Books. ART. V.-1. Siluria. By Sir R. I. Mur- 3. Recent Discoveries in Geology and 4. The Geology of Pennsylvania. A 5. Etudes sur la Métamorphisme des P'Ecole Normale. 6. Memoirs of the Geological Survey of the United Kingdom. THE object we propose to ourselves in this article is to endeavour to explain to non-geological readers what is the present state of geology, both theoretical and practical,-what it is that the geologists of the present day profess to know, and what it is they profess to do. We may say briefly in the first place that they pro fess to know the structure of the external crust of the globe, in some parts pretty accurately, in others generally, and in others again vaguely; and they profess also to know, more or less fully, the way in which that structure was elaborated. What they profess to do is to apply this knowledge to the examination of one disRe-trict of the earth after another, so as to make out their internal constitution, sometimes to a depth of many thousand feet. 7. Geological Survey of Canada. 9. Memoirs of the Geological Survey of 12. On Lavas of Mount Etna formed on Steep Slopes and on Craters of Eleva tion. By Sir C. Lyell. From Phil. Transactions. 1858. 13. Report on the Copper Mines of South Namaqualand. By Andrew Wyley, Esq., Geological Surveyor of Cape of Good Hope. Every fresh district that a geologist examines adds to his store of facts, and strengthens his theoretical ideas as to the origin of the structures he observes. Fact and theory are indissolubly united, it being the test of a sound theory that every fresh fact finds a place in it,-enlarges what was deficient, explains what was obscure, cuts away what was superfluous, reconciles that which appeared contradic tory, and makes the theory different from what it was before only in being more complete and harmonious. Geological theory is by no means perfect, since there are still differences of opinion among geologists upon certain parts of it; while all geologists agree in confessing their ignorance on some other parts. That the agreement among men of science on the principal conclusions does not rest on insufficient grounds, may be inferred from the results being the conjoint but independent work of many hundreds of acute investigators of different nations, working in almost all parts of the globe. Any one of the tribe would be ready to point out the errors committed by any of the others, both for the sake of truth itself and for his own credit. Such corrections, indeed, upon minor points are continually appearing. That man would. acquire an almost unbounded fame who could prove that any important part of the theory of geology was erroneous; yet all who study the subject unite in upholding its general truth, if indeed with respect to some of them it may not be more appropriately said that they have, against their previous convictions, been obliged servations in deep mines, and on the temto confess it. The order in which we shall perature of the water of deep wells, inhere proceed is to state, first, the theo-creases internally at the rate of 1° Fah. for retical conclusions at which geologists every 50 or 60 or 100 feet of descent. have arrived; and secondly, the nature of 4th. The Volcanic action, consisting in the facts on which those conclusions are the ejection of intensely-heated matters based. from a deep source in the interior of the earth, which is now going on, and appears always to have been going on from the earliest geological period to the present day. The theoretical history of the crust of the globe, meaning by the crust a thickness of some ten miles, or about part of the distance from the surface to the centre, is deduced from the examination of its complex structure, which bears evident marks of the way in which it was produced. Like many human histories, however, there is no proper commencement to the geological history of the earth. We can trace it back till we lose ourselves in uncertainty for want of data. We arrive at what we may call a mythical period, when the scanty facts must be grouped together as a mere hypothesis. This hypothesis is that at one time the earth was a molten globe, containing the constituents of our present rocks in a state of fusion. The elements of the atmosphere and the water could then only have existed as greatly expanded gases surrounding the fluid. The hypothesis involves the conclusion, that, when the crust of the earth first cooled, it assumed very nearly the shape and dimensions it now has; that, in the first instance, all the rocks on the crust of the globe were such as had consolidated from fusion,-in other words, were igneous rocks; while, as water could not exist till after these were formed, and does not in itself contain the constituents of any rocks, it follows that all rocks deposited from water, must be derived either mediately or immediately from igneous rocks. This hypothesis certainly explains better than any other the following undoubted facts: 1st. The form of the Earth, which is exactly that which it would have assumed, if the hypothesis be true, namely, the form of a spheroid, bulging th part of its diameter, or to an extent of some 13 miles on all sides about the equator, as if in obedience to a centrifugal force acting on a rotating fluid. 