Chapter I: Part 1

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CHAMBERS'S ELEMENTARY SCIENCE MANUALS.

GEOLOGY

BY

JAMES GEIKIE, LL.D., F.R.S.

OF H.M. GEOLOGICAL SURVEY; AUTHOR OF
'THE GREAT ICE AGE.'

[Logo]

W. & R. CHAMBERS
LONDON AND EDINBURGH
1883

Edinburgh:
Printed by W. and R. Chambers.

PREFACE.

The vital importance of diffusing some knowledge of the leading principles of Science among all classes 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 assistance 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 class 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.

CONTENTS.

PAGE
INTRODUCTORY 7

CLASSIFICATION OF ROCKS 8

MINERALOGY 12
ROCK-FORMING MINERALS 14

PETROLOGY--
MECHANICALLY FORMED ROCKS 17
CHEMICALLY FORMED ROCKS 19
ORGANICALLY DERIVED ROCKS 20
METAMORPHIC ROCKS 21
IGNEOUS ROCKS 23
STRUCTURE AND ARRANGEMENT OF ROCK-MASSES--
Stratification, &c.; Mud-cracks and Rain-prints;
Succession of Strata; Extent of Beds; Sequence of
Beds--Joints; Cleavage; Foliation; Concretions;
Inclination of Strata; Contemporaneous Erosion;
Unconformability; Overlap; Faults; Mode of
Occurrence of Metamorphic and Igneous Rocks;
Mineral Veins 26-46

DYNAMICAL GEOLOGY--
THE ATMOSPHERE AS A GEOLOGICAL AGENT OF CHANGE 46
WATER AS A GEOLOGICAL AGENT OF CHANGE 48
GEOLOGICAL ACTION OF PLANTS AND ANIMALS 60
SUBTERRANEAN FORCES 64
METAMORPHISM 72

PHYSIOGRAPHY 74

PALAEONTOLOGY 77

HISTORICAL GEOLOGY 84

QUESTIONS 89

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 multitudinous 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 masses, 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.

_Classification of Rocks._--All the rocks that we know of are thus capable of being arranged under _five_ classes, as follows:

I. MECHANICALLY FORMED.
II. CHEMICALLY FORMED.
III. ORGANICALLY DERIVED.
IV. METAMORPHIC.
V. IGNEOUS.

3. The MECHANICALLY FORMED class 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 mass 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 accumulated 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 mass 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 coarseness, from a mass in which the constituent 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 accumulated 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 homogeneous, 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 constituent 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 accumulations 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 accumulations 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 class--the MECHANICALLY FORMED ROCKS.

7. CHEMICALLY FORMED ROCKS constitute another well-marked class, 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 accumulated 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 class comprises a number of the most important and useful rock-masses. _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, shells, 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 class, 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 constituent 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 class.

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 glassy 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 masses, like certain varieties of _tuff_, up to coarse, irregular accumulations 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 classes, we have now to investigate the matter more in detail. It will be observed that the classification adopted above is based chiefly upon the external characters of the constituent 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 mass of once molten matter; in another case their collection and intimate association 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 glassy-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 lustre, 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 coarse-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 glassy and unscratchable ingredient is _quartz_; and the remaining material is _felspar_. The mineralogist's analysis 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 (potassium, 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 (potassium, 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 assumed 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 constituents 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 associated together in rocks, of which illustrative 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 analysed 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 constituent, 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 masses 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 associated with ores. With these, and a few other exceptions, all the minerals hitherto discovered contain oxygen as an essential element; and so large is the proportion of this gas which enters into union with other elements to constitute 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.

_Hematite._--This is an oxide of iron. It occurs in mammillary rounded masses, with a fibrous structure, and a dull metallic lustre. _Magnetite_ or magnetic iron ore, _specular iron_, and _limonite_ are also oxides of iron. _Hematite_ shews a red streak when scratched with a knife, which distinguishes it from magnetite.

