Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/379

Rh res 121'-En'rIA1tY.] from the coast of New Jersey southward round the margin of the Gulf of Mexico, whence they riiu up the valley of the Mississippi to beyond the mouth of the Ohio. On the western sea-board they also occur in the coast ranges of California and Oregon, where they sometimes have a thickness of 3000 or 4000 feet, and reach a height of 3000 feet above the sea. Over the Rocky Mountain region Tertiary strata cover an extensive area, but are chieﬂy of fresli-water origin, though containing marine interstratiﬁ- cations. The following are the subdivisicns into which they have been grouped, together with their supposed liuropean equivalents :— . Sumter series: Pliocene. . Yorktown ,, 2: Miocene, with perhaps part of Pliocene. Alabama ,, :Middle and Upper Eocene. 1. Lignitic ,, :Lower Eocene. 1. I.1'_r//u't[c.——.-s already mentioned, it is still matter of dispute whether this formation should not be included wholly or in large measure in the (‘retaceous systeni below. It consists mainly of la -ustrinc strata, with occasional brackish water and marine bands. Its name is derivcl from the beds of fossil fuel which it contains. The inollusca in some of the slicll-bearing beds comprise species of 1.2m‘ ‘ranms, Jnclumr, (r'_a/roclcs, Uimliunz, U3/rcna, .[clam pus, 0.5-lrca, and .1 210111 in. thcr strata contain the modern laciistrine and fluvi- atile genera I’lu/so, I'aIra/(1, Cg/rcna, Corbula, and Univ. An abundant terrestrial flora has been disiiiterred from the Lignitic strata. It resembles in many respects the present flora of North Aim-i'i«-a, comprisinf_-; as it does species of oak, poplar, maple, elm, dogwoi nl, bet-ch,hi"l{nl'y, ilex, plane, lig, cinnamon, laiircl,inagnolia, smilax, tliuja, sequoia, and SC'el'al palms. A few of the species are common to the Middle Tertiary flora of Europe, and a number of them have been met with in the Tertiary beds of the Arctic regions. Some of the seams of vegetable matter are true bituminous coals and even anthracites. It was stated in the account of the North American (‘rctaccous rocks that con.<i«ler-able dill'ereiice of opinion exists as to the line to be (lrawn between these rocks and those of Tertiary age. According to Cope, the 'crtcbrate remains of the Lignitic series bind it indissoliibly to the llesozoic formations. Lesquereux, on the other hand, insists that the vtgetation is unequivocally Tertiar_v. The former writer, a-l'ni.‘ting the force of the evidence furnished by the fossil plants, con -lules. that “ there is no alternative. but to accept the result that a 'l'ertiar_v flora was tonteinporaneoiis with a Cretaceous fauna, estab- lishing an uninterrupted succession of life aeross what is generally r-;_:.irded as one of the greatest breaks in geologic time." The Lignitie series was disturbed along the llocky Mountain region bi-for ‘ the deposition of the succeeding Tertiary forniations, for these lie nneomformably upon it. So great have been the changes in some regions that the strata have assumed the character of hard slates like those of Palzeozoic date, if indeed they have not become in California thoroughly er_vstalline masses. ‘2. Alubanm G'roI(p.—'l‘he strata included in this group are believed to 1'cprescnt the Middle and Upper liocenc of Europe. As their name implies, they are well developed in the State of Alabama, where they consist of two sub-groups,———(1) the Clayborne beds— c-lays, marls, limestones, lignite, and sands, and (2) the Vicksburg beds—lignitic clays, limestones, and marls,—the whole attaining a thickness of nearly 2:30 feet. But the strata thicken into South Carolina. Towards the west the marine fossils give place to leaf—beds and lignites. In the Green River basin the strata attain a thickness of more than 2000 feet, lying on the disturbed Lignitic group, and containing beds of coal, with remains of ﬁsh and of tapi1'oid pachy- derms. The fossils of the Alabama group in the eastern States comprise numerous sharks, some of which are specifically, and more are generically, the same as some of the English Eocene forms, such as Lcmnm clegrms, and Ciu'cluu'ozl0:L mcgalodon. ; also bones of several crocodiles and snakes. In the lacustrine and fluviatile deposits of the west there have been found also the remains of several birds, and a large number of mammals, including marsupials, numerous representatives of the tapirs, with double pairs of nasal lionis, an early form of horse (Ora/u'p]n(s) not larger than a fox, and with four toes in the fore foot, rodents, iiisectivorcs, bats, carnivores, and a number of forms allied to the living lemurs and marmosets. The richness of this fauna is remarkable, particularly in the mammals. 