The Elements of Geology Part 23

The oil horizons of California and Texas are Tertiary; those of Colorado, Cretaceous; those of West Virginia, Carboniferous; those of Pennsylvania, Kentucky, and Canada, Devonian; and the large field of Ohio and Indiana belongs to the Ordovician and higher systems.

Petroleum and natural gas, wherever found, have probably originated from the decay of organic matter when buried in sedimentary deposits, just as at present in swampy places the hydrogen and carbon of decaying vegetation combine to form marsh gas. The light and heat of these hydrocarbons we may think of, therefore, as a gift to the civilized life of our race from the humble organisms, both animal and vegetable, of the remote past, whose remains were entombed in the sediments of the Ordovician and later geological ages.

Petroleum is very widely disseminated throughout the stratified rocks. Certain limestones are visibly greasy with it, and others give off its characteristic fetid odor when struck with a hammer.

Many shales are bituminous, and some are so highly charged that small flakes may be lighted like tapers, and several gallons of oil to the ton may be obtained by distillation.

But oil and gas are found in paying quant.i.ties only when certain conditions meet:

1. A SOURCE below, usually a bituminous shale, from whose organic matter they have been derived by slow change.

2. A RESERVOIR above, in which they have gathered. This is either a porous sandstone or a porous or creviced limestone.

3. Oil and gas are lighter than water, and are usually under pressure owing to artesian water. Hence, in order to hold them from escaping to the surface, the reservoir must have the shape of an ANTICLINE, DOME, or LENS.

4. It must also have an IMPERVIOUS COVER, usually a shale. In these reservoirs gas is under a pressure which is often enormous, reaching in extreme cases as high as a thousand five hundred pounds to the square inch. When tapped it rushes out with a deafening roar, sometimes flinging the heavy drill high in air. In accounting for this pressure we must remember that the gas has been compressed within the pores of the reservoir rock by artesian water, and in some cases also by its own expansive force. It is not uncommon for artesian water to rise in wells after the exhaustion of gas and oil.

LIFE OF THE ORDOVICIAN

During the ages of the Ordovician, life made great advances. Types already present branched widely into new genera and species, and new and higher types appeared.

Sponges continued from the Cambrian. Graptolites now reached their climax.

STROMATOPORA--colonies of minute hydrozoans allied to corals--grew in places on the sea floor, secreting stony ma.s.ses composed of thin, close, concentric layers, connected by vertical rods. The Stromatopora are among the chief limestone builders of the Silurian and Devonian periods.

CORALS developed along several distinct lines, like modern corals they secreted a calcareous framework, in whose outer portions the polyps lived. In the Ordovician, corals were represented chiefly by the family of the CHOETETES, all species of which are long since extinct. The description of other types of corals will be given under the Silurian, where they first became abundant.

ECHINODERMS. The cystoid reaches its climax, but there appear now two higher types of echinoderms,--the crinoid and the starfish.

The CRINOID, named from its resemblance to the lily, is like the cystoid in many respects, but has a longer stem and supports a crown of plumose arms. Stirring the water with these arms, it creates currents by which particles of food are wafted to its mouth. Crinoids are rare at the present time, but they grew in the greatest profusion in the warm Ordovician seas and for long ages thereafter. In many places the sea floor was beautiful with these graceful, flowerlike forms, as with fields of long-stemmed lilies.

Of the higher, free-moving cla.s.ses of the echinoderms, starfish are more numerous than in the Cambrian, and sea urchins make their appearance in rare archaic forms.

CRUSTACEANS. Trilobites now reach their greatest development and more than eleven hundred species have been described from the rocks of this period. It is interesting to note that in many species the segments of the thorax have now come to be so shaped that they move freely on one another. Unlike their Cambrian ancestors, many of the Ordovician trilobites could roll themselves into b.a.l.l.s at the approach of danger. It is in this att.i.tude, taken at the approach of death, that trilobites are often found in the Ordovician and later rocks. The gigantic crustaceans called the EURYPTERIDS were also present in this period.

The arthropods had now seized upon the land. Centipedes and insects of a low type, the earliest known land animals, have been discovered in strata of this system.

BRYOZOANS. No fossils are more common in the limestones of the time than the small branching stems and lacelike mats of the bryozoans,--the skeletons of colonies of a minute animal allied in structure to the brachiopod.

BRACHIOPODS. These multiplied greatly, and in places their sh.e.l.ls formed thick beds of coquina. They still greatly surpa.s.sed the mollusks in numbers.

CEPHALOPODS. Among the mollusks we must note the evolution of the cephalopods. The primitive straight Orthoceras has now become abundant. But in addition to this ancestral type there appears a succession of forms more and more curved and closely coiled, as ill.u.s.trated in Figure 285. The nautilus, which began its course in this period, crawls on the bottom of our present seas.

VERTEBRATES. The most important record of the Ordovician is that of the appearance of a new and higher type, with possibilities of development lying hidden in its structure that the mollusk and the insect could never hope to reach. Scales and plates of minute fishes found in the Ordovician rocks near Canon City, Colorado, show that the humblest of the vertebrates had already made its appearance. But it is probable that vertebrates had been on the earth for ages before this in lowly types, which, being dest.i.tute of hard parts, would leave no record.

THE SILURIAN

The narrowing of the seas and the emergence of the lands which characterized the closing epoch of the Ordovician in eastern North America continue into the succeeding period of the Silurian. New species appear and many old species now become extinct.

