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If these analyses are compared with those of the calcareous ooze given on page 222, it will be seen that the proportion of carbonate of lime is very much greater in the case of the chalk, and that of the silica much less. Now it has been stated that the proportion of calcic carbonate in the ooze decreases with the depth of the bottom from the surface, while samples taken from places where it lies on bottoms of less depth than 500 fathoms contain the largest amount of calcic carbonate, and thus approximate to the chalk in composition. It is only reasonable to conclude that the chalk was deposited in a sea of less depth than 500 fathoms, though doubtless at a considerable distance from land.

The student therefore will do well to remember that though the sediments of calcareous ooze, red mud, and radiolarian ooze, which are now being formed beneath the oceans, are interesting cases of modern deposits, yet they cannot be regarded as specially illustrative of the formation of such rocks as are met with in those parts of the earth's crust to which we have access.

I. is from the grey chalk of Folkestone, II. from the chalk-rock of Cambridgeshire, III. from the upper chalk.

PART II.

STRUCTURAL GEOLOGY,

OR THE STRUCTURE AND RELATIVE POSITION

THE

OF ROCK-MASSES.

HE geological significance of the principal physical operations which are now taking place in or upon the earth's crust has been considered in the first part of this volume. The knowledge thus obtained will enable us to explain the nature and structure of the various rock-masses which make up that portion of the earth's crust that is open to our observation; and to understand how the rocks have been brought into the positions in which they are now found.

When the student is in possession of these facts he is able to enter upon a third branch of geological inquiry, namely, that of Physiography, or the evolution of the physical features of the earth's surface.

IN

SECTION I-Lithology.

CHAPTER I.

CRYSTALS AND ROCK-FORMING MINERALS.

N the preceding pages we have assumed that the reader possesses such an acquaintance with the commoner kinds of rock as may be obtained from any elementary treatise on Geology or Mineralogy. To comprehend what has gone before it was only requisite that he should understand the difference between such common materials as clay, chalk, sandstone, limestone, granite, &c.; but before entering upon the special study of rock-masses, it is certainly necessary to have a more considerable acquaintance with the different kinds of rocks and minerals which occur in the earth's crust. The following chapters have therefore been prepared with the object of imparting to the student just so much mineralogical information as will enable him to comprehend the contents of those that succeed them.

Lithology, or the study of the mineral components of rocks, is based on the science of mineralogy. It is not necessary, however, for geological purposes that the student should make himself acquainted with all the different minerals known to the mineralogist, because those which may be considered as the essential constituents of rocks are comparatively few in number. It is therefore only these special rock-forming minerals which will be described in the following pages.

Again, to understand mineralogy some knowledge of chemistry is required, and if the student has not previously acquired this, it will be desirable for him to read some elementary treatise, such as Roscoe's "Primer of Chemistry" (Macmillan's Series, price one shilling). Chemistry and Mineralogy being equally outside the scope of

this manual, we shall not occupy space by giving more than a brief summary of what the student can learn elsewhere.

Chemical Elements and Compounds.-Chemical analysis has discovered that there are between sixty and seventy simple or elementary substances. All mineral bodies and all organic bodies consist either of one of these elements, or of a compound of two or more of them. Except fifteen these are all metals, and of these metallic elements many are of very rare occurrence. Of the other fifteen some are solid, as sulphur and carbon, others are always found in a gaseous state, such as oxygen, hydrogen, and nitrogen.

Of all these elementary substances not more than nineteen enter into the composition of rock-forming minerals; these are given below, together with the letters which chemists employ to denote them.

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Oxygen is the most abundant element on the earth. It combines with all the other elements except fluorine, and these simple combinations, or oxides, are consequently the most abundant and important compounds in nature. Oxygen combined with hydrogen, in the proportion of one atom of O to two atoms of H, forms water (H2O). In combination with silicon it makes the substance silica (SiO2), and combined with the metals aluminium, calcium, magnesium, it forms alumina (Al,O,), Lime (CaO), and magnesia (MgO). All these substances are oxides, and when they have no special name in ordinary use, like lime for the oxide of calcium, they are spoken of simply as the oxide of iron, or oxide of manganese.

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