2nd. The Specific Gravity of the Earth, which is not greater than 5 or 6 times that of water; whereas its density must apparently have been much greater, if it had not been kept out to its present dimensions by the expanding force of internal heat. 3rd. The present internal Temperature of the Earth, which, according to all ob The conclusion as to the origin of aqueous rocks would also agree well with the supposition, since in any case all those rocks are derived from some others, and, as there are only two classes, those others must have been igneous. Still we hold it safer not to admit at present the original igneous fluidity of the earth as part of the geological theory, but to keep it distinct as an hypothesis which does not affect the truth of the history we have to relate. Whatever may have been the primitive condition of the earth, we can show that a period of such vast extent as to be almost inconceivable by human faculties has elapsed since it first became a habitable globe, with the same general features as at present, having, that is, its surface parcelled out into seas and oceans, islands and continents, the land being diversified by mountain, plain, and valley, with lakes in the hollows, and rivers flowing between the slopes, the whole enveloped in the same atmosphere as now exists, with the same circulation of moisture and a similar system of winds and currents. The mere place and outline of the dry land has frequently changed. Most of our present dry lands have been deep sea, and then dry land, and then deep sea again, several times and the same thing has probably happened to those parts of the earth's sur face that are now covered by water. The solid crust of the earth seems to have been always subject to a gentle fluctuating movement of elevation and depression, affecting first one area and then another, while large parts remain stationary for long periods, until they in their turn are moved and the others left at rest. We may look upon the dry land of any period, therefore, as merely so much of the solid surface of the earth as happens to be taking its turn to stand above the level of the sea. cording, however, to the latitude and the form of the dry land, the extent and direction of its coasts, the altitude and the bearing of its mountain chains, and their relation to prevailing winds and currents, great modifications might be produced in the climate of different parts of the earth. Ac These modifications being allowed for, we may still assert that the general condition of the globe, even at the very beginning of our geological history, was the same as at present. Now let us, merely for the convenience of our narration, suppose the vast length of time, since the Earth assumed its present conditions as a habitable globe, divided into three great portions, which we will call Primary, Secondary, and Tertiary; or if we choose to refer to the life upon the globe and use Greek instead of Latin terms, we may call them Paleozoic, Mesozoic, and Cainozoic-that is, the times of the ancient, the middle, and the modern life. Let us use the term Epoch to designate these great divisions of time,* and let us subdivide each into minor but still vast durations, which we may call Periods. For the names of these Periods we must use words that have been derived from notions that had no connexion with chronology but disregarding altogether the literal meaning of the terms, and taking them merely as names without any particular signification, we shall have the following chronological scheme. A. Primary or Paleozoic Epoch. 2. Lower (or Cambro-) Silurian Period. 4. Devonian Period. B. Secondary or Mesozoic Epoch 7. Triassic Period. 8. Oolitic or Jurassic Period. 9. Cretaceous Period. C. Tertiary or Cainozoic Epoch. 10. Eocene Period. 11. Miocene Period. 12. Pliocene Period. 14. Modern or Human Period. Each of these first thirteen periods had a vast indefinite duration, during which great changes occurred in the structure and physical geography of the surface, as also in the species of animals and plants that inhabited it. Large additions were in each period made in places to the de * Epy, although a vague term and generally used to mark the point at which any space of time commences rather than its whole duration, is now extensively employed in the latter sense. posits that now enter into the crust of the earth, and those additions necessarily involved equally large destruction of other parts that had been previously formed. Let us first speak of the purely physical or inorganic and afterwards of the biological or organic changes. From the commencement of the Primary or Paleozoic Epoch, there have been two great agents at work upon the crust of the globe-the one that of water acting from the outside, the other that of fire or heat acting from the inside. Moving water, whether it be the breakers and currents of seas and lakes, the floods of rivers, the fall of cataracts, the grinding of glaciers, or the transporting powers of icebergs, is always at work in separating rock into blocks and fragments, and wearing it down into gravel, sand, and mud. Water also, especially when it contains carbonic acid gas, which almost all water naturally does, exercises a solvent power on carbonate of lime and other minerals, and thus dissolves the rocks which contain them. In these ways water is always active in transporting the materials of which rocks are composed from higher to lower levels, and finally depositing those