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But more remarkable still, at the time when these trees grew, huge elephants, rhinoceroses, and deer roamed over England, and strayed in the woods; as we know from their remains being found in this layer in many parts of the country! The forest afterwards sank under the waters of a

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lake, and remained submerged for a long time, during which the thick deposit of clay was laid down, containing remains that lived in its waters. After the clay, a new agent acted upon the country. As will afterwards be shewn, the British Isles were subjected to the action of ice, the high mountains being covered with great glaciers, while icebergs floated over the submerged land, carrying boulders, and grinding down the surface of the land over which they passed. Here, away in Norfolk, we have the débris of what perhaps had been grated and rolled from the peaks of Wales and Cumberland! Then followed the deposition of the sand and gravel marked E and F. We observe, however, that F, though appearing on the left, has been washed away from the right side of the valley. After this gravel was laid down, a great river rolled its waters across the country, and wore away the strata, till it formed the deep valley that occupies the centre of the section. The gravel of this great river is still seen at the bottom of the hollow at G. Then followed a period when this valley formed the bed of an immense lake, into which flowed streams bearing the vegetable remains that now form the peat H. In this peat we find the scales and bones of the fish that gambolled in its waters-the perch, the salmon, and that fierce water-wolf, the pike. After this lake dried up, the sea flowed in, and the old river-valley became a bay or gulf, with its yellow sand I, and gravel K. These have subsequently been scooped out by streams, as shewn by the bend in the surface. We have, at last, reached the present time, when Norfolk was raised above the ocean, whose ancient bed is now trodden by the farmers and children of that fertile county.

Such are examples of the old-world stories disclosed by every rocky shore, railway-cutting, and river-cliff. We have been describing not what might have been, but what was; for we can prove every assertion, seeing that the rocks themselves furnish incontrovertible data.

Reasoning regarding Surface Phenomena.

1. Iceberg Action.-In some parts of Britain, as in the central region of Scotland, there are exhibited certain remarkable appearances from which striking deductions may be drawn. We find that all the hills, eminences, and rocks are precipitous towards the west, and exhibit traces of being rendered so by a mighty force wearing them down on that side. In the level ground between these heights we find a thick deposit of fine blue tenacious clay, with embedded stones, rounded and water-worn, which is known as boulder-clay. Also, laid down here and there, sometimes on mountain tops, we find enormous blocks brought from great distances to the west, as shewn by the rock of which they are made. We also discover all across the country certain remarkable scratchings, and deep indentations on the exposed surfaces of the hardest rocks, all pointing in one direction, at right angles to the broken cliffs. We also note that all heights are rounded and worn, as if ground down by some powerful agent moving over them.

The problem before us therefore is-How are all these appearances to be accounted for, and by what agent or agents were they produced?

It is evident, from the existence of the clay, that the whole region referred to must have lain under water. But water alone, even in a powerful current, could not carry the blocks, and make the deep scratches that everywhere are seen. It is evident that these effects must have been produced by something borne on the surface of the water, of size and hardness capable of doing all this. What hard substance, therefore, can be borne by water, that can carry rocks and leave deep scratchings behind it? Nothing but ice in the form of huge bergs; and a current bearing packs of these on its surface fully satisfies all the requirements of the case. Floating along, of enormous size, these ice-mountains wear down all surfaces over which they move; carry from great distances, and drop as they melt, blocks of every size; dash against and wear down into cliffs all opposing eminences; and leave marks on the rock-surfaces identical in appearance with those everywhere exhibited; while the current that bears them along, deposits in the lower grounds the mud and boulder débris they generate in their onward course. Thus we prove incontestibly that icebergs floated over the district exhibiting these phenomena. But was the climate such as to generate icebergs? This is proved, independently of the above reasoning, by various considerations; amongst others, by the fact, that in the boulder clay in various parts shells are found, called boreal

shells, that could only have lived in an arctic climate, and now live only in the northern seas.

2. The Existence of Glaciers in Britain.-In the same way, we can as conclusively prove that these great bergs were broken from glaciers that at that remote epoch filled the upper valleys of the Highlands, to the north and south. This is shewn by such facts as these, which we can but enumerate-the general aspect of these valleys; the striations and polished surfaces pointing always to the highest peaks, whence the icestreams descended; the rounded rocks with steep sides looking down the valleys; the rocky, worn, and broken débris; the moraines or rock-walls driven before the advancing ice, or borne on its surface, and that now run along or across the valleys like ramparts; and other equally striking and conclusive phenomena. Thus we arrive at the undeniable fact, that the British Isles once exhibited in successive periods all the ice-features of Greenland, Norway, and the Alps.

The Rocks as Related to Time.

