The Elements of Geology, William Harmon Norton [e textbook reader TXT] 📗

the interval between the flows in any locality was sometimes long enough for clays to gather in the lakes which filled depressions in the surface. Again and again the surface of the black basalt was reddened by oxidation and decayed to soil, and forests had time to grow upon it before the succeeding inundation sealed the sediments and soils away beneath a sheet of stone. Near the edges of the lava plain, rivers from the surrounding mountains spread sheets of sand and gravel on the surface of one flow after another. These pervious sands, interbedded with the lava, become the aquifers of artesian wells.

In places the lavas rest on extensive lake deposits, one thousand feet deep, and Miocene in age as their fossils prove. It is to the middle Tertiary, then, that the earliest flows and the largest bulk of the great inundation belong. So ancient are the latest floods in the Columbia basin that they have weathered to a residual yellow clay from thirty to sixty feet in depth and marvelously rich in the mineral substances on which plants feed.

In the Snake River valley the latest lavas are much younger. Their surfaces are so fresh and undecayed that here the effusive eruptions may well have continued to within the period of human history. Low lava domes like those of Iceland mark where last the basalt outwelled and spread far and wide before it chilled (Fig. 341). In places small mounds of scoria show that the eruptions were accompanied to a slight degree by explosions of steam. So fluid was this superheated lava that recent flows have been traced for more than fifty miles.

The rocks underlying the Columbia lavas, where exposed to view, are seen to be cut by numerous great dikes of dense basalt, which mark the fissures through which the molten rock rose to the surface.

The Tertiary included times of widespread and intense volcanic action in other continents as well as in North America. In Europe, Vesuvius and Etna began their career as submarine volcanoes in connection with earth movements which finally lifted Pliocene deposits in Sicily to their present height,—four thousand feet above the sea. Volcanoes broke forth in central France and southern Germany, in Hungary and the Carpathians. Innumerable fissures opened in the crust from the north of Ireland and the western islands of Scotland to the Faroes, Iceland, and even to arctic Greenland; and here great plateaus were built of flows of basalt similar to that of the Columbia River. In India, at the opening of the Tertiary, there had been an outwelling of basalt, flooding to a depth of thousands of feet two hundred thousand square miles of the northwestern part of the peninsula, and similar inundations of lava occurred where are now the table-lands of Abyssinia. From the middle Tertiary on, Asia Minor, Arabia, and Persia were the scenes of volcanic action. In Palestine the rise of the uplands of Judea at the close of the Eocene, and the downfaulting of the Jordan valley were followed by volcanic outbursts. In comparison with the middle Tertiary, the present is a time of volcanic inactivity and repose.

EROSION OF TERTIARY MOUNTAINS AND PLATEAUS. The mountains and plateaus built at various times during the Tertiary and at its commencement have been profoundly carved by erosive agents. The Sierra Nevada Mountains have been dissected on the western slope by such canyons as those of King's River and the Yosemite. Six miles of strata have been denuded from parts of the Wasatch Mountains since their rise at the beginning of the era. From the Colorado plateaus, whose uplift dates from the same time, there have been stripped off ten thousand feet of strata over thousands of square miles, and the colossal canyon of the Colorado has been cut after this great denudation had been mostly accomplished.

On the eastern side of the continent, as we have seen, a broad peneplain had been developed by the close of the Cretaceous. The remnants of this old erosion surface are now found upwarped to various heights in different portions of its area. In southern New England it now stands fifteen hundred feet above the sea in western Massachusetts, declining thence southward and eastward to sea level at the coast. In southwestern Virginia it has been lifted to four thousand feet above the sea. Manifestly this upwarp occurred since the peneplain was formed; it is later than the Mesozoic, and the vast dissection which the peneplain has suffered since its uplift must belong to the successive cycles of Cenozoic time.

Revived by the uplift, the streams of the area trenched it as deeply as its elevation permitted, and reaching grade, opened up wide valleys and new peneplains in the softer rocks. The Connecticut valley is Tertiary in age, and in the weak Triassic sandstones has been widened in places to fifteen miles. Dating from the same time are the valleys of the Hudson, the Susquehanna, the Delaware, the Potomac, and the Shenandoah.

In Pennsylvania and the states lying to the south the Mesozoic peneplain lies along the summits of the mountain ridges. On the surface of this ancient plain, Tertiary erosion etched out the beautifully regular pattern of the Allegheny mountain ridges and their intervening valleys. The weaker strata of the long, regular folds were eroded into longitudinal valleys, while the hard Paleozoic sandstones, such as the Medina and the Pocono, were left in relief as bold mountain walls whose even crests rise to the common level of the ancient plain. From Virginia far into Alabama the great Appalachian valley was opened to a width in places of fifty miles and more, along a belt of intensely folded and faulted strata where once was the heart of the Appalachian Mountains. In Figure 70 the summit of the Cumberland plateau (ab) marks the level of the Mesozoic peneplain, while the lower erosion levels are Tertiary and Quaternary in age.

LIFE OF THE TERTIARY PERIOD

VEGETATION AND CLIMATE. The highest plants in structure, the DICOTYLS (such as our deciduous forest trees) and the MONOCOTYLS (represented by the palms), were introduced during the Cretaceous. The vegetable kingdom reached its culmination before the animal kingdom, and if the dividing line between the Mesozoic and the Cenozoic were drawn according to the progress of plant life, the Cretaceous instead of the Tertiary would be made the opening period of the modern era.

