GEOLOGY

There are, It is true, some circumstances in the physical constitution of our own planet which do obscurely point to an origin and a formation, but from them nothing distinct can be concluded. On the other hand, neither the researches of astronomy nor of geology give us the slightest grounds for regarding either our system, or the globe we inhabit, as of eternal duration; in short, if we would speculate to any purpose, either upon the former state of our globe, or upon the succe ion of events which from time to time may have changed the condition and the form of its surface, we must confine our views within limits far more restricted, and to subjects much more within the reach of our capacity, than either the beginning or the end of the world. These, indeed, were almost the sole inquiries of the older geological school, the favourite speculations of a race of geologists now nearly extinct. Within the limits of the present century even geology has undergone an entire change of character, and is, as we have already remarked, brought within the list of the inductive sciences. Geologists now no longer ransack their imaginations for theories of the formation of the globe from chaos, or bewilder themselves with its hypothetical transformations; but they aim at a careful and accurate examination of the records of its former state, and of the indisputable evidences of former life and habitation whic*h the organic remains of its strata afford. These wonderful wrecks of a former state of nature, preserved, like ancient medals and marbles in the ruins of an extinct empire, afford a kind of rude chronology, by the aid of which the succe ive depositions of the strata in which they are found may be marked out in epochs more or le definitely terminated, and each recognized by some peculiarity which enables the diligent observer to recognize the deposits ofany period, in whatever part of the globe they may occur. And, so far at least as investigation has hitherto gone, these deposits are in the same order qf succe ion in every part of the world. But, in using these terms, we must not forget how very little we do actually know of the earth's surface. We talk of certain strata extending over the whole globe, forgetting that nearly three fourths of it are covered by the waters of the ocean, and that consequently of that portion we can know but little,and that little imperfectly; and that of the fourth which remains in the shape of dry land not one thousandth part has been at all examined, or at least not investigated. Again, we speak of the crust of our globe, and of the boivels of the earth, and the depth of the sea, and the loftine of mountains, forgetting how insignificant all these things really are, compared with the entire bulk of our planet; that our deepest mines hardly penetrate to a depth surpa ing the ten thousandth part of the distance from the surfaca to the centre; that the profoundest rece es are mere scratches upon the surface; that the loftiest hills and most inacce ible mountains are only as grains of dust and of sand sprinkled on the surface of one of our moderate-sized artificial globes. Such being the real state of our information, and such the limitations to which all further pursuit is subjected, we may well be animated by the magnitude and extent of the inquiries which this limited range displays. " And finding, even in these restricted bounds, the amplest proofs of order and design, the mind is naturally led to the sublimest inferences respecting what is unseen, and even to the conception of a power and intelligence to whicn we may well apply the term infinite; since we not only see no limit to the instances in which they are manifested,l)ut find, on the contrary, that they continually open npon us in increasing abundance, in proportion as we are enabled to extend our sphere of observation and inquiry; and that as the study of one prepares us to understand and appreciate another, wonder follows on wonder, till our faculties become bewildered in admiration, and our intellect falls back on itself in utter hopele ne of arriving at an end." We may now state of what the external film or cuticle of the globe appears to consist, and examine how far we are justified in surmising respecting the nature of its deeper-seated parts, and of such portions as are out of the reach of actual inspection. We say that the crust of the earth is stratifkd, because it consists chiefly of distinct strata or layers of difl'erent materials. These differ in depth and extent; but, what is most e ential to our present purpose, they follow each other, on the large scale and as ma es, in an apparently regular and uniform succe ion in all places, districts, and countries where they admit oi examination and have been attentively studied. They appear, in most instances, to rest upon, and are blended with, invaded, and in some few instances overflowed as it were by, substances which are not distinctly stratified, and which most geologists have agreed in calling unstrai{fled rocks. The former, or