_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 brass-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 masses 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 lustre. 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 massive, and is dark green or black, with a vitreous lustre. 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 constituents of granite, and enters into the composition of many rocks. _Albite_ is usually white. It often occurs as a constituent of granite, not unfrequently being associated 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 constituent 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 lustre. Mica is one of the common constituents of granite. _Talc_ is a silvery white, grayish, pale or dark-green coloured mineral, with a pearly lustre. 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 lustre, and is soft, and easily cut; it is tough, however, and takes on a good polish. It forms rock-masses in some places. The finer varieties are called _noble serpentine_. _Chlorite_ is another soft, easily scratched mineral, generally of a dark-green colour. It has a pearly lustre. Sometimes it occurs in beds (_chlorite-slate_), and is often found coating the walls of fissures in certain rocks. It has a somewhat greasy feel. The three last-mentioned minerals--talc, serpentine, and chlorite--are all silicates of magnesia. _Zeolites_ is a term which comprises a number of minerals of varying chemical composition, all of which tend to form a jelly when treated with acids. When heated by the blow-pipe they bubble up, owing to the escape of water; hence their name _zeolites_, from _zeo_, I boil, and _lithos_, a stone. The zeolites occur very commonly in cavities in igneous rocks, and also in mineral veins.

Having now mentioned the chief rock-forming minerals, we proceed to a brief description of some of the more typical representatives of the five great classes of rocks referred to at page 8.

PETROLOGY.[C]

[C] _Petros_, a rock, and _logos_, a discourse. Some geologists
restrict this term to the study of the _structure_ and
_arrangement of rock-masses_, and apply the term _lithology_
(_lithos_, a stone, and _logos_, a discourse) to the study of
the _mineralogical composition of rocks_.

MECHANICALLY FORMED ROCKS.

17. (_A._) SEDIMENTARY CLASS.--Three of the most commonly occurring rocks of this class have already been described, but a few details are added here.

_Conglomerate._--This is a consolidated mass of more or less water-worn and rounded stones. These stones may be of any size. When they are very large, the rock is called a _coarse conglomerate_; the finer varieties, in which the stones are small, are known as _pebbly conglomerates_. The ingredients of a conglomerate may consist of any kind of rock, or of a mixture of many different kinds. When they consist entirely of quartz, the rock becomes _quartzose_. The finer-grained conglomerates usually shew lines of deposition or bedding, but in some of the coarser sorts it is often difficult to detect any kind of arrangement. The stones are usually imbedded in a matrix of quartzose grit and sand, but sometimes this is very scanty. When the nature of the material which binds the stones together is very well marked, the rock becomes _ferruginous_, _calcareous_, _arenaceous_, or _argillaceous_, according as the binding or cementing material is _iron_, _lime_, _sand_, or _clay_. _Breccia_ is a rock in which the included fragments are _angular_.

18. _Sandstone_ is, as already remarked, merely consolidated sand. The coarser varieties, in which the grains are as large and larger than turnip-seeds, are termed _grit_. From these coarse varieties the rock passes insensibly, in one direction, into a fine or pebbly conglomerate, and in another into a rock, so fine-grained that a lens is needed to distinguish the component particles. Quartz is the prevailing ingredient--sometimes clear, at other times white. Frequently, however, the grains are coated with an oxide of iron, which gives the resulting rock a red colour. The other colours assumed by sandstone--such as yellow, brown, green, &c.--are also in like manner due to the presence of some compound of iron. When mica or felspar occurs plentifully, we have, in the one case, _micaceous sandstone_, and in the other _felspathic sandstone_. A sandstone in which the grains are cemented by carbonate of lime is said to be _calcareous_. _Freestone_ is a sandstone which can be worked freely in any direction. In most sandstones, the lines of bedding are distinct; when they are so numerous as to render the rock fissile, the sandstone is said to be _shaly_.

_Shale_ is a more or less indurated fissile or laminated clay. When the rock becomes coarse by the admixture of sand, it gradually passes into a _shaly sandstone_. There are many other varieties of clay-rocks--such as _fire-clay_, _pipe-clay_, _marl_, _loam_, &c.--which are sufficiently familiar.

