upper limestone of the Paris basin are obtained the well-known Burr millstones; also a marble susceptible of high polish, and which is very ornamentally marked by the shells imbedded in it. A marl, for manure, is obtained from the disintegration of some of the limestones. Pipe_and potter's clay are dug up both from the London and the Paris basins. From the latter is also obtained the famous plaster of Paris, which is gypsum, or sulphate of lime, reduced to powder and kneaded with water, for the purposes of the plasterer, the stereotype-founder, who makes his moulds of it, and the image-maker, who sends forth into our streets so many cheap and beautiful copies of the finer works of our sculptors. The gypsum itself is largely used for manure. Lignite, or wood-coal, is found in some tertiary strata for instance, near Exeter. Amber is often found with the lignite, and is supposed to be gum that exuded from the same or neighbouring trees that formed the lignite.

CHAPTER XI.

SUPERFICIAL STRATA, &c.

Probably an era betwixt the Tertiary Formations and those of our Period. -From the depths of the earth's crust we have now re-ascended to the surface, and seem, for the moment, as we look around, to have nothing left to occupy our attention but those superficial accumulations with which our eyes and our feet have been familiar from childhood-the gravels, sands, clays, peats, &c., of our own country, and which in other countries are varied by the existence of shell-beds, coral reefs, &c. But on a more careful review we find many things of interest lying about, and the origin of which suggests interesting questions. As we examine these we are led to believe that between the era of the Tertiary strata and that to which we ourselves belong there has existed another-perhaps more than one-period, of a transitional character, to which no name has been attached, but to which belong certain clearly distinguishable phenomena.

Denudation. We have already described the geological meaning of the word "fault." Let us now add that there are often found great faults, or hitches, in the superficial strata, which, if left as they were originally formed, must have caused striking irregularities in the face of the country, through the one side that was uplifted remaining standing up much higher than the other; but there is no such inequality left. The coal-fields of Ashby-dela-Zouch may be instanced. Here there is a fault. We see in them that the beds, or strata, have been forcibly ruptured, and certain strata on one side raised five hundred feet higher than the corresponding and formerly united strata on the other. But if any one walks over the top of both sides he will find them level. What, then, has become of the five hundred feet of rock that must have originally projected above the surface on the raised side? It has been all washed away by the action of water. This is what is meant by Denudation; and we may see how potent an agent it has been in bringing the world to its present state. Professor Ramsay has shown that

certain portions of rock, removed from the top of the Mendip Hills, must have been nearly a mile in thickness, and they were also washed away by

water.

There is another class of cases only to be explained by the power of denudation. We find extensive valleys hollowed out in the sedimentary strata, leaving the sides facing each other at considerable distances, and having sometimes a mass standing up between them to the same height, and evidently forming a part of the original uninterrupted range of rock before the valleys were formed by the removal of their contents. This phenomenon is explained by the action of water in washing away-and making a channel for itself through—the softer portions of the rock.

Diluvium. This term has been applied to the next class of phenomena of which we shall speak. Below the superficial covering of mere vegetable soil, mixed as it generally is with the minute fragments of disintegrated rock, and above the stratified rocks of all eras, we find in all parts of the world, and generally in somewhat low situations, a layer of stiff clay, commonly of a blue colour, but sometimes reddish, varying in thickness from only a few feet to above a hundred feet, and mixed with fragments of rock that bear the marks of having been much rubbed and worn by travel, and which vary in size from that of an egg to the dimension of large isolated rocks, or boulders, weighing many tons. This is sometimes called the Boulder formation, and is supposed to have been the product of some vast deluge-hence the name, Diluvium-or of the sea in a state of unwonted agitation even for those agitated periods.

Boulders, &c.-The fragments in question can generally be traced to their source in certain parent masses, lying often at great distances. This is as true of the largest boulders as of the smallest stones. Thus pieces of the granite of Shap Fell are found fifty miles away from the latter; and one boulder rock, in particular, lies high up the Criffel mountain, on the opposite side of the Solway estuary. Parts of the primitive rocks of the Lammermuir and Cheviot ranges are also scattered through the vale of the Tweed, and in Northumberland. Blocks from the Welsh mountains lie about in the midland counties; and others, on the east coast, are presumed to have travelled thither from Norway. It is not likely that the exact same agency was concerned in the transport of these large masses as sufficed for the smaller ones. The latter might have been driven to and fro by the mere chaotic force of the water; the other could only have been transported to great distances by some additional power. This we find in icebergs, which, as we have already had occasion to state, are now continually transporting masses of rock. These, while imbedded in ice, are broken off from the parent mass, fall into the sea, and are then tossed about till they reach a region of a milder temperature, under which they melt and drop their inclosed burdens. In one of the recent voyages of discovery to the Arctic regions this process of transportation was seen going on. There was a darkcoloured, angular-shaped piece of rock, five or six feet wide and twelve feet high in its visible proportions, to say nothing of what might be concealed in the iceberg that inclosed it, which was between two hundred and three hundred feet high, and at least 1,400 miles from any known land. As a whole, this Boulder formation seems to tell us of a period when much of what is now dry land must have been under water-a fact that appears at once curious and interesting, if we think of it as the latest of an almost infinite number of

