Justin R. Loomis
The Elements of Geology; Adapted to the Use of Schools and Colleges
Published by Good Press, 2019
EAN 4057664561756
Table of Contents
PREFACE
In preparing the following work, it was intended to present a systematic and somewhat complete statement of the principles of Geology, within such limits that they may be thoroughly studied in the time usually allotted to this science.
A sufficient number of leading facts has been introduced to enable the learner to feel that every important principle is a conclusion to which he has himself arrived; and yet, for the purpose of compression, that fullness of detail has been avoided with which more extended works abound. In furtherance of the same object, authorities are seldom cited.
The consideration of geological changes is made a distinct chapter, subsequent to the one on the arrangement of materials. It should, however, be remembered that these processes of arranging and disturbing are not thus separated in time. In nature the two processes are always going on together.
It seemed important to exhibit the science with as much unity and completeness as possible; and hence, discussions upon debatable points in Theoretical Geology, so interesting to mature geologists, would have been out of place here; and yet those more intricate subjects have not been omitted. A large proportion of the work is devoted to the explanation of geological phenomena, in order to convey an idea of the modes of investigation adopted, and the kind of evidence relied on. Where diversities of opinion exist, that view has been selected which seemed most in harmony with the facts; and the connection has not often been interrupted to combat, or even to state, the antagonist view.
Technical terms have, in a few instances, been introduced, and principles referred to, which are subsequently explained. The index will, however, enable the student to understand them, without a separate glossary.
Some may prefer to commence with the second chapter, deferring the study of the elementary substances, minerals and rocks, to the last. Such a course may be pursued without special inconvenience.
Questions have been added, for the convenience of those teachers who may prefer to conduct their recitations by this means. But, when the circumstances of the case admit of it, a much more complete knowledge of the subject will be acquired by pupils who are required to analyze the sections, and proceed with the recitation themselves; while the teacher has only to correct misapprehension, explain what may seem obscure, and introduce additional illustrations.
CHAPTER I.
OF THE MATERIALS WHICH COMPOSE THE CRUST OF THE EARTH.
SECTION I.—ELEMENTARY SUBSTANCES.
There are about sixty substances known to the chemist which are considered as elementary; but most of them are rarely met with, and only in minute quantities. A few of them are, however, so abundant, in the composition of the crust of the earth, as to render some attention to them necessary.
Oxygen is more widely diffused than any other substance. It is an ingredient of water and of the atmosphere, the former containing eighty-eight per cent., and the latter twenty-one. Nearly all rocks contain oxygen in combination with the metallic and metalloid bases, and the average proportion of oxygen which they contain is about forty-five per cent.; so that it will not differ much from the truth to consider the oxygen in the earth’s crust as equal in weight to all the other substances which enter into its composition.
Hydrogen occurs in nature principally in combination with oxygen, forming water. It is also an ingredient in bitumen and bituminous coal.
Nitrogen is confined almost entirely to the atmosphere, of which it forms four-fifths. It enters into the composition of some varieties of coal, and is sparingly diffused in most fossiliferous rocks.
One of the most important substances in nature is carbon. It constitutes the principal part of all the varieties of coal, as well as of graphite, peat and bituminous matter. A much larger amount of carbon exists in the carbonic acid which is combined with the oxides of the metalloids and metals. The most abundant of these compounds is limestone, which contains about twelve per cent, of carbon.
In the neighborhood of volcanoes sulphur is found pure and in a crystalline form. It is a constant ingredient in volcanic rocks, and in several of the most important ores, particularly those of lead, copper and iron. The most abundant sulphate is gypsum, which contains twenty-six per cent, of sulphur. In small quantities it is widely diffused in rocks, and in the waters of the ocean.
Chlorine is found principally as an ingredient of rock-salt, which contains sixty per cent, of it, and of sea-water, which contains one and a half per cent.
Fluorine is found, though very sparingly, in nearly all the unstratified rocks. It forms nearly half of the mineral known as Derbyshire spar.
Of the metals, Iron is the only one that is found abundantly. It enters into the composition of nearly all mineral substances. It is generally combined with oxygen, and occurs less frequently as a carbonate or sulphuret. Of volcanic rocks it forms about twenty per cent. Its ores are sometimes found in the form of dikes or seams, having been injected from below; at other times, in the form of nodules or stratified masses, like other rocks of mechanical origin.
Manganese is likewise extensively diffused, but in very small quantity. The other metals are often met with, but their localities are of very limited extent.
Of the metallic bases of the earths and alkalies, Silicium is the most abundant. It generally occurs in the form of silex, which is an oxide of the metal. There are but few rocks in which it is not found in considerable amount.
Aluminium generally occurs as an