19. (_B._) EOLIAN or AERIAL CLASS.--_Blown-sand_ is found at many places on sea-coasts. It generally forms smooth rounded hummocks, which are sometimes arranged in long lines parallel to the trend of the coast, as, for example, in the Tents Moor, near St Andrews. The _sand-hills_ of deserts also belong to this class.

_Debris_ is the loose angular rubbish which collects at the base of cliffs, on hill-tops, and hill-slopes. Immense accumulations of it occur in lofty mountainous districts and in arctic regions. In Nova Zembla, for example, the solid rock of the country is almost concealed beneath a thick covering of debris. But the various kinds of debris will be more particularly described further on.

_Soil._--An account of this can hardly be given without entering into the theory of its origin, and therefore we reserve its consideration for the present.

CHEMICALLY FORMED ROCKS.

20. _Stalactites_ and _stalagmites_ are carbonates of lime. They vary in colour, being white, or yellow, or brown. Stalactites are usually found adhering to the roofs of limestone caverns, &c., or depending from limestone rocks; stalagmites, on the other hand, commonly occur on the floors of limestone caverns, where they often attain a thickness of many feet.

_Siliceous sinter_ is silica with the addition of water--in other words, a hydrated quartz. It is not a very abundant rock, and is found chiefly in volcanic countries.

_Rock-salt_ has already been described. It occurs either as thin beds, or in the form of thick cake-like masses, often reaching ninety or one hundred feet in thickness. It is rudely crystalline in texture, and is usually discoloured brown and red with various impurities.

ORGANICALLY DERIVED ROCKS.

21. _Limestone_ consists of carbonate of lime, but usually contains some impurities. The varieties of this rock are numerous; some of them are as follows: _Chalk_; _oolite_, a rock built up of little spheroidal concretions, whence its name, _egg_ or _roe stone_ (the coarser oolites are called _pisolite_, or _pea-stone_); _lacustrine limestone_, &c. When much silica is diffused through the rock, we have a _siliceous limestone_; the presence of clay and of carbonaceous matter gives us _argillaceous_ and _carbonaceous limestones_. _Cornstone_ is a limestone containing a large quantity of arenaceous matter or sand. Many limestones are distinguished by the different kinds of organic remains which they yield. Thus, we have _muschelkalk_ or _shell-limestone_, _nummulitic_, _crinoidal_, &c. limestone. The crystalline limestones, such as _statuary marble_, are metamorphosed limestones. Not a few limestones are chemically formed rocks, and many, also, are partly of chemical and partly of organic origin, so that no hard and fast line can be drawn between these two classes of rock.

_Dolomite_, or _magnesian limestone_.--This is a compound of carbonate of lime and carbonate of magnesia. Its colour is usually yellow, or yellowish brown, but gray and black varieties are sometimes met with. It is generally fine-grained, with a crystalline texture, and pearly lustre. It effervesces less freely with acids than pure limestone. In many cases dolomite is merely a metamorphosed limestone.

22. _Coal_ is composed of vegetable matter, but usually contains a greater or less percentage of impurities. The varieties of this substance are very numerous, and differ from each other principally in regard to their bituminous or non-bituminous character. Coal is bituminous or non-bituminous according as it is less or more highly mineralised. Bitumen results from the decomposition of vegetable matter; but, when the mineralising process (to which the formation of coal is due) has proceeded far enough, the vegetable matter gradually loses its bituminous character, and the result is a non-bituminous coal. Varieties of coal are the following: _Lignite_ or _brown coal_; _caking coal_; _cannel_, _parrot_, or _gas coal_; _splint coal_; _cherry_ or _soft coal_; _anthracite_ or _blind coal_, so called because it burns with no flame. _Peat_ may be mentioned as another natural fuel. It is composed of vegetable matter. In some kinds it is so far decomposed, or mineralised, that the eye does not detect vegetable fibres; when thoroughly dried, such peat breaks like a good lignite, and forms an excellent fuel.

METAMORPHIC ROCKS.