extensive risings and fallings of the earth's crust. And how this alternate action and reaction, advance and retreat, seem to foreshadow man's own mental phenomena! May we not hope that it is with him as with his material home that on the whole he progresses grandly and beautifully, however full of reverses and disappointments his course may seem to those who look but for a time on his movements?

Marks on Rock Surfaces, &c.—In immediate connection with the phenomena we have described are certain marks or grooves often found on the surfaces of rocks in a peculiar position, and which appear to have been made by heavy and hard bodies, as they were rapidly hurried along by some irresistible force, as by that of a flood, for instance, bearing upon its bosom great masses of ice.

Crag and Tail.-The rocky elevations, abrupt on one side and gently sloping away on the other, to which geologists have applied the quaint appellation of "Crag and Tail," belong also to the peculiar class of geological effects now under review.

Clay and Gravel Ridges.-The same may be said of the long ridges of clay and gravel which are found in various parts-in Finland, Sweden, and the United States, for instance. We can readily understand how these were formed after perusing Mr. Simpson's Work on the Polar Seas, where he i-cribes the breaking up, during summer, of the ice formed in the previous winter over gravelly districts, of the driving in upon the shore of the ice fragments by the wind or the tidal waves; of their accumulation on the beach in long ridges, which melting, leave the imbedded gravel behind.

The foregoing Phenomena appear to have had One Common Origin.—It is a very remarkable circumstance that all these phenomena seem to have had not only one common general, but also a special origin; that is to say, they all seem to have resulted from the action of a grand watery current sweeping from the north and north-west towards the south-east; for the directions of the diluvial blocks, of the grooved lines, of the crag and tail, and of the clay and gravel ridges, are all of that character. What sort of a current, or flood, this must have been, our readers may judge for themselves, when they know that it included within its range not only Europe, but America.

Glaciers.-The origin of glaciers is so intimately connected with that of icebergs, and also with that of the Boulder formation, that we need not apologize for the introduction of the subject in this place. By glaciers we understand those enormous masses of ice which are formed on the slopes of lofty mountains, and in the intervening valleys, and which remain apparently eternally unchangeable. Let us, with the help of an eminent foreign geologist, Saussure, picture to ourselves the aspect of the most famous of the glacial regions that of the Alps. Let us imagine ourselves at a sufficient height above these stupendous mountains to overlook the whole, and thus be able to embrace at one view the Alps of Switzerland, Savoy, and Dauphiné. What do we then see? A mass of mountains, intersected by numerous valleys, composed of several parallel ranges, the highest in the middle, and the others gradually receding on each side. This central chain appears bristling with craggy rocks which are covered, even in summer, with snow and ice, except where their sides are directly perpendicular; while, in strange contrast, the deep valleys are green and beautiful, well watered, and covered with villages. Looking still more closely into the details of the

S

wonderful scene-the sublimest, perhaps, that earth can afford- —we perceive that the central range consists of lofty peaks and smaller chains, snowtopped, with all the slopes that are not directly vertical covered with ice, while the intervals between form elevated valleys, containing enormous masses of ice, extending downward into the deeper and inhabited valleys of the lesser bordering chains. The chain nearest to the centre presents the same aspect on a smaller scale, but beyond that one we see no more ice or snow, except upon the peaks of some unusually high summits. Such is the home of the glacier. Between Mont Blanc and the borders of the Tyrol there are about four hundred glaciers, of which the smallest are generally two or three miles long, while most of them range from ten to fifteen miles long, and from one to two miles and a quarter broad. Altogether it has been calculated that the glaciers of the Tyrol, Switzerland, Piedmont, and Savoy cover an area of nearly fifteen hundred square miles. The constant increase of ice at the summits of the Alps, where of course the cold is greatest, would add as constantly to their height, were it not for its descent, in the form of glaciers, into and through the valleys. Sublimity is not the only element of the scene; its beauty is scarcely less attractive when we pass from the whole to study the component parts. Thus, for instance (as Sir C. Lyell notices), the ice in descending the steep slopes falling from the abrupt precipices, or in being forced through narrow gorges, is broken into a thousand fantastic or picturesque forms, with lofty peaks and pinnacles projecting upwards. "These snow-white masses are often relieved by a dark background of pines, as in the valley of Chamouni, and are not only surrounded with abundance of the wild rhododendron in full flower, but encroach still lower into the region of cultivation, and trespass on fields where the tobacco-plant is flourishing by the side of the peasant's hut."

