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

THE BRITISH ISLES. The Triassic strata of the British Isles are continental, and include breccia beds of cemented talus, deposits of salt and gypsum, and sandstones whose rounded and polished grains are those of the wind-blown sands of deserts. In Triassic times the British Isles were part of a desert extending over much of northwestern Europe.

The third great system of the Mesozoic includes many formations, marine and continental, which record a long and complicated history marked by great oscillations of the crust and wide changes in the outlines of sea and land.

EARLY CRETACEOUS. In eastern North America the lowest Cretaceous series comprises fresh-water formations which are traced from Nantucket across Martha's Vineyard and Long Island, and through New Jersey southward into Georgia. They rest unconformably on the Triassic sandstones and the older rocks of the region. The Atlantic shore line was still farther out than now in the northern states. Again, as during the Triassic, a warping of the crust formed a long trough parallel to the coast and to the Appalachian ridges, but cut off from the sea; and here the continental deposits of the early Cretaceous were laid.

Along the Gulf of Mexico the same series was deposited under like conditions over the area known as the Mississippi embayment, reaching from Georgia northwestward into Tennessee and thence across into Arkansas and southward into Texas.

In the Southwest the subsidence continued until the transgressing sea covered most of Mexico and Texas and extended a gulf northward into Kansas. In its warm and quiet waters limestones accumulated to a depth of from one thousand to five thousand feet in Texas, and of more than ten thousand feet in Mexico. Meanwhile the lowlands, where the Great Plains are now, received continental deposits; coal swamps stretched from western Montana into British Columbia.

THE MIDDLE CRETACEOUS. This was a land epoch. The early Cretaceous sea retired from Texas and Mexico, for its sediments are overlain unconformably by formations of the Upper Cretaceous. So long was the time gap between the two series that no species found in the one occurs in the other.

THE UPPER CRETACEOUS. There now began one of the most remarkable events in all geological history,—the great Cretaceous subsidence. Its earlier warpings were recorded in continental deposits,—wide sheets of sandstone, shale, and some coal,—which were spread from Texas to British Columbia. These continental deposits are overlain by a succession of marine formations whose vast area is shown on the map, Figure 260. We may infer that as the depression of the continent continued the sea came in far and wide over the coast lands and the plains worn low during the previous epochs. Upper Cretaceous formations show that south of New England the waters of the Atlantic somewhat overlapped the crystalline rocks of the Piedmont Belt and spread their waste over the submerged coastal plain. The Gulf of Mexico again covered the Mississippi embayment, reaching as far north as southern Illinois, and extended over Texas.

A mediterranean sea now stretched from the Gulf to the arctic regions and from central Iowa to the eastern shore of the Great Basin land at about the longitude of Salt Lake City, the Colorado Mountains rising from it in a chain of islands. Along with minor oscillations there were laid in the interior sea various formations of sandstones, shales, and limestones, and from Kansas to South Dakota beds of white chalk show that the clear, warm waters swarmed at times with foraminiferal life whose disintegrating microscopic shells accumulated in this rare deposit.

At this epoch a wide sea, interrupted by various islands, stretched across Eurasia from Wales and western Spain to China, and spread southward over much of the Sahara. To the west its waters were clear and on its floor the crumbled remains of foraminifers gathered in heavy accumulations of calcareous ooze,— the white chalk of France and England. Sea urchins were also abundant, and sponges contributed their spicules to form nodules of flint.

THE LARAMIE. The closing stage of the Cretaceous was marked in North America by a slow uplift of the land. As the interior sea gradually withdrew, the warping basins of its floor were filled with waste from the rising lands about them, and over this wide area there were spread continental deposits in fresh-water lakes like the Great Lakes of the present, in brackish estuaries, and in river plains, while occasional oscillations now and again let in the sea. There were vast marshes in which there accumulated the larger part of the valuable coal seams of the West. The Laramie is the coal-bearing series of the West, as the Pennsylvanian is of the eastern part of our country.

THE ROCKY MOUNTAIN DEFORMATION. At the close of the Cretaceous we enter upon an epoch of mountain-making far more extensive than any which the continent had witnessed. The long belt lying west of the ancient axes of the Colorado Islands and east of the Great Basin land had been an area of deposition for many ages, and in its subsiding troughs Paleozoic and Mesozoic sediments had gathered to the depth of many thousand feet. And now from Mexico well-nigh to the Arctic Ocean this belt yielded to lateral pressure. The Cretaceous limestones of Mexico were folded into lofty mountains. A massive range was upfolded where the Wasatch Mountains now are, and various ranges of the Rockies in Colorado and other states were upridged. However slowly these deformations were effected they were no doubt accompanied by world-shaking earthquakes, and it is known that volcanic eruptions took place on a magnificent scale. Outflows of lava occurred along the Wasatch, the laccoliths of the Henry Mountains were formed, while the great masses of igneous rock which constitute the cores of the Spanish Peaks and other western mountains were thrust up amid the strata. The high plateaus from which many of these ranges rise had not yet been uplifted, and the bases of the mountains probably stood near the level of the sea.

North America was now well-nigh completed. The mediterranean seas which so often had occupied the heart of the land were done away with, and the continent stretched unbroken from the foot of the Sierras on the west to the Fall Line of the Atlantic coastal plain on the east.

THE MESOZOIC PENEPLAIN. The immense thickness of the Mesozoic formations conveys to our minds some idea of the vast length of time involved in the slow progress of its successive ages. The same lesson is taught as plainly by the amount of denudation which the lands suffered during the era.

