Geology Part 1

Geology.

by James Geikie.

PREFACE.

The vital importance of diffusing some knowledge of the leading principles of Science among all cla.s.ses of society, is becoming daily more widely and deeply felt; and to meet and promote this important movement, W. & R. CHAMBERS have resolved on issuing the present Series of ELEMENTARY SCIENCE MANUALS. The Editors believe that they enjoy special facilities for the successful execution of such an undertaking, owing to their long experience--now extending over a period of forty years--in the work of popular education, as well as to their having the co-operation of writers specially qualified to treat the several subjects. In particular, they are happy in having the editorial a.s.sistance of ANDREW FINDLATER, LL.D., to whose labours they were so much indebted in the work of editing and preparing _Chamber's Encyclopaedia_.

The Manuals of this series are intended to serve two somewhat different purposes:

1. They are designed, in the first place, for SELF-INSTRUCTION, and will present, in a form suitable for private study, the main subjects entering into an enlightened education; so that young persons in earnest about self-culture may be able to master them for themselves.

2. The other purpose of the Manuals is, to serve as TEXT-BOOKS IN SCHOOLS. The mode of treatment naturally adopted in what is to be studied without a teacher, so far from being a drawback in a school-manual, will, it is believed, be a positive advantage. Instead of a number of abrupt statements being presented, to be taken on trust and learned, as has been the usual method in school-teaching; the subject is made, as far as possible, to unfold itself gradually, as if the pupil were discovering the principles himself, the chief function of the book being, to bring the materials before him, and to guide him by the shortest road to the discovery. This is now acknowledged to be the only profitable method of acquiring knowledge, whether as regards self-instruction or learning at school.

For simplification in teaching, the subject has been divided into sub-sections or articles, which are numbered continuously; and a series of Questions, in corresponding divisions, has been appended. These Questions, while they will enable the private student to test for himself how far he has mastered the several parts of the subject as he proceeds, will serve the teacher of a cla.s.s as specimens of the more detailed and varied examination to which he should subject his pupils.

NOTE BY THE AUTHOR.

In the present Manual of GEOLOGY it has been the aim of the author rather to indicate the methods of geological inquiry and reasoning, than to present the learner with a tedious summary of results. Attention has therefore been directed chiefly to the physical branches of the science--Palaeontology and Historical Geology, which are very large subjects of themselves, having been only lightly touched upon. The student who has attained to a fair knowledge of the scope and bearing of Physical Geology, should have little difficulty in subsequently tackling those manuals in which the results obtained by geological investigation are specially treated of.

INTRODUCTORY.

1. _Definition._--Geology is the science of the origin and development of the structure of the earth. It treats of the nature and mode of formation of the various materials of which the earth's crust is composed; it seeks to discover what mutations of land and water, and what changes of climate, have supervened during the past; it endeavours to trace the history of the mult.i.tudinous tribes of plants and animals which have successively tenanted our globe. In a word, Geology is the Physical Geography of past ages.

2. _Rocks._--Every one knows that the crust of the earth is composed of very various substances, some of which are hard and crystalline in texture, like granite; others less indurated and non-crystalline, such as sandstone, chalk, shale, &c.; while yet others are more or less soft and incoherent ma.s.ses, as gravel, sand, clay, peat, &c. Now, all these heterogeneous materials, whether they be hard or soft, compact or loose, granular or crystalline, are termed _rocks_. Blowing sand-dunes, alluvial silt and sand, and even peat, are, geologically speaking, rocks, just as much as basalt or any indurated building-stone. The variety of rocks is very great, but we do not study these long before we become aware that many kinds which present numerous contrasts in detail, yet possess certain characters in common. And this not only groups these diverse species together, but serves also to distinguish them from other species of rock, which in like manner are characterised by the presence of some prevalent generic feature or features.