materials on the beds of lakes and seas, either as a sediment or a precipitate, according as the particles were held by it in mechanical suspension or in actual solution. The rocks which are deposited from mechanical suspension are every variety of mud, clay, sand, and gravel, which, when compacted together, either by pressure or any mineral cement, become marls, shales, slates, sandstones, gritstones, puddingstones, and conglomerates. The principal rocks which are deposited by precipitation from chemical solution are gypsum, rock salt, and some varieties of limestone. Other kinds of limestone, and those the largest and most frequent, seem to be entirely made up of the shells, coats, and bones of animals that secreted the carbonate of lime from its aqueous solution, and thus rendered it capable of conversion into a rock, after the hard parts of the animals had into a calcareous sand or mud. Coal, in been broken up, and more or less ground all its varieties, on the other hand, seems the plants which had in like manner seto be chiefly, if not entirely, composed of creted and solidified the carbon of the atmosphere. All the rocks thus deposited under water occur in regular beds or strata, and are therefore called Stratified as well as Aqueous rocks. The amount of the erosion effected by water in any short period of time seems now, and would always have seemed, very | have been derived from the destruction of insignificant; but as the action is unceas- old, either by erosion, or by solution, or ing, its ultimate amount must be great by both combined. when continued through millions of ages. If, therefore, each of the periods mentioned in our chronological scheme had such a duration, it must of necessity follow that large tracts of dry land were gradually washed down, and the materials were converted into corresponding sheets of rock on the bed of the ocean. These would spread over greater or less areas, and consist of coarser or finer materials, according to circumstances; the coarser materials being less widely diffused, the finer spread in broader and thinner sheets, just as we know that water heaps up pebbles and sand into ridges and mounds, while fine mud is floated for scores of miles and may be carried even hundreds of miles by gentle under-currents before it finally settles upon its bed in the sea. In examining the stratified rocks, we find them exactly such as we should expect, assuming that they were deposited by the methods now described. The strata formed in each of the periods mentioned in our table swell out in some part of the world or other to a thickness of several hundred or several thousand feet, and as the beds, which are regularly deposited one upon the other, are always such as must have been derived from the waste of previously existing rocks, we get a measure for the amount of the erosion that took place, and therefore of the time required for the operation. During each period then there were areas of destruction and areas of production, the latter being generally the neighbour of the former. Other districts there would be, either of dry land not subject to the wearing action of water, or of seabottom too remote from the source of supply for any deposition to take place. These we may speak of as areas of neutrality. On many parts of the sea-bottom, however, where no mechanically-transported materials were thrown down, there might be formed the basis of future limestones by the accumulation of the hard coverings of animals. On some portions of the dry land also thick vegetation might be growing which would afterwards form beds of coal either on the spot where they grew or be carried by floods to fresh places. Nevertheless, the production of any kind of aqueous rock in places where no portions of the earth were borne by water must always have been the exception rather than the rule; and in all cases, except that of coal, the new rocks must It would follow then from these data, that, if there had been no counteracting influence to compensate for the effects of the levelling power of moving water, the whole of the dry land that existed at the commencement of the Paleozoic epoch must long since have been worn down and its materials strewed over the bed of the sea, in which case the entire surface of the earth would have been converted into a weary waste of waters. That this is not the condition of the world is due to the action of fire or heat, a power which has always been at work beneath the surface, lifting up the rocks formed on the bed of the sea into dry land, or raising the dry land to a still higher level than it had before. Elevation has taken place in one region during one period, in another region during another period, and was most probably accompanied by corresponding depressions in neighbouring districts. The upheaving was commonly caused by the intrusion of molten rock into the part of the crust that was being raised; and when the pent-up lava occasionally forced its way through the surface, eruptions took place and volcanoes were formed. What may be the proximate cause of this determination of internal heat towards the exterior, now in one part and now in another of the globe, we do not yet know, and perhaps must ever remain in ignorance. We can, however, watch its effects even in our own day. Sweden and Norway are slowly being lifted out of the sea at the rate of from