some association with the events of history, with the creations of fiction, or with the dreams of poetry. There is thus a double charm about them, and it is often hard to say which is their more powerful element. The materials out of which the landscapes of Scotland have been framed, and the influence of subterranean movements upon them, will be described in later chapters in connection with the three regions into which the country may naturally be divided, each district having its own peculiarities of geological structure. But the forces that have acted at the surface, carving these materials into their present forms, have been at work over the whole country, and may therefore be taken together. Accordingly, I propose first to consider the operation of the various agents that have acted upon the surface of the rocks, and as far as possible to take the illustrations of their mode of working from what may now be seen going on in Scotland. CHAPTER II NATURE'S SCULPTURE-TOOLS-AIR, RAIN, RIVERS, SPRINGS, FROST THE process by which the scenery of a country is produced may be compared to sculpture. In the fashioning of a statue, the block of stone must first be lifted out of its bed in the parent rock, and then the sculptor's tools must be used upon it. Apart from the design and workmanship, the aspect of the statue will primarily depend on the nature of the material employed. Long practice has shown that statuary marble is best adapted for the purpose of the sculptor's art. But if conglomerate, or sandstone, or porphyry, or granite were chosen, the effect would in each case be different. So in the case of the dry land. There must first be an uplifting of the ground above sea-level, and then nature's tools will slowly carve its surface into the characteristic terrestrial shapes. The ultimate details of these shapes will be greatly influenced by the nature of the material, each kind of rock revealing its own peculiar characters and influence in the general process of land-sculpture. But much also will depend upon the particular tools which nature may use, and on the energy with which she wields them. The tools that have been most effective in the carving of the land are Air, Rain, Frost, Springs, Rivers, Glaciers, and the Sea. Let us watch each of them at work. AIR AND RAIN. Long exposure to the air tells even upon the most obdurate kind of stone. An old building always shows more or less manifest proofs of decay, insomuch that if these are not conspicuous, we instinctively begin to doubt whether it can really be old. This decay is known by the name of 'weathering.' It is a complex process, partly chemical, partly mechanical. Great and rapid changes of temperature tell powerfully upon the outer surfaces of rocks. Heated during the day under a strong sun, and chilled by quick radiation at night, these surfaces are in such a state of strain that they often crumble down, or even crack and peel off. Still more general and effective is the alternate soaking and desiccation they undergo. Saturated at one time with rain, and then baked in dry weather, the component particles are gradually loosened, and fall away into sand or clay. The influence of frost, too, where the temperature sinks to the freezing point, plays a large part in the process of weathering. The moisture imprisoned between the grains and in the crevices of rocks expands in passing into ice, pushes the grains apart, and thrusts its wedges of ice into the crevices, so that when thaw comes the loosened materials fall asunder. But rain probably plays the most important part of all in the degradation of the general surface of the land. Its influence is twofold, partly in chemically dissolving out the soluble ingredients in the rocks on which it falls, and partly in mechanically washing away the loosened materials. Nowhere can the nature of weathering be more conveniently and instructively studied than upon ancient masonry, and notably among the gravestones of a churchyard. Originally as they left the hands of the mason, the stones of a wall or the slabs and pillars of a monument were smoothly dressed, or even polished. We can, therefore, compare their present with their original condition, and mark the nature and amount of the disintegration they have suffered. Moreover, when the dates of their erection are preserved, we obtain from them a measure of the rate of waste. A few years ago I made some researches among graveyards in towns and in the country in different parts of Scotland, with the view of obtaining some data for the discussion of the question of weathering. Great differences are there observable in the character and amount of disintegration, according to the nature of the stone employed. By far the most easily destructible material is white marble. The smooth polish given to it by the sculptor is effaced in a few years; the surface of the stone becomes rough and granular so as to look like a sandstone, and if the hand is passed over it, the loosened grains of calcite, that. are ready to fall, are at once swept off. Further exposure leads to the furrowing of the marble on the side most exposed to the rain, until the natural inequalities in the texture of the stone entirely replace the artificial surface. In some cases, a crust of soot and dust forms on the marble and apparently protects it, but the stone decays underneath, and, as the crust breaks off, crumbles into mere sand. Some impressive examples of these changes may be seen in the older churchyards of Edinburgh. The handsome monument erected in the Greyfriars' Churchyard to the great Joseph Black at the beginning of this century, though partially protected from the weather, is already in some places illegible. As I examined the tomb and its Latin inscription that records the genius of the discoverer of carbonic acid, I could not but reflect on the curious irony of Nature, that has furnished in the corrosion of his monument her own testimony to the truth of his discovery. As the result of my inquiries, I found that in such a climate as that of Scotland, marble tombs freely exposed to the weather are destroyed in less than a century. The rate of superficial disintegration amounts sometimes to about a third of an inch in that time. The limestones and marbles which occur so abundantly in the FIG. 1.-Granite weathering along its joints near the top of Ben na Chie. Highlands and Lowlands must thus be liable to great and rapid decay.1 Sandstones being largely made use of for building and monumental purposes, many opportunities are afforded of examining their mode of weathering. The more com pact and siliceous kinds are remarkably durable, retaining 1 See an essay on Rock Weathering measured by the decay of tombstones, in my Geological Sketches at Home and Abroad, p. 182. с |