23. _Quartz-rock_, or _quartzite_, is an altered quartzose sandstone or grit; it is generally a white or grayish-yellow rock, very hard and compact. The original gritty character of the rock is distinct, but the granules appear as if they had been fused so far as to become mutually adherent. When the altered sandstone has been composed of grains of quartz, felspar, or mica, set in a siliceous, felspathic, or argillaceous base, we get a rock called _greywacke_, which is usually gray or grayish blue in colour.

24. _Clay-slate_ is a grayish blue, or green, fine-grained hard rock, which splits into numerous more or less thin laminae, which may or may not coincide with the original bedding. Most usually the 'cleavage,' as this fissile structure is termed, crosses the bedding at all angles.

25. _Crystalline limestone_ is an altered condition of common limestone. _Saccharoid marble_ is one of the fine varieties: it frequently contains flakes of mica. _Dolomite_, or magnesian limestone, already described, is probably in many cases an altered limestone; the carbonate of lime having been partially dissolved out and replaced by carbonate of magnesia. _Serpentine_ is also believed by some geologists to be a highly metamorphosed magnesian limestone.

26. _Schists_.--Under this term comes a great variety of crystalline rocks which all agree in having a foliated texture--that is to say, the constituent minerals are arranged in layers which usually, but not invariably, coincide with the original bedding. Amongst the schists come _mica-schist_ (quartz and mica in alternate layers); _chlorite-schist_ (chlorite with a little quartz, and sometimes with felspar or mica); _talc-schist_ (talc with quartz or felspar); _hornblende schist_ (hornblende with a variable quantity of felspar, and sometimes a little quartz); _gneiss_ (quartz, felspar, and mica).

27. _General Character of Metamorphic Rocks._--All these rocks betray their aqueous origin by the presence of more or less distinct lines of bedding. They consist of various kinds of arenaceous and argillaceous deposits, which, under the influence of certain metamorphic actions, to be described in the sequel, have lost their original granular texture, and become more or less distinctly crystallised. And not only so, but their chemical ingredients have in many cases entered into new relations, so as to give rise to minerals which existed either sparingly or not at all in the original rocks. Frequently, it is quite impossible to say what was the original condition of some metamorphic rocks; often, however, this is sufficiently obvious. Thus, highly micaceous sandstones, as they are traced into a metamorphic region, are seen to pass gradually into mica-schist. When the bedding of gneiss becomes entirely obliterated, it is often difficult to distinguish that rock from granite, and in many cases it appears to pass into a true granite.

28. _Granite_ is a crystalline compound of quartz, felspar (usually potash-felspar), and mica. Some geologists consider it to be invariably an igneous rock; but, as just stated, it sometimes passes into gneiss in such a way as to lead us to infer its metamorphic origin. There are certain areas of sandstone in the south of Scotland which are partially metamorphosed, and in these we may trace a gradual passage from highly baked felspathic sandstones with a sub-crystalline texture into a more crystalline rock which in places graduates into true granite. Granite, however, also occurs as an igneous rock.

29. _Syenite_ is a crystalline compound of a potash-felspar and hornblende, and quartz is frequently present. _Diorite_ is a crystalline aggregate of a soda-felspar and hornblende. Both syenite and diorite also occur as igneous rocks.

There are a number of other metamorphic rocks, but those mentioned are the most commonly occurring species.

IGNEOUS ROCKS.

30. _Subdivisions._--In their chemical and mineralogical composition, igneous rocks offer great variety; but they all agree in having felspar for their base. They may be roughly divided into two classes, distinguished by the relative quantity of silica which they contain. Those in which the silica ranges from about 50 to 70 or 80 per cent. form what is termed the _acidic_ group; while those in which the percentage of silica is less constitute the _basic_ group of igneous rocks, so called because they contain a large proportion of the heavier bases, such as _magnesia_, _lime_, oxides of iron and manganese, &c. Igneous rocks vary in texture from homogeneous, compact, and finely crystalline masses up to coarsely crystalline aggregates, in which the crystals may be more than an inch in diameter. Sometimes they are dull and earthy in texture, at other times vesicular. When the vesicles are filled up with some mineral, the rock is said to be _amygdaloidal_, from the almond shape assumed by the kernels filling the cavities. When single crystals of any mineral are scattered through a rock, so as to be readily distinguished from the compact or crystalline base, the rock becomes _porphyritic_.