one-half to one-tenth of an inch per annum. The west coast of Greenland is as gradually sinking. Other parts of the world may be rising and falling without any one suspecting it, for, except on the margin of the sea, where a natural standard of level exists, and can be readily observed, the change might take place and no man be aware of it. The whole coast of Chili again (and probably the whole sea-margin of the volcanic chain of the Andes) is on the ascent, not progressing by a gentle and gradual motion, but by little starts and wrenches. These occur at intervals of some years, and produce on the surface the undulation of the earthquake. The shocks which occur in many other parts of the world are in all likelihood the indication of some sudden movement of elevation or depression in the mass of the rocks below. In Sir C. Lyell's Principles of Geology' ample details will be found of these phenomena, while Mr. Mallett's Earth-mouth of its funnel-shaped channel, and quake Catalogue' will give some notion of heaped up all round it; and thus is formed the frequency of convulsive movements another class of mountains, made, not by during the last two thousand years. Who- uptilting, but by up-piling and conical acever considers what the extent of that cumulation.* power must be that can raise a district The phenomena we have detailed are larger than all the British Islands from a the surface-effects of the internal heat of depth of thousands of feet, and lift the the globe; it has others, however, which vast mass a single inch higher than it was are more subterranean, and which only before, or could thrust up all Chili with become exposed to view when the once the Andes on its back eight feet at one deep-seated parts of the crust are protremendous heave, will not need to ima- truded. It is then found that huge masses gine any other force as requisite to pro- of igneous rock have been thrust in duce the elevation of continents or moun- among the beds of the aqueous rocks betain-chains. If the forces now acting un-low, without having reached the surface. der Lapland continue at the same rate for a thousand centuries, the North Cape will be 5000 feet higher than at present, and a thousand centuries can be but a small part of one of the great geological periods into which our history of the world is divisible. When this force acts over a broad area countries will be elevated en masse; when its effects are exerted along lines, mountain chains will be formed, with the beds tilted up along the axis or central line. When the tilting of a mountain chain is commenced under the sea, and is carried on slowly till the crest of the ridge is brought to the upper surface of the water and begins to appear as dry land, the breakers and currents will act upon it; and if the elevation be very gradual great erosion of the upper strata will take place about the axis of the chain, till, as the upward thrust is continued, lower and lower rocks successively emerge. The consequence will be, that when finally the mountain chain rises into dry land, the lowest rocks will be shown at the surface in the most central part of the chain reared into high inclinations by the elevating force from below, and uncovered by the action of the denuding force from above. All the atmospheric agencies, too, will exert an influence upon these tilted and fractured rocks, and will deepen the gaps and hollows sketched out by the breakers of the ocean, and thus produce the ravines, and glens, and valleys, which furrow the sides of the growing mountains; while the sea perhaps still continues to eat away the base of the lower cliffs, and thus forms all the passes and the precipices which we are apt to attribute to more convulsive causes. Where the fiery interior of the earth bursts out at volcanic orifices, it not only pours forth streams of lava, but accumulates great piles of ashes, cinders, and rock fragments, which are blown from the In this situation the lava has cooled, and produced either large irregularly-shaped bulks or more regularly-formed sheets that stretch between or across the beds, or else has run into veins, often branching, tortuous, or reticulated. Every crack and crevice in the including or overlying rock having been injected, proves that the material was perfectly fluid at the time. All the Granitic and some of the more crystalline of the Trappean rocks have been thus formed, cooling and consolidating below the surface. Mr. Sorby, indeed, has shown the great probability of some of our Granites which are now at the surface having originally cooled at a depth even of 60,000 or 70,000 feet. These igneous masses have exerted just such an effect on the aqueous rocks with which they come in contact as great heat would produce. Coal is converted into the same kind of coke it would form if heated under the pressure of a great superincumbent mass, where there was little or no escape for the gases, and where there could be no flame. Limestone, even when originally a soft chalk, is not slaked as it would be in a kiln where the carbonic acid would be driven off, but made crystalline, as if partially or entirely melted. Clays and sandstones are baked, the former being sometimes converted into a rocklike porcelain. These, then, are aqueous rocks metamorphosed by contact with igneous rocks or by close proximity to them.f Sometimes, when the igneous mass is |