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

which must have been laid near shore are succeeded by shaly sandstones, sandy shales, and shales. Toward the top of the series heavy beds of limestone, extending from the Blue Ridge to Missouri, speak of clear water, and either of more distant shores or of neighboring lands which were worn or sunk so low that for the most part their waste was carried to the sea in solution.

In brief, the Cambrian was a period of submergence. It began with the larger part of North America emerged as great land masses. It closed with most of the interior of the continental plateau covered with a shallow sea.

THE LIFE OF THE CAMBRIAN PERIOD

It is now for the first time that we find preserved in the offshore deposits of the Cambrian seas enough remains of animal life to be properly called a fauna. Doubtless these remains are only the most fragmentary representation of the life of the time, for the Cambrian rocks are very old and have been widely metamorphosed. Yet the five hundred and more species already discovered embrace all the leading types of invertebrate life, and are so varied that we must believe that their lines of descent stretch far back into the pre-Cambrian past.

PLANTS. No remains of plants have been found in Cambrian strata, except some doubtful markings, as of seaweed.

SPONGES. The sponges, the lowest of the multicellular animals, were represented by several orders. Their fossils are recognized by the siliceous spicules, which, as in modern sponges, either were scattered through a mass of horny fibers or were connected in a flinty framework.

COELENTERATES. This subkingdom includes two classes of interest to the geologist,—the HYDROZOA, such as the fresh-water hydra and the jellyfish, and the CORALS. Both classes existed in the Cambrian.

The Hydrozoa were represented not only by jellyfish but also by the GRAPTOLITE, which takes its name from a fancied resemblance of some of its forms to a quill pen. It was a composite animal with a horny framework, the individuals of the colony living in cells strung on one or both sides along a hollow stem, and communicating by means of a common flesh in this central tube. Some graptolites were straight, and some curved or spiral; some were single stemmed, and others consisted of several radial stems united. Graptolites occur but rarely in the Upper Cambrian. In the Ordovician and Silurian they are very plentiful, and at the close of the Silurian they pass out of existence, never to return.

CORALS are very rarely found in the Cambrian, and the description of their primitive types is postponed to later chapters treating of periods when they became more numerous.

ECHINODERMS. This subkingdom comprises at present such familiar forms as the crinoid, the starfish, and the sea urchin. The structure of echinoderms is radiate. Their integument is hardened with plates or particles of carbonate of lime.

Of the free echinoderms, such as the starfish and the sea urchin, the former has been found in the Cambrian rocks of Europe, but neither have so far been discovered in the strata of this period in North America. The stemmed and lower division of the echinoderms was represented by a primitive type, the CYSTOID, so called from its saclike form, A small globular or ovate "calyx" of calcareous plates, with an aperture at the top for the mouth, inclosed the body of the animal, and was attached to the sea bottom by a short flexible stalk consisting of disks of carbonate of lime held together by a central ligament.

ARTHOPODS. These segmented animals with "jointed feet," as their name suggests, may be divided in a general way into water breathers and air breathers. The first-named and lower division comprises the class of the CRUSTACEA,—arthropods protected by a hard exterior skeleton, or "crust,"—of which crabs, crayfish, and lobsters are familiar examples. The higher division, that of the air breathers, includes the following classes: spiders, scorpions, centipedes, and insects.

THE TRILOBITE. The aquatic arthropods, the Crustacea, culminated before the air breathers; and while none of the latter are found in the Cambrian, the former were the dominant life of the time in numbers, in size, and in the variety of their forms. The leading crustacean type is the TRILOBITE, which takes its name from the three lobes into which its shell is divided longitudinally. There are also three cross divisions,—the head shield, the tail shield, and between the two the thorax, consisting of a number of distinct and unconsolidated segments. The head shield carries a pair of large, crescentic, compound eyes, like those of the insect. The eye varies greatly in the number of its lenses, ranging from fourteen in some species to fifteen thousand in others. Figure 268, C, is a restoration of the trilobite, and shows the appendages, which are found preserved only in the rarest cases.

During the long ages of the Cambrian the trilobite varied greatly. Again and again new species and genera appeared, while the older types became extinct. For this reason and because of their abundance, trilobites are used in the classification of the Cambrian system. The Lower Cambrian is characterized by the presence of a trilobitic fauna in which the genus Olenellus is predominant. This, the OLENELLUS ZONE, is one of the most important platforms in the entire geological series; for, the world over, it marks the beginning of Paleozoic time, while all underlying strata are classified as pre-Cambrian. The Middle Cambrian is marked by the genus Paradoxides, and the Upper Cambrian by the genus Olenus. Some of the Cambrian trilobites were giants, measuring as much as two feet long, while others were the smallest of their kind, a fraction of an inch in length.

Another type of crustacean which lived in the Cambrian and whose order is still living is illustrated in Figure 269.

WORMS. Trails and burrows of worms have been left on the sea beaches and mud flats of all geological times from the Algonkian to the present.

BRACHIOPODS. These soft-bodied animals, with bivalve shells and two interior armlike processes which served for breathing, appeared in the Algonkian, and had now become very abundant. The two valves of the brachiopod shell are unequal in size, and in each valve a line drawn from the beak to the base divides the valve into two equal parts. It may thus be told from the pelecypod mollusk, such as the clam, whose two valves are not far from equal in size, each being divided into unequal parts by a line dropped from the beak.