_Cla.s.sification of Rocks._--All the rocks that we know of are thus capable of being arranged under _five_ cla.s.ses, as follows:

I. MECHANICALLY FORMED.

II. CHEMICALLY FORMED.

III. ORGANICALLY DERIVED.

IV. METAMORPHIC.

V. IGNEOUS.

3. The MECHANICALLY FORMED cla.s.s comprises a considerable variety of rocks, all of which, however, come under only two subdivisions--namely, _Sedimentary_, and _Eolian_ or _Aerial_, the former being by far the more important. Of the _Sedimentary_ group, there are three rocks which may be taken as typical and representative--namely, _conglomerate_ or _puddingstone_, _sandstone_, and _shale_. A short examination of the nature of these will sufficiently explain why they come to be grouped together under one head. _Conglomerate_ consists of a ma.s.s of various-sized rounded stones cemented together; each stone has been well rubbed, and rolled, and rounded. It is quite obvious that the now solid rock must at one time have existed in a loose and unconsolidated state, like gravel and shingle. Nor can we resist the conclusion that the stones were at one time rolled about by the action of water--that being the only mode in which gravel-stones are shaped. Again, when we have an opportunity of examining any considerable vertical thickness of conglomerate, we shall frequently observe that the stones are arranged more or less definitely along certain lines. These, there can be no question, are _lines of deposition_--the rounded stones have evidently not been formed and acc.u.mulated all at once, but piled up gradually, layer upon layer. And since there is no force in nature, that we know of, save water in motion, that could so round and smooth stones, and spread them out in successive layers or beds, we may now amplify our definition of conglomerate, and describe it as a _compacted ma.s.s of stones which have been more or less rounded, and arranged in more or less distinct layers or beds, by the action of water_.

4. _Sandstone_ may at the outset be described as a _granular non-crystalline rock_. This rock shews every degree of coa.r.s.eness, from a ma.s.s in which the const.i.tuent grains are nearly as large as turnip-seed, down to a stone so fine in the grain that we need a lens to discover what the particles are of which it is composed. When these latter are examined, they are found to exhibit marks of attrition, just like the stones of a conglomerate. Sharp edges have been worn off, and the grains rounded and rubbed; and whereas lines of deposition are often obscure, and of infrequent occurrence in conglomerate--in sandstone, on the contrary, they are usually well marked and often abundant. We can hardly doubt, therefore, that sandstone has also had an _aqueous_ origin, or in other words, that it has been formed and acc.u.mulated by the force of water in motion. In short, sandstone is merely compacted sand.

5. If it be easy to read the origin of conglomerate and sand in the external character of their ingredients, and the mode in which these have been arranged, we shall find it not less easy to discover the origin of _shale_. Shale is, like sandstone, a granular non-crystalline rock. The particles of which it is built up are usually too small to be distinguished without the aid of a lens, but when put under a sufficient magnifying power, they exhibit evident marks of attrition. In structure it differs widely from sandstone. In the latter rock the layers of deposition, though frequently numerous, are yet separated from each other by some considerable distance, it may be by a few inches or by many yards. But in shale the layers are so thin that we may split the rock into _laminae_ or plates. Now we know that many sedimentary materials of recent origin, such as the silt of lakes, rivers, and estuaries, although when newly dug into they appear to be more or less h.o.m.ogeneous, and shew but few lines of deposition, yet when exposed to the action of the atmosphere and dried, they very often split up into layers exhibiting division planes as minute as any observable in shale.

There is no reason to doubt, therefore, that shale is merely compacted silt and mud--the sediment deposited by water. It becomes evident, therefore, that conglomerate, sandstone, and shale are terms of one series. They are all equally sedimentary deposits, and thus, if we slightly modify our definition of conglomerate, we shall have a definition which will include the three rocks we have been considering.

For they may all be described as _granular non-crystalline rocks, the const.i.tuent ingredients of which have been more or less rounded, and arranged in more or less distinct layers, by the action of water_.

6. The _Eolian_ or _Aerial_ group of rocks embraces all natural acc.u.mulations of organic or inorganic materials, which have been formed upon the land. The group is typically represented by _debris_, such as gathers on hill-slopes and at the base of cliffs, by the _sand-hills_ of deserts and maritime districts, and by _soil_. All these acc.u.mulations owe their origin to atmospheric agencies, as will be more particularly described in the sequel. As the _Sedimentary_ and _Eolian_ rocks are the results of the _mechanical_ action of water and the atmosphere, they are fitly arranged under one great cla.s.s--the MECHANICALLY FORMED ROCKS.