Glacier Motion.-The cause of the motion of the glaciers has been much discussed among scientific men. The result seems to be a tolerably general agreement that the chief agent of motion is gravity, acting upon a plane more or less inclined, and upon a body capable of a certain amount of selfadaptation to the surrounding circumstances, and aided by the melting of the bottom of the glaciers, where they rest upon the earth, through the higher temperature of the latter. This self-moulding power of ice is much more considerable than one would at first suppose. The following interesting experiment was made by the secretary of the Royal Society. He filled with water a hollow shell of iron an inch and a half thick, and having an internal cavity of ten inches diameter. This was exposed to severe frost, the fuse-hole of the shell being placed uppermost. As the water froze would it burst the cell or force the ice out at the hole? The answer was the protrusion of the ice in the form of a cylinder, which grew on, inch by inch, as a larger quantity of the water became frozen. Sir C. Lyell states: "A series of beautiful experiments enabled Professor Forbes to determine, for the first time, the true laws of glacier motion, which were found to agree very closely with those governing the course of rivers, their progress being faster in the centre than at the sides, and more rapid at the surface than at the bottom. This law was verified by carefully fixing a great number of marks in the ice, arranged in a straight line, which gradually assumed a beautiful curve, the middle part pointing down the glacier, and showing a velocity there double and treble that of the lateral parts. He ascertained that the state of advance by night was nearly the same as by day, and that even the

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hourly march of the icy stream could be detected, although the progress might not amount to more than six or seven inches in twelve hours." By the incessant though invariable advance of the marks placed upon the ice, time," says Mr. Forbes, "was marked out as by a shadow on a dial, and the unequivocal evidence which I obtained that even whilst on a glacier we are, day by day, and hour by hour, imperceptibly carried on by the resistless flood of the icy stream, filled me with admiration." In order to show or explain this remarkable regularity of motion, and its obedience to laws so strictly analogous to those of fluids, the same writer proposed the theory that the ice, instead of being solid and compact, is a viscous or plastic body, capable of yielding to great pressure, and the more so in proportion as its temperature is higher, or as it approaches more nearly to the melting point.

English Glaciers. Now, what is true of the existing glaciers of the continental Alps was doubtless also true of the glaciers of our own mountainous regions when they existed; for we have no doubt that they did exist in such districts as the Welsh mountains. The investigations of Agassiz and others, and the known greater extension of the glaciers of the Alps in past times, render it highly probable that this "ice-power," as it has been called, has been actively at work (during former eras) in parts where now no fields of ice are ever found. Applying this fact to our mountainous regions, we see at once the explanation of the smoothed and rounded rocks, the grooved surfaces, the channels parallel to the grooves, and other phenomena, for all these are known to be produced by glaciers on the surfaces they pass by or over. Of course an iceberg is but a floating glacier, and therefore there is no difficulty in understanding how the boulders we have spoken of were originally imbedded in a glacier, which was either carried by its own motion to the water, or to which the waters came in the course of the differing geological phenomena we have described. Thus, to borrow an illustration from the Penny Cyclopædia, the ancient glacier streams of Cumberland may have delivered the detrital blocks of Shap and Carrock into the sea by the breaking off of icebergs, which may then have been drifted by currents to Staffordshire, to the mouth of the Tyne, and the valleys of York and Holderness.

Has England had a low Northern as well as a Tropical Temperature?— All this, however, implies a considerable change in the temperature of England, which, as we have seen, was in all probability tropical during the earlier geological eras, and which, as we shall presently show, remained so during some part of the period at present under review. But such a change involves no great difficulty, when we consider that mere alterations of the relative arrangements of land and water immediately affect the temperature, and changes of that character were obviously of frequent occurrence while the earth was, as it were, making its final arrangements for the state of things which includes man. We seem, therefore, to have had a glacial period among the numerous other periods already spoken of.

Ossiferous Caverns.-But of all the results of the Diluvium phenomena the ossiferous caverns are, perhaps, at once the most popularly and scientifically interesting. These are so called from os, bone, and fero, I bearwords referring to the remarkable contents of the said caverns. They are found in various parts of the world, including our own country. The chief English ones are the following:-Banwell Cave and Hutton Hole, in the Mendip Hills; Kent's Hole, at Torquay; the Peak Cavern, in Niddesdale; and Kirkdale, in Yorkshire. They occur in the limestone strata, which are peculiarly liable to be hollowed out by the action of springs and subterranean