The plants of the Tertiary belonged, for the most part, to genera now living; but their distribution was very different from that of the flora of to-day. In the earlier Tertiary, palms flourished over northern Europe, and in the northwestern United States grew the magnolia and laurel, along with the walnut, oak, and elm. Even in northern Greenland and in Spitzbergen there were lakes covered with water lilies and surrounded by forests of maples, poplars, limes, the cypress of our southern states, and noble sequoias similar to the "big trees" and redwoods of California. A warm climate like that of the Mesozoic, therefore, prevailed over North America and Europe, extending far toward the pole. In the later Tertiary the climate gradually became cooler. Palms disappeared from Europe, and everywhere the aspect of forests and open lands became more like that of to-day. Grasses became abundant, furnishing a new food for herbivorous animals.

ANIMAL LIFE OF THE TERTIARY. Little needs to be said of the Tertiary invertebrates, so nearly were they like the invertebrates of the present. Even in the Eocene, about five per cent of marine shells were of species still living, and in the Pliocene the proportion had risen to more than one half.

Fishes were of modern types. Teleosts were now abundant. The ocean teemed with sharks, some of them being voracious monsters seventy- five feet and even more in length, with a gape of jaw of six feet, as estimated by the size of their enormous sharp-edged teeth.

Snakes are found for the first time in the early Tertiary. These limbless reptiles, evolved by degeneration from lizardlike ancestors, appeared in nonpoisonous types scarcely to be distinguished from those of the present day.

MAMMALS OF THE EARLY TERTIARY. The fossils of continental deposits of the earliest Eocene show that a marked advance had now been made in the evolution of the Mammalia. The higher mammals had appeared, and henceforth the lower mammals—the monotremes and the marsupials—are reduced to a subordinate place.

These first true mammals were archaic and generalized in structure. Their feet were of the primitive type, with five toes of about equal length. They were also PLANTIGRADES,—that is, they touched the ground with the sole of the entire foot from toe to heel. No foot had yet become adapted to swift running by a decrease in the number of digits and by lifting the heel and sole so that only the toes touch the ground,—a tread called DIGITIGRADE. Nor was there yet any foot like that of the cats, with sharp retractile claws adapted to seizing and tearing the prey. The forearm and the lower leg each had still two separate bones (ulna and radius, fibula and tibia), neither pair having been replaced with a single strong bone, as in the leg of the horse. The teeth also were primitive in type and of full number. The complex heavy grinders of the horse and elephant, the sharp cutting teeth of the carnivores, and the cropping teeth of the grass eaters were all still to come.

Phenacodus is a characteristic genus of the early Eocene, whose species varied in size from that of a bulldog to that of an animal a little larger than a sheep. Its feet were primitive, and their five toes bore nails intermediate in form between a claw and a hoof. The archaic type of teeth indicates that the animal was omnivorous in diet. A cast of the brain cavity shows that, like its associates of the time, its brain was extremely small and nearly smooth, having little more than traces of convolutions.

The long ages of the Eocene and the following epochs of the Tertiary were times of comparatively rapid evolution among the Mammalia. The earliest forms evolved along diverging lines toward the various specialized types of hoofed mammals, rodents, carnivores, proboscidians, the primates, and the other mammalian orders as we know them now. We must describe the Tertiary mammals very briefly, tracing the lines of descent of only a few of the more familiar mammals of the present.

THE HORSE. The pedigree of the horse runs back into the early Eocene through many genera and species to a five-toed, [Footnote: Or, more accurately, with four perfect toes and a rudimentary fifth corresponding to the thumb.] short-legged ancestor little bigger than a cat. Its descendants gradually increased in stature and became better and better adapted to swift running to escape their foes. The leg became longer, and only the tip of the toes struck the ground. The middle toe (digit number three), originally the longest of the five, steadily enlarged, while the remaining digits dwindled and disappeared. The inner digit, corresponding to the great toe and thumb, was the first to go. Next number five, the little finger, was also dropped. By the end of the Eocene a three-toed genus of the horse family had appeared, as large as a sheep. The hoof of digit number three now supported most of the weight, but the slender hoofs of digits two and four were still serviceable. In the Miocene the stature of the ancestors of the horse increased to that of a pony. The feet were still three-toed, but the side hoofs were now mere dewclaws and scarcely touched the ground. The evolution of the family was completed in the Pliocene.

The middle toe was enlarged still more, the side toes were dropped, and the palm and foot bones which supported them were reduced to splints.

While these changes were in progress the radius and ulna of the fore limb became consolidated to a single bone; and in the hind limb the fibula dwindled to a splint, while the tibia was correspondingly enlarged. The molars, also gradually lengthened, and became more and more complex on their grinding surface; the neck became longer; the brain steadily increased in size and its convolutions became more abundant. The evolution of the horse has made for greater fleetness and intelligence.

THE RHINOCEROS AND TAPIR. These animals, which are grouped with the horse among the ODD-TOED (perissodactyl) mammals, are now verging toward extinction. In the rhinoceros, evolution seems to have taken the opposite course from that of the horse. As the animal increased in size it became more clumsy, its limbs became shorter and more massive, and, perhaps because of its great weight, the number of digits were not reduced below the number three. Like other large herbivores, the rhinoceros, too slow to escape its enemies by flight, learned to withstand them. It developed as its means of defense a nasal horn.

Peculiar offshoots of the line appeared at various times in the Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks, resembling in aspect the hippopotamus, lived