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TABULAR VIEW OF THE SUCCESSIVE GEOLOGICAL PERIODS.
III. The Modern Epoch.
II. The Middle Epoch.
I. The Ancient Epoch.
10. The Period of the caverns and gravel; with Carnivora, the Megaceros and other gigantic ruminating animals, and the elephants of Europe; and of various gigantic animals in Asia, America, Australia and New Zealand. (Newer Tertiary.)
9. The Period of various large animals of the Middle Rhine valley, succeeded by that of the mastodon and elephants in North America, England, Northern Europe and India. (Middle Tertiary.)
8. The Period of the pachyderms of the Paris basin, and of the sub-tropical (?) fruits and animals of the London and Hampshire Basins. (Older Tertiary.)
7. The Periods of the Chalk and Greensand; during the deposit of which there was probably a deep sea, covering a large proportion of the existing land.
6. The Periods of the gigantic land reptiles, the flying reptiles, the gigantic crocodilians, and the first introduction of mammalian animals. (Wealden and Oolite.)
5. The Periods of the frog-like, bird-like, and marine reptiles. (Lias and Trias.)
4. The Periods marked by the presence of vegetables and the first introduction of reptilian animals. (Permian and Carboniferous.)
3. The Period of fishes. (Devonian.)
2. The Period of invertebrated animals. (Silurian.)
1. The Period antecedent to the introduction
THE FIRST OR ANCIENT EPOCH.
THE PERIOD ANTECEDENT TO THE INTRODUCTION OF LIFE.
JUDGING from the general appearance of the solar system, and combining the result of astronomical observations on distant bodies in the universe with the appearance presented by various rocks on the earth's surface, it seems not unlikely that, at a very early period of its history, our globe existed as an intensely heated body in a fluid state, (the fluidity being the result of igneous fusion,) and that it gradually cooled at the surface, perhaps by exposure in space, contracting in dimensions as it cooled and hardened. In this manner, it may be, a succession of thin solid crusts were formed, each in succession shrinking and cracking, until at length, when a certain balance was arrived at between the thickness of the crust, the rate of cooling, and the amount of internal heat, there would be left a rough uneven surface, having many elevations and depressions, its temperature being sufficiently reduced to allow of the existence of some such atmosphere as now surrounds it, and also permit the permanent presence of water
in a fluid state reposing in the hollows, and forming seas, lakes, and oceans. During the whole of this time, and until the existence of water in the liquid state, and the establishment of a sea, and perhaps long afterwards, it is likely that there were no living beings on the earth; because, so far as we know, neither animal nor vegetable can exist, and perform its functions, at the temperature of water actually boiling,*— although, at a temperature not much short of that, some small animalcules, and even some animals of higher organization, would seem capable of enjoying life. Thus, therefore, according to this view,-and the reader will understand that it is merely offered as the most probable explanation of certain appearances observed, the first period of the existence of the earth as a planet was marked by a chaotic state of igneous fusion, and characterised by frequent disturbances of the surface consequent upon cooling from such a state. Let us consider for a moment what kind of rocks are exhibited to us when we examine these earliest records of our globe, and let us see also how far we are able to examine them.
In the first place, we often find, as the basis of all other rocks in mountain chains, and throughout some extensive tracts of country, a well-known rock called granite; a rock whose structure is crystalline, and which bears strong marks of having cooled slowly from a state of intense heat. This rock is found in all parts of the world, and sometimes in widely ex
* This refers, of course, to the boiling temperature of water at the earth's surface with the present atmosphere. There is no proof of any change in the gaseous condition or pressure of the air, neither do we know what would be the condition of the surface with a steam atmosphere.
tended masses. It generally exhibits its own charac
teristic features with sufficient distinctness to leave no doubt as to its nature; and it may be found in our own island, as, for instance, in Cornwall, Wales, and Scotland; and in other parts of Europe, as in the Scandinavian mountains, the Hartz, the range of mountains separating Northern Germany from Bavaria and Bohemia, in the Alps both of Switzerland and the Tyrol, in the Pyrenees, and in the Carpathians. In Asia it forms the centre of the Caucasus; it occupies a large part of the Himalayan, Uralian, and Altai mountains; and is found also in Siberia. In Africa it appears in Upper Egypt, in the Atlas mountains, and at the Cape of Good Hope; and it may be traced along the Western part of the whole of the two Americas, and appears again in the Southern islands and in Australia.
A rock so universally extended might, almost for that reason, be looked upon as the foundation and the main solid frame-work of our globe. It must not be lost sight of, however, that in many cases the granite has been, if not formed, at least placed in its present position, in a pasty or fluid state,* long subsequent to the early period of which we are now speaking; and thus, though we may safely consider the granite as frequently the oldest rock, we should always remember that a material so widely extended and so important, may be elaborated and expelled from the deep recesses of Nature's store-house at any time, and even at the present day.
I shall not detain the reader any longer with an * In either case the result of intense heat acting under enormnous pressure.
account of the rocks which have been called "Primitive," but shall quit this subject with a remark which, from the manner in which I have spoken of granite, may perhaps be necessary. It is this: that there are many kinds of granite and granitic rock, some of them very different from ordinary granite in appearance, and that there are also many other so-called primitive rocks very different in structure; but these varieties do not prevent the account I have given from being sufficiently accurate for my purpose, and I trust the reader will not suppose, when he has read through this little volume, that he has learnt everything in Geology.
Next in order to the crystalline rocks, and almost as widely extended, there are two or three others, often themselves crystalline, but bearing evident marks of what is called "mechanical structure," or, in other words, of having been deposited from water. These may be described as gneiss, mica-schist, and clay-slate.* If we imagine common granite coarsely pounded, and thrown into a vessel of water, it will arrange itself at the bottom of the vessel in a condition very much like that of gneiss, which is indeed nothing else than stratified granite. If the water in which the pounded rock is thrown is moving along at a slow rate, and that part of the granite called felspar happens to be somewhat decomposed, as it often
Under this name 66 clay-slate," ," I only mean here to include those slates, whether of distinctly crystalline structure or not, which present no marks of having contained fossils. That there are such, no Geologist will, I suppose, doubt; but when the name clay-slate is given, as it is sometimes, to fossiliferous beds, they ought to be referred at once to the period indicated by the kind of fossils discovered.