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Looking on granite as the rock which lies at the base of all the other rocks of which we have certain knowledge, and tracing the various deposits which overlie it, we get a view of the composition of the earth's crust. It is nevertheless to be kept in mind that this remark does not extend to all granite. There is proof that in some quarters it is not older than the coal measures; in others it is even of much more recent date. Different masses also vary a good deal in their structure and chemical characteristics.
It would be out of place here to enter on any of the theories which have been urged in regard to the mode in which this rock had been formed—whether by the action of fire or of water. Enough for present purposes that we have proof, that it is composed of minerals which may have singly been the main constituents of rocks which lay nearer the beginning than the oldest members of this one did. We, however, can get no standing point nearer the origin of the material earth than this. But even there we meet with much suggestive of the presence of the all-wise and the all-powerful God. He is seen controlling mighty forces, called by himself into being and action, and making all obedient to his sovereign will. The work of rearing the earth on foundations laid by him had begun, and—for man need not shrink from the frank acknowledgment of this, or from all the consequences—the forces then created have continued in action even till this hour, subserving ever his purposes, waiting on him as ministering spirits always ready at his bidding. To the divinely-guided action of these, men have agreed to give the name of “natural law.” This, which is virtually one of the modes in which God makes himself known, has been made an idol of by many, who have put it in a place of honour which he claims by whom it is unerringly guided. It has, notwithstanding, ever been as the hand by which the mind of the Almighty, as creator, found expression. Through it we see him controlling all matter, wielding its atoms in order to most remarkable results, and, while accepting the ever-varying circumstances, themselves divinely ruled, through this same natural law he makes these all subservient to sublime, beautiful, and useful ends. The most remote point then, in the history of the crust of the earth, is not less suggestive of a pre-existent Creator, than the results of the six days work are.
The relation in which the stratified rocks lie to the oldest granite may be represented thus :
The unbroken relation between the unstratified granite and the overlying stratified rocks shown in this cut, is not to be expected as constantly occurring in the field. The present is not the natural condition of the earth's crust. Forces which at different periods have been brought into terrible action, and others which have been constantly at work up through all the geologic periods, and are still at work, have broken it up at different epochs; and, as the results of volcanic action and denudation, we have the present rugged and unequal character of the surface of the earth. Granitic and trappean masses have penetrated the stratified rocks—in some instances raising them from a horizontal into a vertical position, in others forcing them into well-marked contortions, and in others pervading the broken strata, as shown in the accompanying figure.
Turning from the unstratified crystalline rocks which may thus be regarded as “the foundations of the earth,” we next meet, in the
ascending scale, a series of deposits generally known as the primary stratified rocks. These bear evidence of having been formed from the ingredients of the great unstratified masses now naturally underlying them. And it is worthy of notice at this point, that the Creator has shown his power and his control over matter, as much in the varied
results which he has produced, in bringing the action of natural laws to bear differently on the same substances, as he would have done by direct and absolute acts of creation. The strata now under notice supply an illustration. There is not the least likelihood that these rocks were created as they now are. Matter sufficient for their formation was already stored up in the unstratified granitic masses, and all that was needed was the exposure of these to the wearing power of heat and cold, to prevailing atmospheric influences, and the action of water. Thus a process of disintegration would be begun and carried on, until the loose ingredients were sufficiently abundant to form strata of a certain thickness. These, under laws everywhere at work around us, would in time assume the stratified character which they now have. Thus too, by the accumulation of the loose particles of granitic rocks under one condition, we obtain the well known gneiss; under another condition we have the mica schists; and under yet another condition clay slate is the result. These, with one or two more into which quartz (rock crystal) enters as a chief ingredient, are named the metamorphic rocks. They are so called because of the marks which they bear of having undergone a hardening process after having been naturally laid down, which has given them the appearance of having been subjected to great heat.
The Cambrian series lies above the metamorphic rocks, with which indeed they have several mineralogical resemblances, though in other respects they differ widely. It would not be profitable to enter here on the much controverted relation of this series to the highly fossiliferous rocks that next meet us in the ascending scale. This would not agree with the present design, which is not the discussion of questions in physical geology, but simply the indication of the succession of strata, in order to an intelligent acquaintance with the theories of creation which follow. In the alleged uppermost layers of the Cambrian the first traces of life on the earth are met with. The importance of . bearing in mind the order in time at which one form of life and another must have been ushered into existence by the Creator, will be apparent
Oldhamia antiqua. when it is remembered that several of the schemes of so-called reconciliation between the Book of nature and the Word of God must be mainly tried by this.
The strata in which these first traces of life appear, are known to geologists as the bottom or Longmynd rocks, a name given to them by those who hold that they lie at the bottom of the Silurian series; they have their chief development in Britain in the Longmynd hills in Shropshire. Whatever may be their place in the Cambrian system, there is no doubt that they lie naturally deeper down in the earth's crust, than those strata hitherto known as the Lower Silurian. The
traces of organized being now referred to, consist of what seems to be a fragment, perhaps the tail-plate, of a trilobite, a characteristic fossil of the Silurian, and the holes bored by an annelid resembling the arenicola or the lob-worm of our coasts, used by fishermen as bait. These are in many respects full of interest. They show that at that incalculably remote period the world existed in a condition like the present, in regard to the action of natural laws and the play of the elements. The sea ebbed and flowed on sandy shores, and the light looked down from the deep blue sky even as now. Sir Roderick Murchison has pointed out, not only that the Longmynd slab discovered by Mr. Salter, with the marks of the trail and the burrows of the lob-worm, has distinct traces of the wave ripple and of the rain drop, but also that, as may be seen on the partially dried sand of our own beaches, "the holes left by the Annelides are most conspicuous on the sheltered sides of the ripples," and that “even cracks produced by the action of the sun on a half-dried surface” are evident.
In what are regarded the equivalents in age of the Longmynd rocks in Ireland, Dr. Oldham discovered in the schists of Bray Head a zoophyte, which was named by the late Edward Forbes Oldhamia antiqua, in honour of its discoverer. Associated with this were abundant traces of the lob-worms and occasionally of fucoids—plants resembling our sea-weeds. The cuts given above are copied from the last edition of “Siluria," and represent the first traces of life now noticed.
When we leave the uppermost Cambrian and enter the Silurian system, we find ourselves everywhere surrounded with teeming evidences of former life. Corals, crustaceans, and molluscs everywhere abounded in its sea, which must have in many places reached a great depth. In its uppermost layers fishes have left traces of their presence, and here and there sea-weeds must have flourished, while evidences of land vegetation are at least not awanting, for a few seed cases and twigs of land plants have been detected. “The discovery," says Murchison, "by Professor Nicol of some imperfect reed-like plants in the rock (Peeblesshire silurian), with a minute vascular or tubular structure in the burnt residue of anthracite, has led to the suggestion that some sort of grassy vegetation existed on the adjacent lands during this ancient period.” Well marked remains have been found at the top of the Silurian, at Stoke Edith and other places in Herefordshire, of Lepidodendra, species of gigantic club mosses, and characteristic plants of the great coal formation.
On Plate I. the figures of certain silurian fossils are given. The