The Elements of Geology; Adapted to the Use of Schools and Colleges.

by Justin R. Loomis.

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 circ.u.mstances of the case admit of it, a much more complete knowledge of the subject will be acquired by pupils who are required to a.n.a.lyze the sections, and proceed with the recitation themselves; while the teacher has only to correct misapprehension, explain what may seem obscure, and introduce additional ill.u.s.trations.

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 quant.i.ties. 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 princ.i.p.ally 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 const.i.tutes the princ.i.p.al 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 quant.i.ties it is widely diffused in rocks, and in the waters of the ocean.

_Chlorine_ is found princ.i.p.ally 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 ma.s.ses, like other rocks of mechanical origin.

_Manganese_ is likewise extensively diffused, but in very small quant.i.ty. 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 oxide, in which form it is alumina.

It is the base of the different varieties of clay and clay-slate. It is also a const.i.tuent of felspar and mica.

_Pota.s.sium_ is an ingredient of felspar and mica, and hence is found in all the primary and in most of the volcanic rocks, as well as in the stratified rocks derived from them.

_Sodium_ is a const.i.tuent of a variety of felspar which is somewhat abundant in volcanic rocks. Its princ.i.p.al source is the extensive beds of rock-salt, and the same substance in a state of solution in the waters of the ocean.

_Calcium_ const.i.tutes about forty per cent, of limestone, and is an ingredient in nearly all igneous rocks. This metal, in the state of an oxide, is lime.

_Magnesium_ is somewhat abundant, but less so than calcium. It is one of the bases of dolomite and magnesian limestone, and is an ingredient of talc and all talcose rocks.

The substances now enumerated const.i.tute nearly the entire mineral ma.s.s of the crust of the earth. They may be arranged in the following order:--

I. NON-METALLIC SUBSTANCES.

Oxygen. Hydrogen. Nitrogen.

Carbon. Sulphur. Chlorine.

Fluorine.

II. METALS.

Iron. Manganese.

III. METALLIC BASES OF THE EARTHS AND ALKALIES.

Silicium. Aluminium. Pota.s.sium.

Sodium. Calcium. Magnesium.

These substances, chemically combined, form _Simple Minerals_.

SECTION II.--SIMPLE MINERALS.

All substances found in the earth or upon its surface, which are not the products of art or of organic life, are regarded by the mineralogist as _simple minerals_. About four hundred mineral species are known, and the varieties are much more numerous; but only a small number of them are so abundant as to claim the attention of the geologist. An acquaintance with the following species is, however, necessary.

_Quartz_ is probably the most abundant mineral in nature. It is composed wholly of silex. Its specific gravity is 2.65. It is the hardest of the common minerals, gives sparks with steel, scratches gla.s.s, and breaks into irregular angular fragments under the hammer. When crystallized, its most common form is that of a six-sided prism, terminated by six-sided pyramids. When pure, it is transparent or translucent, and its l.u.s.tre is highly vitreous. The transparent variety is called _rock crystal_. When purple, it is _amethyst_. When faint red, it is _rose quartz_. When its color is dark brown, or gray, and it has a conchoidal fracture, it is _flint_. When quartz occurs in white, tuberous ma.s.ses, of a resinous l.u.s.tre and conchoidal fracture, it is _opal_. The precious opal is distinguished by its lively play of colors. _Jasper_ is opaque, and contains a small per cent, of oxide of iron, by which it is colored dull red, yellowish red or brown. The light-colored, ma.s.sive, translucent variety is _chalcedony_. The flesh-colored specimens are _carnelian_. When composed of layers of chalcedony of different colors, it becomes _agate_. Several of the varieties of quartz, such as amethyst, opal, carnelian and agate, are used to considerable extent in jewelry.

_Felspar_ is composed of silex, alumina and pota.s.sa. It resembles quartz, but it is not as hard, cleaves more readily, and is not generally transparent. Its specific gravity is 2.47. Its l.u.s.tre is feebly vitreous, but pearly on its cleavage faces. Its color is sometimes green, but generally dull white, and often inclined to red or flesh-color.

_Mica_ is composed of the same ingredients as felspar, together with oxide of iron. Its specific gravity is nearly three. It is often colorless, but frequently green, smoky, or black. It may be known by its capability of division into exceedingly thin, transparent, elastic plates.

_Hornblende_ is composed of silex, alumina and magnesia. Its specific gravity is a little above three. Its color is generally some shade of green. When dark green or black, whether in a ma.s.sive or crystalline state, it is _common hornblende_. When light green, it is _actinolite_.

The white variety is _tremolite_. When it is composed of flexible fibres, it is _asbestus_; and when the fibres have also a silky l.u.s.tre, it is _amianthus_.

_Augite_ or _Pyroxene_ has, till recently, been considered as a variety of hornblende. Its specific gravity is slightly different; its composition is the same, and in general appearance it is not easily distinguished from hornblende. It has, however, been made a distinct species, because its crystalline form is different.