Among the standard jokes of ancient Athens was that of the simpleton who, with the intent of selling his house, carried about a brick as a specimen. In this and the following chapter I propose to follow his example, and, for the purpose of giving my reader a correct notion of the structure displayed in the stratified portion of the earth's crust, to select therefrom a single formation whose details will connect together the subjects discussed in the previous pages. And in so doing it will, I trust, be found that what was ludicrous in the hands of the Greek becomes sober sense in those of the geologist. The "brick," then, which I would humbly present to the thoughtful consideration of the reader as really a specimen of the house of which it forms a part, has been termed the "Carboniferous System," and consists of a series of stratified rocks sometimes nearly 15,000 feet thick. The plants and animals found in these strata have been already described somewhat in detail, and we have turned aside to look at the processes whereby such masses of sedimentary rock came to be accumulated. But we shall probably better understand the habits of the animals and the general aspect of the vegetation, as well as the agencies at work in depositing vast beds of mineral matter, if we take a coal-field and analyse it stratum by stratum, marking as we go their varied and ever-changing character, and the corresponding diversity of the included organic remains. Such an examination will bring before us some of the more striking and important laws of geological research, and while of use to the young observer, may be not without some share of interest to the general reader. Before beginning, however, let me endeavour to illustrate the principles that will guide us by a simple though hypothetical story.