The Length of Geological Periods, or Geological Time.-In studying Geology, it is necessary to have an accurate notion regarding the nature of the periods spoken of. It is to be strictly observed that in Geology, time cannot be measured by years. When we examine any stratum of rock, with all its enclosed organisms, it is natural to inquire how long this mass of rock took to be deposited. We can judge of this only in the following way. From observation of river-action as at present exhibited, we see with what extreme slowness rock-masses are worn down into sand; how a thousand years make an almost imperceptible change on a boulder, and even on the gravel by the shore. Yet we know that the sandstone before us, often hundreds of feet in thickness, is composed of grains of rock ground down by water-action, transported by rivers to the seabottom, and deposited there till other strata were heaped upon it; and that in after-ages the grains united, and were hardened by pressure into the rock we see. What incalculable ages, therefore, must this sandstone bed have taken to be thus formed! The more we think of these slowworking causes, the more are we astonished at the enormous periods of time that must have elapsed before the formation of even the thinnest layer of rock. Geological periods, therefore, are quite indefinite in the matter of years; but from various considerations, we can arrive at certain very definite conceptions regarding the length of time required for the formation of the various rock-systems. This inability to assert a definite number of years in regard to any formation, is no defect in the science, for the knowledge of this would add not one item to the conception we

already have of the immense periods presented to our contemplation by geology.

The Relative Ages of Rocks.-When we speak of the ages of rocks, we can do so only by comparison with others. Our ideas on this point are merely relative. We can assert, as we have already done in analysing the rock-sections, that one layer must have been formed before another; or that, after its formation, and before the deposition of a certain other rock, a rise or fall in the strata took place; or that, at a certain point in the series, a volcanic eruption threw up a mass of igneous rock; and make like statements based on comparison of the rocks with one another. Our conceptions, therefore, regarding the connection in age between the various rock-formations are merely relative, one being proved to have taken place before, or after, or during the formation of another.

The Order of the Rock-formations.-By long-continued and widely extended observations in various parts of the globe, based on numberless data of composition, structure, inclination, and fossil contents, geologists have been able to form a definite list of the various rock-formations from the earliest to the most recent, arranged in the order of time. They have divided the whole of the rocks composing the crust of the earth into sections called 'systems,' and these again into 'groups,' in a certain welldefined order-so that when a rock is presented to their observation in any part of the globe, they can state, with more or less certainty, the system to which it belongs, and the period in the past history of the earth at which it was deposited. Regarding these rock-systems, one point is to be very strictly noted. Suppose that we represent the various rock-systems by the letters of the alphabet-the earliest by A, the second by B, and so onwards to the last and most recent, represented by Z. Now, the various rock-systems always stand in this relative historic order; so that the formation indicated by M is always after L, and before N, and cannot occur in any other relation to these two systems, wherever they may be found. At the same time, certain formations, one or more, may be awanting in certain parts of the world, not having been deposited there; so that one or more systems may not be represented in these districts. Thus, L and M may be awanting. What two systems will then be found together? Certainly and unvaryingly, K and N. But here the historic order is not violated, as it would be if N preceded K. The various rock-systems are, therefore, always presented in an unvarying succession in the order of their formation; although in different parts of the globe certain strata, and even whole systems, may not be found.

Classification of the Rocks-Description of the Rock-Systems.

We now proceed to describe, in the order of their formation, the different kinds of rocks that compose the crust of the earth, and their fossil contents. As already said, geologists have divided all the rocks into different classes, according to their relative position and the fossils they contain. The whole of the rocks are divided into twelve great systems, and each of these into separate groups, to which names have been given, more or less descriptive of the strata to which they are applied. These systems we shall describe in order, beginning with the earliest, down to the most recent, giving the appearance and composition of the rocks, the uses to which they are applied, and the fossils they contain. We shall also endeavour to realise the state of the earth at each successive epoch, the scenery then exhibited, and the plants and animals that then enlivened the landscape. Before doing so, it is interesting to ask—

What Rock is oldest and lowest ?-What rock is the foundation on which all others rest? On the answer to this question, geologists are not agreed. We know the oldest sedimentary rock; and though the above question were to remain unanswered, the science of geology proper would be as complete as it requires to be. But some approach can be made to an answer. Granite seems to underlie all other rocks; at least, as far as yet ascertained, no other rock has been found under the granite, and this rock is erupted through all other strata; but we cannot assert definitely that this is the primitive or oldest rock, but only that, as far as yet discovered, no other has been found to underlie it. All are familiar with this beautiful variegated rock as used for paving our streets, and as polished for gravestones and hewn into pillars. It has two chief varieties, the gray and the red, according to the prevailing mineral in its composition. It composes the mass of the chief mountain ranges, and forms part of some of the grandest scenes in nature. It is generally considered to be of igneous origin. It shews no stratification, occurs only in great mountain masses, or in veins intersecting other strata, and seems to point to a period in the past history of our globe when it was a great molten ball, with conditions that we can scarcely imagine, so different must they have been from those now existing. But it seems to be undeniable, that before any of the stratified rocks were deposited, and before any life existed, the globe was in a state of molten fusion through intense heat; and that we have before us, in the granite, the remains of that igneous condition, which now exists as a cold, crystallised, variegated rock, containing valuable minerals, and of great industrial value.

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