7. CHEMICALLY FORMED ROCKS const.i.tute another well-marked cla.s.s, of which we may take _rock-salt_ as a typical example. This rock has evidently been deposited in water, but not in the manner of a sedimentary bed. It is not built up of water-worn particles which have been rolled about and acc.u.mulated layer upon layer, but has been slowly precipitated during the gradual evaporation of water in which it was previously held in solution. Its formation is therefore a chemical process. Various other rocks come under the same category, as we shall afterwards point out.

8. The ORGANICALLY DERIVED cla.s.s comprises a number of the most important and useful rock-ma.s.ses. _Chalk_ may be selected as a typical example. Even a slight examination shews that this rock differs widely from any of those mentioned above. Conglomerate, sandstone, shale, &c.

are built up of pebbles, particles, grains, &c. of various inorganic materials. But chalk, when looked at under the microscope, betrays an organic origin. It consists, chiefly, of the hard calcareous parts of animal organisms, and is more or less abundantly stocked with the remains of corals, sh.e.l.ls, crustaceans, &c. in every degree of preservation; indeed, so abundant are these relics, that they go to form a great proportion of the rock. _Coal_ is another familiar example of an organically derived rock, since it consists entirely of vegetable remains.

9. The METAMORPHIC cla.s.s, as the name implies, embraces all those rocks which have undergone some decided change since the time of their formation. This change generally consists in a re-arrangement of their const.i.tuent elements, and has frequently resulted in giving a crystalline texture to the rocks affected. Hence certain sedimentary deposits like sandstone and shale have been changed from granular into crystalline rocks, and the like has happened to beds of limestone and chalk. _Mica-schist, gneiss_, and _saccharoid marble_ are typical of this cla.s.s.

10. The IGNEOUS rocks are those which owe their origin to the action of the internal forces of the earth's crust. Most of them have been in a state of fusion, and betray their origin by their crystalline and sometimes gla.s.sy texture, and also, as we shall see in another section, by the mode of their occurrence. _Lava_, _basalt_, and _obsidian_ are characteristic types of this group of igneous rocks. Another group embraces a large variety of igneous rocks which are non-crystalline, and vary in texture from fine-grained, almost compact, bedded ma.s.ses, like certain varieties of _tuff_, up to coa.r.s.e, irregular acc.u.mulations of angular stones imbedded in a fine-grained or gritty matrix, like _volcanic breccia_ and _volcanic agglomerate_.

MINERALOGY.

11. Having learned that all the rocks met with at the surface of the earth's crust are capable of being arranged under a few cla.s.ses, we have now to investigate the matter more in detail. It will be observed that the cla.s.sification adopted above is based chiefly upon the external characters of the const.i.tuent ingredients of the rocks, and the mode in which these particles have been collected. In some rocks the component materials are crystalline, in others they are rounded and worn; in one case they have been brought together by precipitation from an aqueous solution, or they have crystallised out from a ma.s.s of once molten matter; in another case their collection and intimate a.s.sociation is due to the mechanical action of the atmosphere or of water, or to the agency of the organic forces. We have next to inquire what is the nature of those crystals and particles which are the ingredients of the rocks?

The answer to this question properly belongs to the science of mineralogy, with which, however, the geologist must necessarily make some acquaintance.

12. _Granite--its composition._--It will tend to simplify matters if we begin our inquiry by selecting for examination some familiar rock, such as _granite_. This rock, as one sees at a glance, is crystalline, nor is it difficult to perceive that three separate kinds of ingredients go to compose it. One of these we shall observe is a gray, or it may be, clear gla.s.sy-looking substance, which is hard, and will not scratch with a knife; another is of a pink, red, gray, or sometimes even pale green colour, and scratches with difficulty; while the third shews a glistering metallic l.u.s.tre, and is generally of a brownish or black colour. It scratches easily with the knife, and can be split up into flakes of extreme thinness. If the granite be one of the coa.r.s.e-grained varieties, we shall notice that these three ingredients have each more or less definite crystalline forms; so that they are not distinguished by colour and hardness alone. The metallic-looking substance is _mica_; the hard gray, or gla.s.sy and unscratchable ingredient is _quartz_; and the remaining material is _felspar_. The mineralogist's a.n.a.lysis of granite ends here. But there is still much to be learned about quartz, felspar, and mica; for, as the chemist will tell us, these are not 'elementary substances.' Quartz is a compound, consisting of two elements, one of which is a non-metallic body (silicon), and the other an invisible gas (oxygen). Felspar[A] is a still more complex compound, being made up of two metals (pota.s.sium, aluminium) and one non-metallic body (silicon), each of which is united to an invisible gas (oxygen).

