Geological facts; or, the crust of the earth, what it is, and what are its uses

by W. G. Barrett

DIAGRAM II.

Here, in the words of another writer, we would add for the reader’s guidance, that “the unstratified or igneous rocks occur in no regular succession, but appear amidst the stratified without order or arrangement; heaving them out of their original horizontal positions, breaking up through them in volcanic masses, and sometimes overrunning them after the manner of liquid lava. From these circumstances they are, in general, better known by their mineral composition than by their order of occurrence. Still it may be convenient to divide them into three great classes; granite, trappean, and volcanic—granite being the basis of all known rocks, and occurring along with the primary and transition strata; the trappean, of a darker and less crystalline structure than the granite, and occurring along with the secondary and tertiary rocks; and the volcanic, still less crystalline and compact, and of comparatively recent origin, or still in process of formation.” This the student will observe by another reference to the previous diagram; but, in looking at the one now before him, we must also add for his further guidance,—for we are presuming that we address those who need initiation into the rudiments of this science, and the circumstance that we never met with a preliminary treatise that quite satisfied us, or helped such intelligent youth as were prying into the apparently cabalistic mysteries of the earth’s structural divisions, is one strong inducement to the present undertaking;—we must add, that “it must not be supposed, however, that all the stratified rocks always occur in any one portion of the earth’s crust in full and complete succession as represented” in Diagram II. “All that is meant is, that such would be their order if every group and formation were present. But whatever number of groups may be present, they never happen out of their regular order of succession; that is, clay-slate never occurs above coal, nor coal above chalk. Thus in London, tertiary strata occupy the surface; in Durham, magnesian limestone; in Fife, the coal measures; and in 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:—

DIAGRAM III.

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.