Geologists tell us, then, that the materials of which the Earth’s Crust is composed, are not heaped together in a confused mass, but are disposed with evident marks of definite and systematic arrangement. This is an important truth, of which many examples are familiar to us all, though perhaps we do not all attend to their significance. Thus in a quarry, we see commonly enough first a bed of limestone, then above that a bed of gravel, and higher still a bed of clay: and even the limestone itself is not usually a compact mass, but is arranged in successive layers, something like the successive courses of masonry in a building. Now it appears that a very large proportion of the Earth’s Crust is made up in this way of successive layers, or _strata_, as they are called by Geologists. These _strata_ are composed of various substances, such as clay, chalk, sand, lime, and coal; and they present everywhere the same general appearances. They are known under the common name of Aqueous Rocks,[13] because it is believed that they were originally formed under water; and here it is that the professors of Geology first come into collision with the popular notions that formerly prevailed. They hold that these stratified rocks were not arranged as we see them now, when the Earth first came from the hands of its Creator, but have been formed, during the lapse of unnumbered ages, by the operation of natural causes. Nay more, they have divided the rocks into sundry classes, and they undertake to explain the particular process by which each several variety has been produced. First in order and importance are those which derive their existence from the mechanical force of moving water. The materials of which they are composed first existed in the form of minute particles, which were transported by the action of water from one place to another; then they were spread out over a given surface, just as we now see layers of sand, or mud, or gravel deposited near the mouths of rivers, or in the estuaries of the sea, or even upon the land itself during temporary inundations. Lastly, after a long interval came the slow but certain process of consolidation. The fine sand was cemented together and became sandstone; the loose gravel by a similar process was transformed into a solid mass, known by the name of Conglomerate or Pudding-stone; while the soft mud by simple pressure was converted into a kind of slaty clay, called Shale. Thus from age to age Nature was ever building up new strata, and consolidating the old.