Mica, again, contains no fewer than four metals (pota.s.sium, magnesium, iron, calcium) and one non-metallic body (silicon), each of which is in like manner chemically united to its share of oxygen. Thus the rock-forming substances, quartz, felspar, and mica, have each a definite chemical composition.

13. _Minerals._--Now, any inorganic substance which has a definite chemical composition, and crystallises in a definite crystalline or geometric form, is termed a _mineral_. Having once discovered that quartz is composed of silicon and oxygen--that is, silica--and that the faces of its crystals are arranged in a certain definite order, we may be quite sure that any mineral which has not this composition and form cannot be quartz. And so on with mica and felspar, and every other mineral. The study of the geometric forms a.s.sumed by minerals (crystallography) forms a department of the science of mineralogy. But, in the great majority of cases, the mineral ingredients of the rocks are either so small individually, or so broken, and rounded, and altered, that crystallography gives comparatively little aid to the practical geologist in the field. He has, therefore, recourse to other tests for the determination of the mineral const.i.tuents of rocks. Many of these tests, however, can only be applied by those who have had long experience. The simplest and easiest way for the student to begin is to examine the forms and appearance of the more common minerals in some collection, and thereafter to accustom his eye to the aspect presented by the same minerals when they are a.s.sociated together in rocks, of which ill.u.s.trative specimens are now to be met with in most museums. The microscope is largely employed by geologists for determining the mineralogical composition of certain rocks; and, indeed, many rocks can hardly be said to be thoroughly known until they have been sliced and examined under the microscope, and a.n.a.lysed by the chemist. But with a vast number such minute examination is not required, the eye after some practice being able to detect all that is needful to be known.

[A] There are various kinds of felspar; the one referred to above is _orthoclase_, or potash-felspar.

ROCK-FORMING MINERALS.

14. Nearly all the minerals we know of contain oxygen as a necessary ingredient, there being only a very few minerals in which that gas does not occur in chemical union with other elements. Three of these minerals, _sulphur_, the _diamond_, and _graphite_, consist of simple substances, and are of great commercial importance, but none of them is of so frequent occurrence, as a rock const.i.tuent, as the minerals presently to be described. _Sulphur_ occurs sometimes in thin beds, but more frequently in small nests and nodules, &c. in other rocks, or in joints, and fissures, and veins. It is frequently found in volcanic districts. The _diamond_, which consists of pure _carbon_, is generally met with in alluvial deposits, but sometimes, also, in a curious flexible sandstone, called _itacolumite_. _Graphite_ is another form of carbon. It occurs both in a crystalline and amorphous form, the latter, or non-crystalline kind, being extensively used for lead-pencils.

_Rock-salt_ is a _chloride of sodium_, and appears sometimes in ma.s.ses of a hundred feet and more in thickness. Another mineral which contains no oxygen is the well-known _fluor-spar_. It occurs chiefly in veins, and is often a.s.sociated with ores. With these, and a few other exceptions, all the minerals. .h.i.therto discovered contain oxygen as an essential element; and so large is the proportion of this gas which enters into union with other elements to const.i.tute the various minerals of which the rocks are composed, that it forms at least one-half of all the ponderable matter near the earth's surface. When the student learns that there are probably no fewer than six or seven hundred different minerals, he will understand how impossible it is to do more in a short geological treatise than point out a few of the most commonly occurring ones. And, indeed, a knowledge of the chief rock-forming minerals, which are few in number, is all that is absolutely requisite for geological purposes. Some of these we accordingly proceed to name.[B]

[B] It is needless to describe the minerals minutely here. The student can only learn to distinguish the different species by carefully examining actual specimens.

