Page:EB1911 - Volume 09.djvu/695

Rh non-local rocks of enormous dimensions included in the argillaceous or sandy matrix. The occurrence of these large boulders together with the scarceness of fossils has suggested a glacial origin for the formation; but the evidence hitherto collected is not conclusive. C. W. von Gümbel has classified the Eocene of the northern Alps (Bavaria, &c.) as follows:—

The Häring group of northern Tirol contains lignite beds of some importance. In the southern and S.E. Alps the following divisions are recognized.

In the central and northern Apennines the Eocene strata have been subdivided by Prof. F. Sacco into an upper Bartonian, a middle Parisian and a lower Suessonian series. In the middle member are the representatives of the Flysch and the Macigno. These Eocene strata are upwards of 5500 ft. thick. In northern Africa the nummulitic limestones and sandstones are widely spread; the lower portions comprise the Libyan group and the shales of Esneh on the Nile (Flandrien), the Alveolina beds of Sokotra and others; the Mokattam stage of Egypt is a representative of the later Eocene. Much of the N. African Eocene contains phosphatic beds. In India strata of Eocene age are extensively developed; in Sind the marine Ranikot beds, 1500 to 2000 ft., consisting of clays with gypsum and lignite, shales and sandstones; these beds have, side by side with Eocene nummulites, a few fossils of Cretaceous affinities. Above the Ranikot beds are the massive nummulitic limestones and sandstones of the Kirthar group; these are succeeded by the nummulitic limestones and shales at the base of the Nari group. In the southern Himalayan region the nummulitic phase of Eocene deposit is well developed, but there are difficulties in fixing the line of demarcation between this and the younger formations. The lower part of the Sirmur series of the Simla district may belong to this period; it is subdivided into the Kasauli group and the Dagshai group with the Subáthu group at the base. Beneath the thick nummulitic Eocene limestone of the Salt Range are shales and marls with a few coal seams. The marine Eocene rocks of N. America are most extensively developed round the coast of the Gulf of Mexico, whence they spread into the valley of the Mississippi and, as a comparatively narrow strip, along the Atlantic coastal plain to New Jersey.

The series in Alabama, which may be taken as typical of the Gulf coast Eocene, is as follows:—

The above succession is not fully represented in the Atlantic coast states.

On the Pacific coast marine formations are found in California and Oregon; such are the Tejon series with lignite and oil; the Escondido series of S. California (7000 ft.), part of the Pascadero series of the Santa Cruz Mountains; the Pulaski, Tyee, Arago and Coaledo beds—with coals—in Oregon. In the Puget formation of Washington we have a great series of sediments, largely of brackish water origin, and in parts coal-bearing. The total thickness of this formation has been estimated at 20,000 ft. (it may prove to be less than this), but it is probable that only the lower portion is of Eocene age. The most interesting of the N. American Eocene deposits are those of the Rocky Mountains and the adjacent western plains, in Wyoming, Nevada, Nebraska, Colorado, &c.; they are of terrestrial, lacustrine or aeolian origin, and on this account and because they were not strictly synchronous, there is considerable difficulty in placing them in their true position in the time-scale. The main divisions or groups are generally recognized as follows:—

The Fort Union beds of Canada and parts of Montana and N. Dakota are probably the oldest Eocene strata of the Western Interior; they are some 2000 ft. thick and possibly are equivalent to the Midwayan group. But in these beds, as in those known as Arapahoe, Livingston, Denver, Ohio and Ruby, which are now often classed as belonging to the upper Laramie formation, it is safer to regard them as a transitional series between the Mesozoic and Tertiary systems. There is, however, a marked unconformity between the Eocene Telluride or San Miguel and Poison Canyon formations of Colorado and the underlying Laramie rocks.

Many local aspects of Eocene rocks have received special names, but too little is known about them to enable them to be correctly placed in the Eocene series. Such are the Clarno formation (late Eocene) of the John Day basin, Oregon, the Pinyon conglomerate of Yellowstone Park, the Sphinx conglomerate of Montana, the Whitetail conglomerate of Arizona, the Manti shales of Utah, the Mojave formation of S. California and the Amyzon formation of Nevada.

Of the Eocene of other countries little is known in detail. Strata of this age occur in Central and S. America (Patagonia-Megellanian series—Brazil, Chile, Argentina), in S. Australia (and in the Great Australian Bight), New Zealand, in Seymour Island near Graham Land in the Antarctic Regions, Japan, Java, Borneo, New Guinea, Moluccas, Philippines, New Caledonia, also in Greenland, Bear Island, Spitzbergen and Siberia.

Organic Life of the Eocene Period.—As it has been observed above, the name Eocene was given to this period on the ground that in its fauna only a small percentage of living species were present; this estimation was founded upon the assemblage of invertebrate remains in which, from the commencement of this period until the present day, there has been comparatively little change. The real biological interest of the period centres around the higher vertebrate types. In the marine mollusca the most noteworthy change is the entire absence of ammonoids, the group which throughout the Mesozoic era had taken so prominent a place, but disappeared completely with the close of the Cretaceous. Nautiloids were more abundant than they are at present, but as a whole the Cephalopods took a more subordinate part than they had done in previous periods. On the other hand, Gasteropods and Pelecypods found in the numerous shallow seas a very suitable environment and flourished exceedingly, and their shells are often preserved in a state of great perfection and in enormous numbers. Of the Gasteropod genera Cerithium with its estuarine and lagoonal forms Potamides, Potamidopsis, &c., is very characteristic; Rostellaria, Voluta, Fusus, Pleurotoma, Conus, Typhis, may also be cited. Cardium, Venericardia, Crassatella, Corbulomya, Cytherea, Lucina, Anomia, Ostrea are a few of the many Pelecypod genera. Echinoderms were represented by abundant sea-urchins, Echinolampas, Linthia, Conoclypeus, &c. Corals flourished on the numerous reefs and approximated to modern forms (Trochosmilia, Dendrophyllia). But by far the most abundant marine organisms were the foraminifera which flourished in the warm seas in countless myriads. Foremost among these are the Nummulites, which by their extraordinary numerical development and great size, as well as by their wide distribution, demand special recognition. Many other genera of almost equal importance as rock builders, lived at the same time: Orthophragma, Operculina, Assilina, Orbitolites, Miliola, Alveolina. Crustacea were fairly abundant (Xanthopsis, Portunus), and most of the orders and many families of modern insects were represented.

When we turn to the higher forms of life, the reptiles and mammals, we find a remarkable contrast between the fauna of the Eocene and those periods which preceded and succeeded it. The great group of Saurian reptiles, whose members had held dominion on land and sea during most of the Mesozoic time, had completely disappeared by the beginning of the Eocene; in their place placental mammals made their appearance and rapidly became the dominant group. Among the early Eocene mammals no trace can be found of the numerous and clearly-marked orders with which we are familiar to-day; instead we find obscurely differentiated forms, which cannot be fitted without violence into any of the modern orders. The early placental mammals were generalized types (with certain non-placental characters) with potentialities for rapid divergence and development in the direction of the more specialized modern orders. Thus, the Creodonta foreshadowed the Carnivora, the