Brachiopods include two orders. In the most primitive order—that of the INARTICULATE brachiopods—the two valves are held together only by muscles of the animal, and the shell is horny or is composed of phosphate of lime. The DISCINA, which began in the Algonkian, is of this type, as is also the LINGULELLA of the Cambrian. Both of these genera have lived on during the millions of years of geological time since their introduction, handing down from generation to generation with hardly any change to their descendants now living off our shores the characters impressed upon them at the beginning.

The more highly organized ARTICULATE brachiopods have valves of carbonate of lime more securely joined by a hinge with teeth and sockets (Fig. 270). In the Cambrian the inarticulates predominate, though the articulates grow common toward the end of the period.

MOLLUSKS. The three chief classes of mollusks—the PELECYPODS (represented by the oyster and clam of to-day), the GASTROPODS (represented now by snails, conches, and periwinkles), and the CEPHALOPODS (such as the nautilus, cuttlefish, and squids)—were all represented in the Cambrian, although very sparingly.

Pteropods, a suborder of the gastropods, appeared in this age. Their papery shells of carbonate of lime are found in great numbers from this time on.

Cephalopods, the most highly organized of the mollusks, started into existence, so far as the record shows, toward, the end of the Cambrian, with the long extinct ORTHOCERAS (STRAIGHTHORN) and the allied genera of its family. The Orthoceras had a long, straight, and tapering shell, divided by cross partitions into chambers. The animal lived in the "body chamber" at the larger end, and walled off the other chambers from it in succession during the growth of the shell. A central tube, the SIPHUNCLE, passed through from the body chamber to the closed tip of the cone.

The seashells, both brachiopods and mollusks, are in some respects the most important to the geologist of all fossils. They have been so numerous, so widely distributed, and so well preserved because of their durable shells and their station in growing sediments, that better than any other group of organisms they can be used to correlate the strata of different regions and to mark by their slow changes the advance of geological time.

CLIMATE. The life of Cambrian times in different countries contains no suggestion of any marked climatic zones, and as in later periods a warm climate probably reached to the polar regions.

CHAPTER XVII

THE ORDOVICIAN AND SILURIAN
[Footnote: Often known as the Lower Silurian.]

THE ORDOVICIAN

In North America the Ordovician rocks lie conformably on the Cambrian. The two periods, therefore, were not parted by any deformation, either of mountain making or of continental uplift. The general submergence which marked the Cambrian continued into the succeeding period with little interruption.

SUBDIVISIONS AND DISTRIBUTION OF STRATA. The Ordovician series, as they have been made out in New York, are given for reference in the following table, with the rocks of which they are chiefly composed:

    5 Hudson . . . . . . . . shales
    4 Utica . . . . . . . . shales
    3 Trenton . . . . . . . limestones
    2 Chazy . . . . . . . . limestones
    1 Calciferous . . . . . sandy limestones

These marine formations of the Ordovician outcrop about the Cambrian and pre-Cambrian areas, and, as borings show, extend far and wide over the interior of the continent beneath more recent strata. The Ordovician sea stretched from Appalachia across the Mississippi valley. It seems to have extended to California, although broken probably by several mountainous islands in the west.

PHYSICAL GEOGRAPHY. The physical history of the period is recorded in the succession of its formations. The sandstones of the Upper Cambrian, as we have learned, tell of a transgressing sea which gradually came to occupy the Mississippi valley and the interior of North America. The limestones of the early and middle Ordovician show that now the shore had become remote and the lands had become more low. The waters now had cleared. Colonies of brachiopods and other lime-secreting animals occupied the sea bottom, and their debris mantled it with sheets of limy ooze. The sandy limestones of the Calciferous record the transition stage from the Cambrian when some sand was still brought in from shore. The highly fossiliferous limestones of the Trenton tell of clear water and abundant life. We need not regard this epicontinental sea as deep. No abysmal deposits have been found, and the limestones of the period are those which would be laid in clear, warm water of moderate depth like that of modern coral seas.

The shales of the Utica and Hudson show that the waters of the sea now became clouded with mud washed in from land. Either the land was gradually uplifted, or perhaps there had arrived one of those periodic crises which, as we may imagine, have taken place whenever the crust of the shrinking earth has slowly given way over its great depressions, and the ocean has withdrawn its waters into deepening abysses. The land was thus left relatively higher and bordered with new coastal plains. The epicontinental sea was shoaled and narrowed, and muds were washed in from the adjacent lands.

THE TACONIC DEFORMATION. The Ordovician was closed by a deformation whose extent and severity are not yet known. From the St. Lawrence River to New York Bay, along the northwestern and western border of New England, lies a belt of Cambrian-Ordovician rocks more than a mile in total thickness, which accumulated during the long ages of those periods in a gradually subsiding trough between the Adirondacks and a pre-Cambrian range lying west of the Connecticut River. But since their deposition these ancient sediments have been crumpled and crushed, broken with great faults, and extensively metamorphosed. The limestones have recrystallized into marbles, among them the famous marbles of Vermont; the Cambrian sandstones have become quartzites, and the Hudson shale has been changed to a schist exposed on Manhattan Island and northward.

In part these changes occurred at the close of the Ordovician, for in several places beds of Silurian age rest unconformably on the upturned Ordovician strata; but recent investigations have made it probable that the crustal movements recurred at later times, and it was perhaps in the Devonian and at the close of the Carboniferous that the greater part of the deformation and metamorphism was accomplished. As a result of these movements,— perhaps several times repeated,—a