Primary or Palæozoic.

SUPERPOSITION OF STRATA.

-PERMIAN, or MAGNESIAN LIMESTONE.

29

CARBONIFEROUS, Coal Measures, Mountain Limestone,

&c.

DEVONIAN, or OLD RED SANDSTONE.

UPPER SILURIAN. Ludlow Rocks, Wenlock Rocks, &c.

It is not meant that there is any pit or mine fourteen miles deep, to exhibit a section of these strata: it is not meant that we should find all these rocks in any one place where we might sink a shaft. If we dig at Dover or Brighton, we may find the chalk on the surface, and all the beds above it absent; if we sink a well at Kentish Town, it may happen that, after passing through Tertiary strata, Chalk, Upper Greensand, and Gault, we come unexpectedly on sandstones which seem to be of Paleozoic age, all the intermediate strata-Lower Greensand, Oolite, Lias, &c.-being missing. But we are not in danger of supposing that the Gault was laid down. immediately after the deposition of the Paleozoic sandstones, because in other places we find the Lias above the Paleozoic rocks, the Oolite above the Lias, the Lower Greensand above the Oolite, and the Gault above the Lower Greensand; while we know that the lower rocks must have been laid down before the upper could be laid upon them. We have, again, all the advantage which very deep shafts could give us, in the circumstance that since the strata were laid down, many of

1 Mr J. Prestwich, F.R.S., Annual Address to Geological Society, February 16, 1872.

them have become tilted and exposed at the surface of the earth. If we set up twenty or thirty books in a row, without support at the ends, and by a blow make them fall from right to left, each volume will be seen passing under its right-hand neighbour; and we shall have in such a tilted row a rough representation of the sedimentary strata, as they are passed over when we journey by the Great Western Railway from London into Cornwall or South Wales.

When we investigate the composition of the strata we find that, speaking generally, the more ancient are the more simple, the less ancient the more complex; the materials of several older rocks often lending their aid to form one newer stratum. Each newer bed must of course have been formed from older ones, worn away for the purpose, just as the beds forming on the seafloor to-day consist of materials which the rivers are washing down from the lands they drain. Here we see one cause of the absence of some of the strata in some localities; they may have been deposited, but afterwards washed away to form new beds. In other cases, they may never have been laid down; for the sediment of a river, though that river be as large as the Ganges, is never spread over all the ocean floors of the world, and the sediment from different rivers would usually form strata of different kinds, and containing life of different forms. It is well known that the stream of the Mississippi is charged with sediment of a different colour from that of the Arkansas and Red River, which are tinged with red mud derived from rocks of porphyry in "the far west;" and it has been noticed that the waters of the Uruguay, draining a granitic country, are clear and black, while those of the Parana are

HOW SUPERPOSITION OCCURS.

31

red.1 Again, even if one stratum were spread over every sea floor, the lands exposed above the water would escape receiving it; and by the time it was their turn to be covered over, the rivers would be bringing down very dissimilar material.

This brings us to the question how the strata formed at the bottom of oceans, lakes, and estuaries ever came to be dry land; and by what process the chalk (for instance), after succeeding in getting itself exposed as dry land, should have the London clay and other strata deposited upon it? Were the British Isles to sink down bodily 600 feet, nearly the whole of England and Ireland would be submerged; while Scotland and Wales would be reduced in size through the encroachment of the waters on all their coasts. On the other hand, were the same islands with the neighbouring seas lifted 600 feet higher, the whole of the German Ocean, the Irish Sea, and the English Channel, would become dry land, so that the British Isles would all be united together and to the Continent.2 Now there is evidence that the British Islands have experienced oscillations of level, and that subsidence and upheaval are continually going on in many parts of the world. Several large areassome of them several thousands of miles in circumference-such as Scandinavia, the west coast of South America, and certain archipelagoes in the Pacific, are slowly rising at this day; while other regions, such as Greenland, and parts of the Pacific and Indian Oceans, in which atolls or circular coral islands are found, are as gradually sinking.

1 Darwin's Journal, 10th thousand, p. 139.

2 See the maps illustrating this in Sir C. Lyell's Antiquity of Man.

These elevations and depressions remind us that, besides the agency of water, there have been subterranean forces at work to remodel the face of the earth. The earthquake and the volcano, visiting in succession every zone, and filling the earth with monuments of ruin and disorder, are nevertheless the agents of a conservative principle essential to the stability of the system. We need not, however, go into any details. concerning their operations.

From the fact that the materials for each new stratum of rock have been derived from previous strata, we may infer that the seas have never covered all the earth at one time, that there always has been land above water; and the truth of this inference is confirmed by the discovery of fossil trees and terrestrial plants imbedded. in rocks of every age. Occasionally lacustrine and fluviatile shells, insects, or the bones of amphibians or land reptiles, point to the same conclusion.

What, then, is our conclusion from all these facts? It is that the earth has had a history extending through many millions of years; its physical geography, its land surfaces, its climates continually changing; one phase of things slowly dying into another, like pictures of a dissolving view. The picture we gaze on to-day is changing also, no exceptional character belongs to it; and as it is itself the outgrowth of all that went before it, so is it a stage of departure for new developments. There has not been a cycle of changes, returning to the starting-point, and then repeating itself; but through the constant action of the same forces as those now in operation, nature has been made to assume "beauty ever blushing, ever new."

A few words more will connect this section with the

GEOLOGICAL CHANGE CAUSED BY SOLAR.

33

last, and show that the evolution of the earth's crust is but a part of that evolution which is universal. Assuming the truth of the Nebular Hypothesis, all geologic changes are either direct or indirect results of the unexpended heat caused by nebular condensation. The earth, after becoming a separate globe, would be still fluid and incandescent, and the first thin "crust" of the earth would result from the slow cooling of the molten mass. On the view held by some geologists, that the interior is still in a melted state, and that earthquakes and volcanic action are to be traced to the perturbations of the molten ocean; the elevations and subsidences thus caused, the eruptions of volcanoes and the extrusions of igneous rock through strata, are shown to be indirect results of the original heat of the unformed solar system. And similarly with regard to aqueous agency: If we ask whence comes the power of the river current, bearing sediment down to the sea? the reply is, The gravitation of water throughout the tract which this river drains. If we ask, How came the water to be raised over this tract? the reply is, It fell in the shape of rain. If we ask, How came the rain to be in that position whence it fell? the reply is, The vapour from which it was condensed was drifted there by the winds. If we ask, How came this vapour to be at this elevation? the reply is, It was raised by evaporation. And if we ask, What force thus raised it? the reply is, The sun's heat. Just that amount of gravitative force which the sun's heat overcame in raising the atoms of water, is given out again in the fall of those atoms to the same level. Similarly with the winds that transport the vapours hither and thither since atmospheric currents result from differences of temperature-between

C