Although at first sight the interior of the earth appears a confused scene, after careful observation we readily detect in it a regularity and order from which much instructive light is thrown on its past vicissitudes. The deposition of the aqueous rocks and the projection of the volcanic have unquestionably taken place since the settlement of the earth in its present form. They are, indeed, of an order of events which are going on under the agency of intelligible causes, down to the present day. We may therefore consider these generally as recent transactions. But advancing to the far distant antecedent era of its existence, we may consider it to have been a globe of its present size enveloped in the crystalline rock already described, with the waters of the present seas and the present atmosphere around it, though these were probably in considerably different conditions, both as to temperature and their constituent materials, from what they now are. We may thus presume that, without this primitive case of granitic texture, the great bulk of the matters of our earth were agglomerated, whether in a fluid or solid state is uncertain; but there cannot be any doubt that they continue to exist in a condition of great heat and compression, having a mean density of more than double that of the minerals on the surface.
Judging from the results and still observable conditions, it may be inferred that the heat retained in the interior of the globe was more intense, or had greater freedom to act, in some places than in others. These become the scenes of volcanic operations, and in time marked their situations by the extrusion from below of trap and basalts—rocks composed of the crystalline matter, fused by intense heat, and developed on the surface in various conditions, according to the particular circumstances under which it was sent up; some, for example, being thrown up under water, and some in the open air, which contingencies would make considerable difference in its texture and appearance. It would, however, be a mistake to infer that, previous to these eruptions, the earth was a smooth ball, with air and water playing round it. Geology tells us plainly that there were great irregularities—lofty mountains, interspersed with deep seas—and by which, perhaps, the mountains were wholly or partially covered. But it is a fact worthy of observation that the solids of our globe cannot for a moment be exposed to water or the atmosphere without becoming liable to change. They instantly begin to wear down. The matter so worn off being carried into the neighbouring depths and there deposited, became the components of the successive series of stratified rocks, extending from the basal envelope of granite to the earth's surface, and which it will be proper briefly to describe.
DEPOSITS OR ROCK FORMATIONS.
The first of the series is the Gneis and Mica Slate System, of which examples are exposed to view in the Highlands of Scotland and the west of England. These earliest stratified rocks contain no matters which are not to be found in the primitive granite. They are the same in material—silica, mica, quartz, or hornblende—but changed into new forms and combinations, and hence called by Mr. LYELL metamorphic rocks. Some of them are composed exclusively of one of the materials of granite; the mica schist, for example, of mica; the quartz rocks, of quartz. In the metamorphic rocks no organic remains have been found, and they are geologically below all the rocks that do contain traces of animal life.
From the primary rocks we pass into the next ascending series, called the Clay Slate and Grauwacke Slate System, which in some places is found resting immediately on the granite, the antecedent bed being there wanting. This deposit has been well examined, because some of its slate beds have been extensively quarried for domestic purposes. By some geologists it is called the Silurian System, it being largely developed at the surface of a district of western England formerly occupied by the Silures. It is found also in North Wales and in the north of England, in beds of great thickness, and in Scotland, but there the Silurian rocks are more feebly represented.
The Old Red Sandstone, or Devonian System, comes next. It forms the material of the grand and rugged mountains which fringe many parts of our Highland coasts, and ranges, on the south flank of the Grampians, from the eastern to the western sea of Scotland. There is no part of geology and science more clear than that which refers to the ages of mountains. It is as certain that the Grampian mountains are older than the Alps and Apennines, as it is that civilisation had reached Italy and enabled her to subdue the world, while Scotland was the abode of barbarism. The Pyrenees, Carpathians, and other ranges of continental Europe are all younger than these Scotch hills, or even the insignificant Mendip Hills of southern England. Stratification tells this tale as plainly, and more truly, than LIVY tells the story of the Roman republic. It tells us that at the time when the Grampians sent streams and detritus to straits where now the valleys of the Forth and Clyde meet, the greater part of Europe was a wide ocean.
The last three series of strata contain the remains of the earliest occupants of the globe, and of which we shall soon speak. They are of enormous thickness—in England, not much less than 30,000 feet, or nearly six miles.
We have now arrived at the secondary rocks, of which the lowest group is the Carboniferous Formation, so called from its remarkable feature of numerous interspersed beds of coal. It commences with beds of the mountain limestone, which in England attains a depth of 800 yards. Coal is altogether composed of the matter of a terrestrial vegetation, transmuted by putrefaction of a peculiar kind beneath the surface of water, and in the absence of air. From examples seen at the present day at the mouths of such rivers as the Mississippi, which traverse extensive sylvan regions, it is thought that the vegetation, the rubbish of decayed forests, was carried by rivers into estuaries, and there accumulated into vast natural rafts, until it sank to the bottom, where an overlayer of sand or mud would prepare it for becoming a stratum of coal. Others conceive that the vegetation first went into the condition of peat moss, that a sink in a level then exposed it to be overrun by the sea and covered with a layer of sand or mud; that a subsequent uprise made the mud dry land, and fitted it to bear a new forest, which afterwards, like its predecessors, became a bed of peat—that, in short, by repetitions of this process the alternate layers of coal, sand and shell constituting the carboniferous group were formed.
The Magnesian Limestone deposits succeed the carboniferous, and sometimes pass into them by insensible gradations. In the south of England they are represented by conglomerates, and partly composed of the solid and more or less rounded fragments of the older strata. They afford a proof of what geologists have often occasion to remark of the long periods of time during which the ancient works of nature were perfected; for the older rocks were solid as they are now, and their organic remains petrified at the time these conglomerates were forming.
We can only briefly glance at the remaining chapters of geological history. The New Red Sandstone forms the base of the great central plains of England, and is surmounted by the oliferous marls and red arenaceous beds which pass under the succession of great oolitic terraces that stretch across England from the coasts of Dorsetshire to the north-eastern coast of Yorkshire. It marks the commencement of an important era, being the strata in which land animals are first found. The Oolte System which follows marks the beginning of mammalia, and in some of its beds in Buckinghamshire are found the exuviæ of tropical trees. Near Weymouth, in the well-known dirt beds, are found trees with their silicified trunks growing up in the position of nature, and their roots embedded in the soil on which they grew.