Here perhaps is a stratum of stiff clay, and there is a deposit of sandstone. Here hard granite bars the path, and there an easy road is found through crumbling chalk. Here a great thickness of pebbles appears, bound together into a rock-like conglomerate; and there seams of coal alternate with limestone. Each whispers a tale of Earth’s past history; and not the same tale. They were not all made in the same workshop, or after the same mode. Each has its own separate biography. And the various layers do not lie flat and even, one upon another. Once upon a time they may so have lain, but since those days they have been moved and shoved about, tilted and lowered, heaved up and dragged down, so that some of the upper layers have in places disappeared below the pile, and in other places some of the undermost layers have been pushed to the top. It is by studying the latter, where they happen to crop up within reach, that the geologist can learn a little about deeper portions of the earth-crust and about the rocks of which it is made. By that word “rock” must be understood, not only such hard kinds as granite and marble, but softer materials, such as chalks, clays, sands, and even muds. Rocks generally have been roughly divided into two classes--the Stratified and the Unstratified. Stratified Rocks, known also as Sedimentary, have been put together in the past, grain by grain, layer by layer, under water, built gradually through ages, and slowly welded into firmness. Unstratified Rocks, known also as Igneous, from the Latin word for “fire,” have been melted down by great heat into a liquid state, and have then cooled into solid crystallized masses. When these fire-made rocks have been heaved up from lower depths and are exposed to the wearing effects of rain and wind, rivers and waves, they too have to part with much of their material, like stratified rocks, only more slowly, owing to their harder make. These materials are carried seaward, to be used in the building of fresh stratified rocks. Then again, some kinds of stratified rocks, when exposed to intense heat, will crystallize into igneous rocks. So the one kind may be the child of the other; and many rocks partake of the characteristics of both. With regard to the wearing away of rocks, the destruction of land by water, it was said in the close of the last chapter that, but for certain “counteracting forces,” the whole of the Continents might in the course of ages be slowly carried off by the Ocean, and be buried in the mighty deep. This sounds like what, in modern parlance, may be termed “a very large order.” Who could imagine a feeble substance like water having any effect upon massive granite cliffs? Few people grasp the tremendous battering force of ocean-waves upon a rocky coast, and fewer still realise the wasting power of running rivers, or of endless successions of raindrops.