Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 27

Let us walk in front of the precipice which frowns along the hill-side near the village of Panama, on the west. It is no more instructive than a thousand other cliffs, but it may be more convenient to reach. The cliff rises fifty or sixty feet and presents a broken and rugged front. The brown and yellowish rock is composed of fine silicious grains, with small imbedded pebbles, and thus answers the description of a conglomeritic sandstone. The face of the cliff shows several yawning fissures extending from bottom to top. The winter snow drifts into these in such abundance as to remain, sometimes, till midsummer. One of these chasms is known, therefore, as the “Ice House.” You observe that this precipice is composed of layers of sandstone piled one above the other. These are strata, and the whole formation is stratified. [Notice that one of these layers is a stratum—not “a strata;” and we must never say “stratas.”] You observe, also, that some of the strata are composed of laminæ which run obliquely across the stratum. This is oblique lamination. It is of the same nature as we saw in the semi-stratified drift. We concluded that such mode of arrangement was caused by torrential action. A similar explanation is allowable here, but the water was less turbulent; it was, perhaps, wave action along a beach.

At Watkins’ Glen, at the south end of Seneca Lake, is a wild, deep gorge cut by a stream which rushes down from the highland on its way to the lake. It is a striking example of erosion, and the materials carried away are deposited in Seneca Lake. The rocks here are shales. They are thin-bedded, and soft enough to be cut with a knife. We see no oblique lamination. This is a fine example of another sort of strata. At Rochester, where the Central Railroad crosses the Genesee river, a few rods above the Falls, we look down into a gorge eroded by the river. The high walls of the gorge are distinctly stratified; and here many of the strata are composed of limestone. No traces of oblique lamination can be found in limestones. If we go to Portland, in Connecticut, we may look down into wide and deep excavations in a sandstone rock of a brownish color, and very evenly bedded. Near Cleveland, and at Berea, Ohio, are extensive quarries in a grayish and bluish gray sort of sandstone. At Cincinnati, back of the city, we find a steep slope formed of beds of limestone, shale, and clay. Descending the Mississippi from St. Paul to St. Louis, we see high cliffs of buffish strata overlooking the river at frequent intervals—now on the west, now on the east. At St. Paul the rocks are distinctly stratified limestone. At Davenport and St. Louis we find other kinds of limestones.

Now, I have directed your attention to these few examples out of hundreds for the purpose of enabling you to understand that everywhere solid rocks underlie the Drift; and that they are, at least very generally, stratified rocks, and are composed chiefly of sandstones, limestones, and shales. Let us consider how these solid strata have been produced. None of these have we ever seen making; but I think we have seen a process similar to rock-making in the beds of alluvial matter deposited by an overflowing stream. In traveling down the lower Mississippi, we can see from the deck of the steamer that the material of the alluvial banks is horizontally stratified. More strictly we should say that it is laminated; but the nature of the geological work is the same in either case. Now, if those alluvial banks should become firmly consolidated, they would present the appearance of some of the rocky cliffs—those in Watkins’ Glen, for instance. You have also learned how large quantities of sediments borne down by rivers are carried out to sea many miles, and slowly deposited on the ocean’s bottom. These deposits must necessarily be in layers, each of which is spread evenly over the bottom. You remember that the distance to which materials of a certain degree of coarseness may be carried before sinking to the bottom, depends on the velocity of the motion of the water. At a certain place in the sea the velocity is undoubtedly more rapid at one time than another. The motion is caused by winds, by tides, and by currents. Therefore, a coarser sheet of materials will be laid down at one time, and a finer sheet at another. The alternations of coarser and finer render the bedded arrangement conspicuous. Very likely the colors of the sediments will vary also; since, from one direction, they may be supplied by pulverized limestone, from another by pulverized sandstone, and from another by pulverized shale, which may be blue, red, or black. We noticed, too, in our walk under the sea, that sedimentary materials are spread over all the slope of the ocean’s floor, within fifty or a hundred miles of the land—often much farther, if the shore is “shelving” or the currents are favorable.

