Perthshire, the old red sandstone and clay-slate; so that it would be fruitless to dig for chalk in Durham, for magnesian limestone in Fife, or for coal in Perthshire. It would not be absurd, however, to dig for coal in Durham, because that mineral underlies the magnesian limestone; or for old red sandstone in Fife, because that formation might be naturally expected to occur under the coal strata of that country, in the regular order of succession.”[8] Still, after reading all this, we can easily imagine, not so much an air of incredulity taking possession of the countenance of our courteous reader as a feeling somewhat like this, with which we have often come into contact in those geological classes of young persons which it has been our pleasure to conduct: “Well, all that’s very plain in the book; I see granite lies at the bottom, and pushes itself up to the top very often; and I see in the diagrams that coal and chalk are not found in the same place, and that different localities have their different formations, and the various formations have their different fossils, but I confess that I cannot realize it. I know the earth is round like an orange, a little flattened at the poles—what is called an oblate spheroid; but all this surpasses my power of comprehension; can’t you make it plainer?” Well, let us try; on page 27 is a diagram, representing no ideal, but an actual boring into the earth. London is situated on the tertiary formation, in what is called geologically the basin of the London clay, that is almost on the very top of the crust, or external covering that lies on the vast mass of molten and other matter beneath. Here is first a drawing and then a section that may represent this basin:— [Illustration: DIAGRAM III. ] [Illustration: DIAGRAM IV. ] The water which falls on the chalk hills flows into them, or into the porous beds adjoining, and would rise upwards to its level but for the superincumbent pressure of the bed of clay above it, _cccc_. Under these circumstances, in order to procure water, Artesian[9] wells are sunk through the bed of clay, perhaps also through the chalk, but at any rate till the depressed stratum of chalk is reached; and this gives exit to the subterranean water, which at once rises through the iron tubes inserted in the boring to the surface. By these borings through the clay, water is obtained where it would be impossible to sink a well, or where the expense would prohibit the attempt. To explain this matter, here is a diagram (No. V.) which represents the Artesian well at the Model Prison at Pentonville, London, the strata upon which London is built, and which we can apply to the diagram on page 21, that the theory of the earth’s crust may be the more thoroughly understood before we proceed. [Illustration: DIAGRAM V. ]