the stratified rocks, from their texture and contents have apparently been for moil under water; and some of them, esi)ecially the uppermost series, abound in fo il remains of the organized kingdom. The unstrati/kd rocks, on the other hand, are in many in492 ' stances of indisputable volcanic origin; and others, from their position, texture, and effects upon their neighbours, are also presumed to be of igneous formation. Water, therefore, on the one hand, and fire on the other, seem to have been the great agents to which the present aspects of the earth's surface are referable. In proceeding to enumerate the principal divisions of the stratified rocks in their order from above downwards, we must remind the reader that the series is frequently rendered irregular by the deficiency of one or more of its members; and that sometimes whole series of strata seem to have been removed from certain districts by causes which we shall afterwards endeavour to trace, constituting what have been called denudations. In this diagram A represents the crystallized or U7i stratified rocks, presumed to be of igneous origin, upon which lie the stratified rocks, B, C, and D. The rocks A were formerly called primitive, and were supposed to be the original crystalline crust of the globe, upon which the strata BCD had been subsequently deposited; but it is now supposed that the rocks B, C, and D have been upheaved by the subsequent intrusion of A by volcanic agencies from beneath. The substances which usually constitute the ground we tread upon are produced chiefly by causes in daily operation—soils and detritus of various kinds washed down from the neighbouring hills by the influence of rain and torrents, and forming what have been generally called «/luvial deposits: these are more or le a ociated with matters probably of a similar but a more ancient origin, and which, suspected by some to be relics of the deluge, have been termed diluvial deposits; such as gravel and boidder stones. We then find in many parts of England and elsewhere occasional patches of what have been termed tertiary rocks, which often are of singular interest, and frequently consist of detached and insulated deposits surrounded by other rocks, and, in reference to their neighbours, occupying a position very like that of the water of lakes, inland seas, and gulfs, in relation to a continent; and, like such waters, being often very deep, but of limited extent: hence they are often designated as basins. They consist of clays and calcareous and arenaceous matters, in which are embedded remains of marine animals, blended with freshwater species, quadrupeds, and even birds. These lie upon chalk, which Is the newest rock of the secondary series. They constitute the supra cretaceous rocks of which the basins of Paris, London, and the Isle of Wight, and parts of Suffolk, furnish Instances. Chalk constitutes the uppermost of the secondary series of the European area; and it is immediately succeeded by certain var i. eties of sand and sandstone, included by Mr. De la Beche in his Cretaceous group. • We then (in the same area) come.to the Oolitic g:roup, composed of calcareous freestone and of lias, resting upon the varieties of red sandstone, and succeeded by coal, mountain limestone, and old red sandstone; and these are followed by the Greywacke group, forming the transition series of Werner, a part of which has been ably described under the name of Silurian system by Mr.Murchison, resting upon a lower portion termed Cam brian system by Profe or Sedgwick. The Greywacke may be conveniently divided into three portions: — 1. Upper, equivalent to the old red sandstone of England; 2. Intermediate, equal to the Silurian system; and 3. Inferior, equivalent to the Cam brian series. These are often made up of fragments of the older series, or contain a few ancient organic relics. They rest upon the inferior order of stratified non fo iliferous rocks, and upon the unstratified or primary series. In endeavouring to trace out the history of the respective strata to which we have adverted, it would, perhaps, be most philosophical to commence our inquiries with the causes that are now active in producing alterations upon the surface of the globe; to consider the action of changes of temperature, and of water and other agents, that tend to degrade and disintegrate the surface, to wear down its asperities, and to modify the present order of things, — among which earthquakes and volcanos form a prominent feature. But the narrative will be more clear if we first sketch out the history of the for7nations, considering them in reference to their position, their composition, their organic relics, and the purposes to which they are applied in the arts; and then, having adverted to their relative durabilities and aspects as mountain ma es, we may consider the action of rivers, and of tho ocean, both as destructive and as renovating agents, tracing out their connection with meteoric phenomena, and with the eflects of volcanic fires. Wc shall thus be Page 506