in such wonderful variety of type were the dominant animals of the eartl1’s surface, alike on land and sea, ever since the commencement of the Lias, now wane before the increase of the mammalia, which advance in ever-augmenting diversity of type until man appears at their head. The name Tertiary, given in the early days of geology before much was known regarding fossils and their history, has retained its hold on the literature of the science. It is sometimes replaced by the term Cainozoic (recent life), which expresses the great fact that it is in the series of strata comprised under this designation that most recent species and genera have their earliest representatives. Taking as the basis of classiﬁcation the percentage of living species of mollusca found in the different groups of the Tertiary series, Lyell proposed a scheme of arrangement which has been generally adopted. The older Tertiary formations, in which the number of still living species of shells is very small, where, in fact, we seem to see as it were the ﬁrst beginnings of the modern life, he named Eocene (dawn of the recent), including under that title those parts of the Tertiary series of the London and Paris basins wherein the proportion of existing species of shells was only 3% per cent. The middle Tertiary beds in the valleys of the Loire, Garonne, and Dordogne, containing 17 per cent. of living species, were termed I-'l[z'ocene (less recent)- The younger Tertiary formations of Italy were included under the designation Pliocene (more recent), because they contained a majority or from 35 to 95 per cent. of living species. This newest series, however, was further subdivided into Older Pliocene (35 to 50 per cent. of living species) and Newer Pliocene (90 to 95 per cent.). This classiﬁca- tion, with various modiﬁcations and amplifications, has been adopted for the Tertiary group not of Europe only but of the whole globe. As the North American development of the Tertiary series differs in so many respects from that of Europe, it will be most conveniently considered by itself after the European classiﬁcation has been described. EOCENE. GREAT BRITAIN.-—Tlle Eocene rocks of Britain are on- tirely conﬁned to the south-east of the island, where they occupy two great depressions of the chalk, known respect- ively as the London and Hampshire basins. They have been arranged into the groups shown in the subjoined table. ("$3 3 Henipstead beds ............................ .. 179 ft. - ‘E 3 Bembridrre, ............................ .. 11.: ,, Lpper ii 2: E  iﬁsbornc O ,,, ............................ .. 70 ,, (,2; s "‘ lleadon ,, ............................ .. 200.,