embrace all the geological records, ancient as well as modern, in one general scheme of classification. Such a scheme will enable us to see the continuity of the materials of geological history, and will fix definitely for us the character and relative position of all the chief rocks out of which the visible part of the globe is composed.

The gradual change that overtakes everything on the face of the earth is expressed in all languages by familiar phrases which imply that the mere passing of time is the cause of the change. As Sir Thomas Browne quaintly said more than two hundred years ago, "time antiquates antiquities, and hath an art to make dust of all things." We speak of the dust of antiquity and the gnawing tooth of time. We say that things are time-eaten, worn with age, crumbling under a weight of years. Nothing suggests such epithets so strikingly as an old building. We know that the masonry at first was smooth and fresh; but now we describe it as weather-beaten, decayed, corroded. So distinctive is this appearance that it is always looked for in an ancient piece of stone-work; and if not seen, its absence at once suggests a doubt whether the masonry can really be old. No matter of what varieties of stone the edifice may have been built, a few generations may be enough to give them this look of venerable antiquity. The surface that was left smoothly polished by the builders grows rough and uneven, with scars and holes eaten into it. Portions of the original polish that may here and there have escaped, serve as a measure of how much has actually been removed from the rest of the surface.

Now, if in the lapse of time, stone which has been artificially dressed is wasted away, we may be quite certain that the same stone in its natural position on the slope of

FIG. 1.--Weathering of rock, as ing and insert them into a shown by old masonry. (The natural crag or cliff of the "false-bedding" and other orisame kind of stone, their ginal structures of the stone are revealed by weathering.) peculiar time - worn aspect would be found to be so ex

actly that of the rest of the cliff that probably no one would ever suspect that a mason's tools had once been upon them.

From this identity of surface between the time-worn stones of an old building and the stone of a cliff we may confidently infer that the decay so characteristic of ancient masonry is as marked upon natural faces of rock. The gradual disappearance of the artificial smoothness given by the mason, and its replacement by the ordinary natural rough surface of the stone, shows that this natural surface must also be the result of decay. And as the peculiar crumbling character is universal, we may be sure that the decay with which it is connected must be general over the globe.

But the mere passing of time obviously cannot change anything, and to say that it does is only a convenient figure of speech. It is not time, but the natural processes which require time for their work, that produce the widespread decay over the surface of the earth. Of these natural processes, there are four that specially deserve consideration, -changes of temperature, saturation and desiccation, frost, and rain.

(1.) Changes of Temperature. - In countries where the days are excessively hot, with nights correspondingly cool, the surfaces of rocks heated sometimes, as in parts of Africa, up to more than 130° Fahr. by a tropical sun, undergo considerable expansion in consequence of this increase of temperature. At night, on the other hand, the rapid radiation quickly chills the stone and causes it to contract. Hence the superficial parts, being in a perpetual state of strain, gradually crack up or peel off. The face of a cliff is thus worn slowly backward, and the prostrate blocks that fall from it are reduced to smaller fragments and finally to dust. Where, as in Europe and the settled parts of North America, the contrasts of temperature are not so marked, the same kind of waste takes place in a less striking

manner.

(2.) Saturation and Desiccation. Another cause of the decay of the exposed surfaces of rocks is to be sought in the alternate soaking of them with rain and drying of them in sunshine, whereby the component particles of the stone are loosened and fall to powder. Some kinds of stone freshly quarried and left to this kind of action are rapidly disintegrated. The rock called shale (see p. 202) is peculiarly liable to decay from this cause. The cliffs into which it sometimes rises show at their base long trails of rubbish entirely derived from its waste.

(3.) Frost.-A third and familiar source of decay in stone

C

exposed to the atmosphere is to be found in the action of Frost. The water that falls from the air upon the surface of the land soaks into the soil and into the pores of rocks. When the temperature of the air falls below the freezing point, the imprisoned moisture expands as it passes into ice, and in expanding pushes aside the particles between which it is entangled. Where this takes place in soil, the pebbles and the grains of sand and earth are separated from each other by the ice that shoots between them. They are all frozen into a solid mass that rings like stone under our feet; but, as soon as a thaw sets in, the ice that formed the binding cement passes into water which converts the soil into soft earth or mud. This process, repeated winter after winter, breaks up the materials of the soil and enables them to be more easily made use of by plants and more readily blown away by wind or washed off by rain. Where the action of frost affects the surface of a rock, the particles separated from each other are eventually blown or washed away, or the rock peals off in thin crusts or breaks up into angular pieces, which are gradually disintegrated and removed.

(4.) Rain. One further cause of decay may be sought in the remarkable power possessed by Rain of chemically corroding stones. In falling through the atmosphere, rain absorbs the gases of the air, and with their aid attacks surfaces of rock. With the oxygen thus acquired, it oxidises those substances which can still take more of this gas, causing them to rust. As a consequence of this alteration, the cohesion of the particles is usually weakened, and the stone crumbles down. With the carbon-dioxide or carbonic acid, it dissolves and removes some of the more soluble ingredients in the form of carbonates, thereby also usually loosening the component particles of the stone. In general, the influence of rain is to cause the exposed parts of rocks to rot from the surface inward. Where the ground is protected with vegetation, the decay is no doubt retarded; but in the absence of vegetation, the outer crust of the decayed layer is apt to be washed off by rain, or when dried to powder may be blown away and scattered by wind. As fast as it is removed from the surface, however, it is renewed underneath by the continued soaking of rain into the stone.

Hence one of the first lessons to be learnt when from the common evidence around us we seek to know what has been the history of the ground on which we live is one of ceaseless decay. All over the land, in all kinds of climates, and from various causes, bare surfaces of soil and rock yield to the influences of the atmosphere or weather. The decay thus set in motion is commonly called "weathering." That it may often be comparatively rapid is familiarly and instructively shown in buildings or open-air monuments of which the dates are precisely known. Marble tombstones in the graveyards of large towns, for example, hardly keep their inscriptions legible for even so long as a century. Before that time, the surface of the stone has crumbled away into a kind of sand. Everywhere the weather-eaten surfaces, the crumbling crust of decayed stone, and the scattered blocks and trains of rubbish, tell their tale of universal waste.

It is well to take numerous opportunities of observing the process of this decay in different situations and on various kinds of materials. We can thus best realise the important part which weathering must play in the changes of the earth's surface, and we prepare ourselves for the consideration of the next question that arises, What becomes