The beginning of the Mesozoic saw a system of lofty mountain ranges stretching from New York into central Alabama. The end of this long era found here a wide peneplain crossed by sluggish wandering rivers and overlooked by detached hills as yet unreduced to the general level. The Mesozoic era was long enough for the Appalachian Mountains, upridged at its beginning, to have been weathered and worn away and carried grain by grain to the sea. The same plain extended over southern New England. The Taconic range, uplifted partially at least at the close of the Ordovician, and the block mountains of the Triassic, together with the pre- Cambrian mountains of ancient Appalachia, had now all been worn to a common level with the Allegheny ranges. The Mesozoic peneplain has been upwarped by later crustal movements and has suffered profound erosion, but the remnants of it which remain on the upland of southern New England and the even summits of the Allegheny ridges suffice to prove that it once existed. The age of the Mesozoic peneplain is determined from the fact that the lower Tertiary sediments were deposited on its even surface when at the close of the era the peneplain was depressed along its edges beneath the sea.

PLANT LIFE OF THE TRIASSIC AND JURASSIC. The Carboniferous forests of lepidodendrons and sigillafids had now vanished from the earth. The uplands were clothed with conifers, like the Araucarian pines of South America and Australia. Dense forests of tree ferns throve in moist regions, and canebrakes of horsetails of modern type, but with stems reaching four inches in thickness, bordered the lagoons and marshes. Cycads were exceedingly abundant. These gymnosperms, related to the pines and spruces in structure and fruiting, but palmlike in their foliage, and uncoiling their long leaves after the manner of ferns, culminated in the Jurassic. From the view point of the botanist the Mesozoic is the Age of Cycads, and after this era they gradually decline to the small number of species now existing in tropical latitudes.

PLANT LIFE OF THE CRETACEOUS. In the Lower Cretaceous the woodlands continued of much the same type as during the Jurassic. The forerunners now appeared of the modern dicotyls (plants with two seed leaves), and in the Middle Cretaceous the monocotyledonous group of palms came in. Palms are so like cycads that we may regard them as the descendants of some cycad type.

In the UPPER CRETACEOUS, cycads become rare. The highest types of flowering plants gain a complete ascendency, and forests of modern aspect cover the continent from the Gulf of Mexico to the Arctic Ocean. Among the kinds of forest trees whose remains are found in the continental deposits of the Cretaceous are the magnolia, the myrtle, the laurel, the fig, the tulip tree, the chestnut, the oak, beech, elm, poplar, willow, birch, and maple. Forests of Eucalyptus grew along the coast of New England, and palms on the Pacific shores of British Columbia. Sequoias of many varieties ranged far into northern Canada. In northern Greenland there were luxuriant forests of magnolias, figs, and cycads; and a similar flora has been disinterred from the Cretaceous rocks of Alaska and Spitzbergen. Evidently the lands within the Arctic Circle enjoyed a warm and genial climate, as they had done during the Paleozoic. Greenland had the temperature of Cuba and southern Florida, and the time was yet far distant when it was to be wrapped in glacier ice.

INVERTEBRATES. During the long succession of the ages of the Mesozoic, with their vast geographical changes, there were many and great changes in organisms. Species were replaced again and again by others better fitted to the changing environment. During the Lower Cretaceous alone there were no less than six successive changes in the faunas which inhabited the limestone-making sea which then covered Texas. We shall disregard these changes for the most part in describing the life of the era, and shall confine our view to some of the most important advances made in the leading types.

Stromatopora have disappeared. Protozoans and sponges are exceedingly abundant, and all contribute to the making of Mesozoic strata. Corals have assumed a more modern type. Sea urchins have become plentiful; crinoids abound until the Cretaceous, where they begin their decline to their present humble station.

Trilobites and eurypterids are gone. Ten-footed crustaceans abound of the primitive long-tailed type (represented by the lobster and the crayfish), and in the Jurassic there appears the modern short- tailed type represented by the crabs. The latter type is higher in organization and now far more common. In its embryological development it passes through the long-tailed stage; connecting links in the Mesozoic also indicate that the younger type is the offshoot of the older.

Insects evolve along diverse lines, giving rise to beetles, ants, bees, and flies.

Brachiopods have dwindled greatly in the number of their species, while mollusks have correspondingly increased. The great oyster family dates from here.

Cephalopods are now to have their day. The archaic Orthoceras lingers on into the Triassic and becomes extinct, but a remarkable development is now at hand for the more highly organized descendants of this ancient line. We have noticed that in the Devonian the sutures of some of the chambered shells become angled, evolving the Goniatite type. The sutures now become lobed and corrugated in Ceratites. The process was carried still farther, and the sutures were elaborately frilled in the great order of the Ammonites. It was in the Jurassic that the Ammonites reached their height. No fossils are more abundant or characteristic of their age. Great banks of their shells formed beds of limestone in warm seas the world over.

The ammonite stem branched into a most luxuriant variety of forms. The typical form was closely coiled like a nautilus. In others the coil was more or less open, or even erected into a spiral. Some were hook-shaped, and there were members of the order in which the shell was straight, and yet retained all the internal structures of its kind. At the end of the Mesozoic the entire tribe of ammonites became extinct.

The Belemnite (Greek, belemnon, a dart) is a distinctly higher type of cephalopod which appeared in the Triassic, became numerous and varied in the Jurassic and Cretaceous, and died out early in the Tertiary. Like the squids and cuttlefish, of which it was the prototype, it had an internal calcareous shell. This consisted of a chambered and siphuncled cone, whose point was