PART II. ROCKS, AND HOW THEY TELL THE CHAPTER X. THE MORE IMPORTANT ELEMENTS AND MINERALS OF THE EARTH'S CRUST. In the foregoing Part of this volume, we have been engaged in considering the working of various processes by which the surface of the earth is modified at the present time, and some of the more striking ways in which the record of these changes is preserved. We have seen that, on the whole, it is by deposits of some kind, laid down in situations where they can escape destruction, that the story of geological revolution is chronicled. In one place, it is the stalagmite of a cavern, in another, the silt of a lake-bottom, in a third, the sand and mud of the sea-floor, in a fourth, the lava and ashes of a volcano. In these and countless other examples, materials are removed from one place and set down in another, and in their new position, while acquiring novel characters, they retain more or less distinctly the record of their source and of the conditions under which their transference was effected. In these chapters, reference has intentionally been avoided as far as possible to details that required some knowledge of minerals and rocks, in order that the broad principles of geology, for which such knowledge is not absolutely essential, might be clearly enforced. It is obvious, however, that as minerals and rocks form the records in which the history of the earth has been preserved, this history cannot be profitably studied until some acquaintance with these materials has been made. What now lies before the reader, therefore, in order that he may be able to apply the knowledge he has gained of geological processes to the elucidation of former geological periods, is to make himself familiar with, at least, the more common and important minerals and rocks. This he can only do satisfactorily by handling the objects themselves, until he acquires such an acquaintance with them as to be able to recognise them where he meets with them in nature. At first, the number and variety of these objects may appear to be almost endless, and the learner may be apt to despair of ever mastering more than an insignificant portion of the wide circle of inquiry and observation which they present. But though the detailed study of this subject is more than enough to tax the whole powers of the most indefatigable student, it is not by any means an arduous labour, and assuredly a most interesting one, to acquire so much knowledge of the subject as to be able to follow intelligently the progress of geological investigation, and even to take personal part in it. This accordingly is the task to which he is invited in this and the following chapters. Before considering the characters presented by the various rocks that form the visible part of the earth's crust, we may find it of advantage to inquire into the general chemical composition of rocks, for by so doing we learn that though the chemist has detected more than sixty substances which he has been unable to decompose, and which, therefore, he calls elements, only a small proportion of these enter largely into the composition of the outer part of the globe. In fact, there are only about sixteen elements that play an important part as constituents of rocks; these together constitute about ninety-nine parts of the terrestrial crust. Half of them are metals; and the other half are metalloids or non-metals, as in the two subjoined lists, the most abundant being in each case placed first. Some of those elements occur in the free state, that is, not combined with any other element. Carbon, for instance, is found pure in the form of the diamond, and also as graphite. But in the great majority of cases, they assume various combinations. Most abundant are oxides, or compounds of oxygen with another element. Compounds of sulphur and a metal are known as sulphides; and similar compounds with chlorine are chlorides. Some of the compounds form further combinations with one or more elements. Thus the acid-forming oxides unite with water to form what are called acids, which, combining with metallic oxides or bases, form with them compounds termed salts. Sulphur and oxygen, for example, uniting in certain proportions with water, constitute sulphuric acid (H2SO4) which, parting with its displaceable hydrogen and combining with the metal calcium, forms the salt known as calcium-sulphate, or sulphate of lime (CaSO4). METALLOIDS. Of the non-metallic elements, by far the most abundant and important is Oxygen. In its free state, it exists as a gas which has been detected by itself at active volcanic vents. But with such rare exceptions, it is always found mixed or combined with one or more elements. Thus, mixed with nitrogen, it constitutes the atmosphere, of which it forms not less than 23 per cent by weight. It takes a still larger share in the composition of water, which consists of 88.88 per cent of oxygen and 11.12 of hydrogen. There is a continual removal of oxygen from air and water in the processes of weathering described in chapter ii. Substances which can take more of this element abstract it especially from damp air or from water. A knife or any other piece of iron, for example, will remain unchanged for an indefinite length of time if kept in dry air; but as soon as it is exposed to moisture, in which there is always some dissolved air, it begins to rust. The familiar brown crust which then forms on its surface, and slowly eats into the very centre of the iron, is due to a chemical union of oxygen with the iron, forming an oxide of iron with water. Among the rocks of the earth's crust, a large proportion are liable to undergo a similar change, and so enormous has been the extent of this change in the past history of the earth, that somewhere about one-half of the outer and accessible part of the crust consists of oxygen, which was probably at first in the atmosphere. Next in importance to oxygen among the metalloids, is Silicon, which is never met with in the free state. It has been artificially obtained, however, in the form of a dull brown powder. In nature, it always occurs united with oxygen, forming the familiar substance known as silica (SiO2), which constitutes more than a half of all the known part of the earth's crust. Silica or silicic acid is indeed the fundamental compound of the crust, forming by itself entire masses of rock, and entering as a principal constituent into FIG. 44. Group of Quartz-crystals (Rock-crystal). the majority of rocks. It occurs abundantly as the mineral quartz, the colourless transparent forms of which are known as rock-crystal (Fig. 44), and also in combination with various metallic bases as the important family of silicates. It is present in solution in most natural waters, both those of the land and of the sea, and is secreted by plants (diatoms, grasses) and animals (radiolarians, sponges). It is thus ready to be carried by percolating water into the heart of rocks, and to be deposited in their interstices and cavities. Its |