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igneous rocks immediately beneath, and partly with those incumbent upon them. They seem to have been originally formed by deposition, but afterwards altered by the action of fire. The chief varieties are gneiss, slate, and mica-schist. Next in order, as we proceed upwards, are the rocks called primary. They include the Silurian strata, the old red sandstone, the mountain limestone, and the coal measures. Above these are the secondary rocks, comprehending the new red sandstone, oolite (a kind of limestone), and chalk. Last of all come the tertiary rocks, consisting of several later formations, over which are spread various sands, gravels, clays, peat-mosses, and other accumulations of historic times.
It does not necessarily follow that all these strata are found together in every, or even in any locality. One or more of the series may be wanting, but the natural order of those which do occur is never inverted. Thus, if we take the letters of the alphabet to represent the strata in the order of their formation, H may come immediately after A, but can never come before F or G. We may have such a sequence as A, P, T, but never A, T, P. This is a point of great importance. For example, the coal measures lie below the new, but always above the old red sandstone. If, then, in any locality, we have the latter at the surface of the ground, it is hopeless to search for coal there; whereas, if the former be at the surface, there is a chance, but no certainty, that beds of coal will be found below.
It is obvious that all the strata, when originally formed, must have been horizontal; but, instead of this, they are now found with every variety of slope, having been upheaved and disrupted by forces acting from beneath. In many cases the igneous rocks have been forced up through the superincumbent strata, and form huge hills, on whose summits they appear at the surface, while the edges of the broken strata are ranged in order along the flanks. The
* It ought to be mentioned that there is a great want of uniformity among geologists in regard to the names by which different strata are distinguished. The igneous rocks, for example, are often called primary, and those here denominated primary, are then ranked among the secondary. By others, again, a totally different set of names is employed.
actual state of the earth's crust will be best understood by a reference to the accompanying illustration. At a (fig. 5), the igneous rocks are on the surface. The earliest or lowest strata begin to appear at b, and the edges of others are passed over in succession as we proceed towards c. The same strata occur, with a still greater slope, between 9 and
h; but here they are covered by later formations (f, k), whose horizontal position shows that they must have been deposited after the forces, by which the surrounding rocks were upheaved, had ceased to operate. The eminences at c are formed of volcanic matter, ejected through the opening seen at e; and the dark lines between b and d represent fissures in the various rocks, produced by earthquakes and similar convulsions, and then filled up, from subterranean treasures, with metallic ores and other substances in a state of fusion. It is from such fissures, usually called veins, that most of the metals and their ores are obtained.
THE most remarkable phenomena, which an examination of the earth's crust presents to us, have not yet been referred to. At an early, period in the formation of stratified rocks, vegetables had begun to clothe the surface of the earth, and animals to people its waters. Of these, and especially of the latter, the remains of successive generations were
buried in the deposits from which rocks were formed, and are still preserved, petrified and encased in the surrounding stone, as memorials of the early inhabitants of our globe. Such remains are known by the general name of fossils.
It is only by examining the fossils which rocks contain, that we can determine accurately to what period the rocks themselves belong. Strata which were unquestionably deposited at the same epoch are often composed of different materials; and, on the other hand, strata composed of the same materials sometimes differ widely in the date of their formation. This would lead to very great uncertainty in the whole science of geology were it not that fossil remains, of which each stratum has its own peculiar collection, afford evidence of a much more exact and reliable character.
In the lower strata of the primary rocks, fossils are not plentiful, and those that do occur represent only the humbler varieties of animal life. But, as we ascend, skeletons of huge fishes begin to be met with, and these are followed by a few species of reptiles, which seem to have been the first living inhabitants of the land. At the remote epoch to which we now refer, the earth must have been covered with a luxurious and splendid vegetation. The layers of coal, which abound in the upper primary formations, consist entirely of fossilized vegetables; and, as these layers are sometimes sixty feet in thickness, it seems difficult to explain how an accumulation of trees, plants, and foliage, could ever be produced in such enormous quantity. More than two hundred species of plants have been distinguished in the British coal measures, and far greater numbers in those of other countries. Thus we see that a beneficent Creator had stored up for the use of man, when as yet the race of man was not, vast supplies of this substance, so necessary to his comfort and progress in civilization.
The fossils of the secondary rocks are extremely varied and numerous, but still mostly limited to the lower orders of the animal creation. The first certain traces of birds are found in the chalk formation. Their number, however, is
very limited, until we come to the tertiary period. Then, for the first time, appear some of the same species of animals which still exist, and their number gradually increases till the latest strata are reached. The former inhabitants of the globe differed from the present in many of the minuter details of their structure, though they agreed in all the more essential principles.
But besides the occasional occurrence, in all these strata, of organic remains, immense layers of rock are entirely formed of animal and vegetable substances. Coal has been already mentioned. The limestone, with which it is often associated, is composed, to a large extent, of minute shells and corals, the remains of animals which could have lived only in the sea. Even at the present day, the coral is building up, throughout the Pacific Ocean, rocks and islands of great extent, which may become the continents of future A celebrated naturalist mentions a stratum of rock in Germany, not less than fourteen feet in thickness, which is composed exclusively of the shells of animalcules, so minute that 40,000,000,000 of them would not fill a space greater than a cubic inch!
Since each stratum contains the remains of the organized tribes which inhabited the earth at the time of its deposition, we have, in these remains, so many museums presenting to us specimens of the zoology and botany of the globe in successive ages of its history. It is remarkable that among the numberless varieties of these fossils, no trace of man has ever been found, except in the accumulations which have been deposited since the present order of things commenced. Thus science coincides with revelation in testifying that man is the latest, as he is the noblest, of God's creatures upon earth.
Many have rashly and impiously argued that the discoveries of geology are at variance with the history of creation revealed to us in the Bible, but the most eminent geologists are of a different opinion. It is not our business here to enter on such a question, but we may rest assured that the Bible will never suffer from the discovery of truth.
QUESTIONS FOR EXAMINATION,
Of what does geology treat? What is meant by the crust of the earth? What substances do geologists call rocks? What do we know of the earth's interior? State some of the effects of volcanoes. What rocks owe their origin to subterranean fire? What are aqueous rocks? Explain how the dry land is being disintegrated? What would follow if this disintegration were not counteracted? What counteracts it? State some of the effects of earthquakes. Why are aqueous rocks sometimes found far from the sea? What are strata? Which rocks are stratified? Which unstratified? What are transition rocks? primary rocks? secondary rocks? tertiary rocks? Why were all the strata originally horizontal? Why not so now? What are fossils? What rocks are formed entirely of fossilized vegetables? of fossilized animals? Are there any human fossils? What is inferred from their absence?
GOD HATH A VOICE.
[ELIZA COOK, the daughter of a tradesman in Southwark, London, was born in 1817. The "Old Arm Chair," "Old Farm Gate," and many other of her pieces are stamped with the truest character of poetry, and are universally admired.]
GOD hath a voice that ever is heard
In the peal of the thunder, the chirp of the bird;
God hath a presence, and that ye may see