THE APPALACHIAN REGION. Where the Silurian system is most fully developed, from New York southward along the Appalachian Mountains, it comprises four series:

4 Salina ... shales, impure limestones, gypsum, salt 3 Niagara ... chiefly limestones 2 Clinton ... sandstones, shales, with some limestones 1 Medina ... conglomerates, sandstones

The rocks of these series are shallow-water deposits and reach the total thickness of some five thousand feet. Evidently they were laid over an area which was on the whole gradually subsiding, although with various gentle oscillations which are recorded in the different formations. The coa.r.s.e sands of the heavy Medina formations record a period of uplift of the oldland of Appalachia, when erosion went on rapidly and coa.r.s.e waste in abundance was brought down from the hills by swift streams and spread by the waves in wide, sandy flats. As the lands were worn lower the waste became finer, and during an epoch of transition--the Clinton-- there were deposited various formations of sandstones, shales, and limestones. The Niagara limestones testify to a long epoch of repose, when low-lying lands sent little waste down to the sea.

The gypsum and salt deposits of the Salina show that toward the close of the Silurian period a slight oscillation brought the sea floor nearer to the surface, and at the north cut off extensive tracts from the interior sea. In these wide lagoons, which now and then regained access to the open sea and obtained new supplies of salt water, beds of salt and gypsum were deposited as the briny waters became concentrated by evaporation under a desert climate.

Along with these beds there were also laid shales and impure limestones.

In New York the "salt pans" of the Salina extended over an area one hundred and fifty miles long from east to west and sixty miles wide, and similar salt marshes occurred as far west as Cleveland, Ohio, and G.o.derich on Lake Huron. At Ithaca, New York, the series is fifteen hundred feet thick, and is buried beneath an equal thickness of later strata. It includes two hundred and fifty feet of solid salt, in several distinct beds, each sealed within the shales of the series.

Would you expect to find ancient beds of rock salt inclosed in beds of pervious sandstone?

The salt beds of the Salina are of great value. They are reached by well borings, and their brines are evaporated by solar heat and by boiling. The rock salt is also mined from deep shafts.

Similar deposits of salt, formed under like conditions, occur in the rocks of later systems down to the present. The salt beds of Texas are Permian, those of Kansas are Permian, and those of Louisiana are Tertiary.

THE MISSISSIPPI VALLEY. The heavy near-sh.o.r.e formations of the Silurian in the Appalachian region thin out toward the west. The Medina and the Clinton sandstones are not found west of Ohio, where the first pa.s.ses into a shale and the second into a limestone. The Niagara limestone, however, spreads from the Hudson River to beyond the Mississippi, a distance of more than a thousand miles. During the Silurian period the Mississippi valley region was covered with a quiet, shallow, limestone-making sea, which received little waste from the low lands which bordered it.

The probable distribution of land and sea in eastern North America and western Europe is shown in Figure 287. The fauna of the interior region and of eastern Canada are closely allied with that of western Europe, and several species are identical. We can hardly account for this except by a shallow-water connection between the two ancient epicontinental seas. It was perhaps along the coastal shelves of a northern land connecting America and Europe by way of Greenland and Iceland that the migration took place, so that the same species came to live in Iowa and in Sweden.

THE WESTERN UNITED STATES. So little is found of the rocks of the system west of the Missouri River that it is quite probable that the western part of the United States had for the most part emerged from the sea at the close of the Ordovician and remained land during the Silurian. At the same time the western land was perhaps connected with the eastern land of Appalachia across Arkansas and Mississippi; for toward the south the Silurian sediments indicate an approach to sh.o.r.e.

LIFE OF THE SILURIAN

In this brief sketch it is quite impossible to relate the many changes of species and genera during the Silurian.

CORALS. Some of the more common types are familiarly known as cup corals, honeycomb corals, and chain corals. In the CUP CORALS the most important feature is the development of radiating vertical part.i.tions, or SEPTA, in the cell of the polyp. Some of the cup corals grew in hemispherical colonies (Fig. 288), while many were separate individuals (Fig. 289), building a single conical, or horn-shaped cell, which sometimes reached the extreme size of a foot in length and two or three inches in diameter.

HONEYCOMB CORALS consist of ma.s.ses of small, close-set prismatic cells, each crossed by horizontal part.i.tions, or TABULAE, while the septa are rudimentary, being represented by faintly projecting ridges or rows of spines.

CHAIN CORALS are also marked by tabulae. Their cells form elliptical tubes, touching each other at the edges, and appearing in cross section like the links of a chain. They became extinct at the end of the Silurian.

The corals of the SYRINGOPORA family are similar in structure to chain corals, but the tubular columns are connected only in places.

To the echinoderms there is now added the BLASTOID (bud-shaped).

The blastoid is stemmed and armless, and its globular "head" or "calyx," with its five petal-like divisions, resembles a flower bud. The blastoids became more abundant in the Devonian, culminated in the Carboniferous, and disappeared at the end of the Paleozoic.

The great eurypterids--some of which were five or six feet in length--and the cephalopods were still masters of the seas. Fishes were as yet few and small; trilobites and graptolites had now pa.s.sed their prime and had diminished greatly in numbers.

Scorpions are found in this period both in Europe and in America.

The limestone-making seas of the Silurian swarmed with corals, crinoids, and brachiopods.

With the end of the Silurian period the AGE OF INVERTEBRATES comes to a close, giving place to the Devonian, the AGE OF FISHES.

CHAPTER XVIII

THE DEVONIAN