These various indications compel us to adopt the conclusion that water has been the agent by which the materials of the stratified rocks have been spread out in broad beds or strata. But, though river overflows must leave the sediments in a bedded condition, these beds are not exactly like those seen in great formations of limestone and sandstone. River sediments never have so wide an extent as the strata which underlie a continent; nor are they generally so evenly bedded as our ordinary rock-strata. We must conclude, therefore, that the watery action which arranged the sediments from which our rock-strata have been formed, was a very widely operating action. There is no watery action known sufficiently wide-spread except the action of the ocean. In the ocean, sediments are now settling down in sheets a thousand miles broad. This conclusion is a somewhat startling one. It implies that, wherever rocky strata exist, there the ocean’s waters have stood. Rocky strata are found hundreds of feet above the level of the ocean, and the fact seems incompatible with our conclusion. The average level of all the northern and northwestern states is from six hundred to a thousand feet above the sea. If the underlying strata were deposited by the ocean, then either the ocean has greatly subsided in later times, or regions which were once sea-bottom have been extensively uplifted.

These subjects have attracted the attention of thoughtful observers for a century—indeed, for two or three centuries. The question has been much discussed; but no doubt is longer entertained that the sea has covered all the land, and that the exposure of land has resulted from upheaval of portions of the ancient sea-bottom. Many confirmations of this view will be discovered as we proceed. Thus by a very simple and easy process of observation and reasoning we have reached a very fundamental principle in geological science; and you understand the evidence on which it rests.

Now, if all the strata which underlie the land are formed from marine sediments, the time required for their accumulation must have been enormous. We have made observations along the sea-shore, and have formed some conception of the rate of sedimentation over a belt near the land. There are times when violent winds cause the waves to wear down the shore at such a rate that the sea, for a mile from shore, becomes turbid with sediments. This has been seen often at Long Branch and Coney Island. But these periods are of short duration, and the deposits at the distance of ten miles from land are no longer conspicuous. In the vicinity of coral reefs and islands the attrition of the waves imparts a milky complexion to the sea, especially during the prevalence of a storm, and the calcareous particles are floated sometimes a hundred miles and more. But it is apparent that, as a rule, the sea floats too little sediment to build up a formation in any other than a very gradual manner. We noticed, also, in our walk under the sea, that the bottom sediments grew thin with distance from the shore, and that those of continental origin ceased entirely at about two miles in depth. When now we remember that the stratified rocks are over a hundred thousand feet in thickness, we perceive immediately that the process of sedimentation has been an extremely long one.

We have then to consider what changes may have taken place in the conditions of the world during so long a period. Probably the nature of the sediments has been changed from time to time by these changes in the physical conditions of the planet. We do not wish to anticipate conclusions to be rested on facts which have not yet fallen under our observation; but every body has noticed that the surface of the earth is undergoing changes; and these, in thousands of years, must aggregate amounts which transform the aspects of the planet. We have lived to see lakelets filled; new channels formed for great rivers; ocean beaches consumed by the waves; hundreds of miles of continental coasts upraised or sunken—as in Chili, Scandinavia, and Greenland; new islands bursting into view; whole provinces shattered by earthquakes. Suppose our observation extended back a million years, and the tenor of events had been the same as in modern times; is it not certain that changes must have aggregated to such an extent that, waking at times to distinct consciousness of the greatly changed conditions, we should from æon to æon have felt ready to declare a new chapter of the world’s history had begun? I think so—reasoning only from the physical data, which, so far, have engaged our attention. But we shall hereafter make the acquaintance of many other facts which will confirm this conclusion. Geologists have considered these facts, and have settled on the principle that the long history of sedimentation has been divided into æons corresponding to successive conditions of the world. Names have been assigned to these æons. Thus, the first series of sediments formed the strata which lie deepest of all. They are called Eozoic, and the æon during which they were accumulating is the Eozoic Æon. We will not pause here to inquire what these sediments rested on—in other words, what kind of rocks formed the bed of the sea, at the beginning of that Æon. The ocean must have had some solid bottom; but of course, it was a bottom formed when there was no ocean; for otherwise, the Eozoic strata would not be the bottom strata.

The Eozoic Great System of strata is at least fifty thousand feet thick. In the next æon the changed conditions gave origin to changed strata. They constitute a Great System known as the Palæozoic; and the time during which this system of strata was accumulated, is the Palæozoic Æon. Next after this, came the Mesozoic Æon, during which the Mesozoic Great System of strata was accumulated. Lastly, followed the Cæn´-o-zo-ic Æon, which continues to the present. The strata formed constitute the Cænozoic Great System. Now, before we take another walk, these names must be well learned.