3. Yorktown Group. ——Under this name are classed strata of sand aml clay, which extend over a large area in the seaward part of the eastern States. Their organic remains (comprising mollusks, with remains of sharks, seals, walruses, whales, 820.) show them to have been chiefly laid down in a shallow sea in Miocene time. Westward, in the Upper Missouri region, and across the Rocky Mountains into (‘alifornia and Oregon, strata assigned to the same geological period were laid down in great lakes, and attain thicknesses of 1000 to 4000 or 5000 feet. The organic remains of these ancient lakes embrace examples of thrcc—tocd horses (A-nchz'then'um), of horned tapir-like !0CO>F~ GEOLOGY 365 animals (Titanothcrmm), of forms related to the hog, rhinoceros, camel, lama, deer, musk-ox, hare, squirrel, beaver, hya-na, wolf, panther, and tiger. The intermediate types indicated by Cuvier among the Paris Tertiary beds have been greatly increased from the American Tertiary groups by the researches of Marsh and Leidy. 4. Sumter Group or 1’l£occ)w.—In the Carolina States beds of loam, clay, or sand, lying in hollows of the older Tertiary deposits, and containing from 40 to 60 per cent. of living marine shells, are referred to the Pliocene age. In the Upper Missomi region, the Yorktown group is overlaid by other fresh-water beds, which are believed to he Pliocene. These strata contain land and fresh-water shells, probably of existing species. But their most remarkable organic remains are the inaminalia, which Continue the wonderful series in the strata underneath. Dana thus summarizes the facts :— “ Leidy has determined a large number of Pliocene mammals, all now extinct. They include three species of camel (Procamclus) ; a rhinoceros as large as the lndiaii species ; a Il1'.l.StOtlU]1 smaller than JI. Amcricanus, L., of the Quaternary; an elephant I__l-.'. American us), occurring also in the Quaternary ; four or five species of the horse family, one of which was closely like the modern horse; a species of deer (Crjrrus II 'a7'7'cn1', L.) ; others near the miisk-deer of Asia ; species of Mrrcchyus, allied to Orcodon (a genius intcrinc-liatc between deer, camel, and hog); a wolf larger than any living species; a small fox; a tiger (Fclis augustus, L.) as large as the Bengal tiger, besides other carnivores; asmall beaver ; a porcupine. The collection of animals has a strikingly Oriental character, except in the preponderance of herbivores.” V. POST-TEl’.TIAPtY OR QUATEI{.'ARY. Under this division are included the various superficial deposits in which all the inollusca are of still living species. It is usually subdivided into two series——(I) an older group of deposits in which many of the mammals are of extinct species,——to this group the names of Pleistocene, Post- Pliocene, or Diluvial have been given ; and (2) a later series, wherein the mammals are all of still living species, to which the name of Ilecent or Alluvial has been assigned. These subdivisions, however, are confessedly very artiﬁcial, and it is often exceedingly diﬂicult to draw any line between them. In Europe and North America a tolerably sharp demarca- tion can usually be made between the Pliocene formations and those now to be described. The Crag deposits of the south east of England show traces of a gradual lowering of the temperature during later Pliocene times. This change of climate continued to augment until at last thoroughly arctic conditions prevailed, under which the oldest of the Post- Tertiary or Pleistocene deposits were accimnilated. It is hardly possible to arrange these deposits in a strict chronological order, because we have no means of deciding, in many cases, their relative antiquity. The following table is rather an enumeration of the more important of them than an arrangement in their exact sequence :~— River alluvia, peat mosses, lake deposits, blown i sand, marine deposits, raised beaches. Brick-earth, valley-gravels, old marine te1'raccs, cavern-deposits. Moraine-stuff, Kaine or Esker series, Clyde-beds, Upper Boulder clays, Interglacial beds, Lower Till, ice-worn rock-siirfaces. Pre—glacial forests and land surfac Recent .... .. Pleistocene J or Glacial.. U L PLi3IsToci~:.'i3 on GLACIAL. Under the name of the Glacial Period or Ice Age, a re- markable geological episode in the history of the northern hemisphere is denoted. We have seen in the foregoing section on the Crag deposits that there is evidence of a gradual refrigeration of the climate at the close of the Tertiary ages. This change of temperature affected the higher latitudes alike of the Old and the New World. It reached such a height that the whole of the north of Europe was buried under snow and ice, extending southwards even as far as Saxony. The Alps and Pyrenees were loaded with vast snow-ﬁelds, from which enormous glaciers descended into the plains, overriding ranges of l11il101‘.l1lllS on their way. The greater portion of Britain was similarly ice-covered.