15. _Quartz._--This mineral has already been partially described. It is the most abundant of all the rock-forming minerals, and occurs in three forms: (1) _crystallised quartz_ or _rock crystal_; (2) _chalcedony_, both of which are composed of silica--that is, silicon and oxygen; and (3) _hydrated quartz_--that is, silica with the addition of water.

_Hemat.i.te._--This is an oxide of iron. It occurs in mammillary rounded ma.s.ses, with a fibrous structure, and a dull metallic l.u.s.tre.

_Magnet.i.te_ or magnetic iron ore, _specular iron_, and _limonite_ are also oxides of iron. _Hemat.i.te_ shews a red streak when scratched with a knife, which distinguishes it from magnet.i.te.

_Iron pyrites._--This is a sulphide of iron of very common occurrence.

Its crystalline form is cubical. When broken, it emits a sulphurous smell. The bra.s.s-yellow coloured cubes so often seen in roofing-slates are familiar examples of the mode of its occurrence. But it is also frequently found in ma.s.ses having a crystalline surface.

16. SULPHATES.--Only two sulphates may be noticed--namely, _gypsum_, which is a sulphate of lime, with its varieties, _selenite_, _satin-spar_, and _alabaster_; and _barytes_, a sulphate of baryta.

_Barytes_ scratches easily with the knife, and from its great specific gravity is often called _heavy-spar_. Gypsum is softer than barytes.

CARBONATES.--Two of these only need be mentioned: _calcite_ or _calc-spar_, a carbonate of lime, which scratches with the knife, and effervesces readily with dilute hydrochloric acid; and _arragonite_, also a carbonate of lime, but denser than calcite.

SILICATES.--These are by far the most abundantly occurring minerals. The species are also exceedingly numerous, but we may note here only a few of the more important. They are composed of silica and various bases, such as lime, potash, magnesia, soda, alumina, &c. _Augite_ or _pyroxene_ is a black or greenish-black mineral, found, either as crystals, which are generally small, or as rounded grains and angular fragments, in basaltic and volcanic rocks. It never occurs in granite rocks. It is brittle, and has a vitreous or resinous l.u.s.tre. There are a number of varieties or sub-species of augite. _Hornblende_, like augite, also includes a great many minerals. When the crystals are small, it is often difficult to distinguish hornblende from augite. Common hornblende occurs crystallised or ma.s.sive, and is dark green or black, with a vitreous l.u.s.tre. It is generally sub-translucent. It usually crystallises in igneous rocks which contain much quartz or silica; while augite, on the other hand, crystallises in igneous rocks which are of a more basic character--that is to say, rocks in which silica is not so abundantly present. _Felspar_ is a generic term which embraces a number of species, such as _orthoclase_ or _potash-felspar_, _albite_ or _soda-felspar_, and _anorthite_ or _lime-felspar_. _Orthoclase_ is white, red or pink, and gray. It is one of the ordinary const.i.tuents of granite, and enters into the composition of many rocks. _Albite_ is usually white. It often occurs as a const.i.tuent of granite, not unfrequently being a.s.sociated in the same rock with pink felspar or orthoclase. In syenite and greenstone it occurs more commonly than orthoclase. _Anorthite_ occurs in white translucent or transparent crystals. It is not so common a const.i.tuent of rocks as either of the other felspars just referred to. _Mica_: this term includes several minerals, which all agree in being highly cleavable into thin elastic flakes or laminae, which have a glistening metallic l.u.s.tre. Mica is one of the common const.i.tuents of granite. _Talc_ is a silvery white, grayish, pale or dark-green coloured mineral, with a pearly l.u.s.tre. It splits readily into thin flakes, which are flexible, but not elastic, and may be readily scratched with the nail. It is unctuous and greasy to the touch. It occurs in beds (_talc-slate_), and is often met with in districts occupied by metamorphic crystalline rocks. _Serpentine_ is generally of a green colour, but brown, red, and variously mottled varieties occur. It has a dull l.u.s.tre, and is soft, and easily cut; it is tough, however, and takes on a good polish. It forms rock-ma.s.ses in some places. The finer varieties are called _n.o.ble serpentine_.