ACIDIC OR FELSPATHIC GROUP.

31. _Trachyte_ (_trachys_, rough) is a pale or dark-gray rock, harsh and rough to the touch, in which felspar is the predominant mineral. It is a common product of eruption in modern volcanoes.

_Clinkstone_ or _phonolite_ is a greenish-gray, compact, felspathic rock, somewhat slaty or schistose, and weathers with a white crust. It gives a clear metallic sound under the hammer. It is a rock not met with among the older formations of the earth's crust, being confined to Tertiary (see table, p. 85) or still more recent times.

_Obsidian_ or _volcanic glass_ is usually black, brown, or green, and usually resembles a coarse bottle-glass. When it becomes vesicular, it passes gradually into the highly porous rock called _pumice_. It is eminently a geologically modern volcanic rock.

_Felstone_ is a reddish-gray, bluish, greenish, or yellowish, hard, compact, flinty-looking rock, composed of potash-felspar and silica. It is generally splintery under the hammer. Some varieties are slaty, and are frequently mistaken for clinkstone, which they closely resemble. When the quartz in felstone is distinctly visible either as grains or crystals, the rock passes into a _quartz-porphyry_.

_Granite_ is recognised as an igneous as well as a metamorphic rock. Sometimes the veins and dykes which proceed from or occur near a mass of granite contain no mica--this kind of rock is called _elvan_ or _elvanite_.

_Porphyrite_ or _felspathite_ includes a number of rocks which have a felspathic base, through which felspar crystals are scattered more or less abundantly. Sometimes hornblende, or augite, or mica is present. The colour is usually dark--some shade of blue, green, red, puce, purple, or brown--and the texture varies from compact and finely crystalline up to coarsely crystalline. Porphyrites are usually porphyritic, and frequently amygdaloidal.

AUGITIC AND HORNBLENDIC OR BASIC GROUP.

32. _Basalt_ is a dark or almost black compact homogeneous rock, composed of felspar and augite with magnetic iron. An olive-green mineral called _olivine_ is very frequently present. The coarser-grained basalts are called _dolerite_. The columnar structure is not peculiarly characteristic of basalt. Many basalts are not columnar, and not a few columnar rocks are not basalts.

_Greenstone_ or _diorite_ is usually a dull greenish rock, sometimes gray, however, speckled with green. It is composed of soda-felspar and hornblende. The fine-grained compact greenstones are called _aphanite_.

_Syenite_, like granite, is recognised as an igneous as well as a metamorphic rock. There are several other rocks which come into the basic group, but those mentioned are the more common and typical species.

33. _Fragmental Igneous Rocks._--All the igneous rocks briefly described above are more or less distinctly crystalline in texture. There is a class of igneous rocks, however, which do not present this character, but when fine-grained are dull and earthy in texture, and frequently consist merely of a rude agglomeration of rough angular fragments of various rocks. These form the FRAGMENTAL group of igneous rocks. The ejectamenta of loose materials which are thrown out during a volcanic eruption, consist in chief measure of fragments of lava, &c. of all sizes, from mere dust, sand, and grit, up to blocks of more than a ton in weight. These materials, as we shall afterwards see, are scattered round the orifice of eruption in more or less irregular beds. The terms applied to the varieties of ejectamenta found among modern volcanic accumulations, will be given and explained when we come to consider the nature of geological agencies. In the British Islands, and many other non-volcanic regions, we find besides crystalline igneous rocks, abundant traces of loose ejectamenta, which clearly prove the former presence of volcanoes. These materials are sometimes quite amorphous--that is to say, they shew no trace of water action--they have not been spread out in layers, but consist of rude tumultuous accumulations of angular and subangular fragments of igneous rocks. Such masses are termed _trappean agglomerate_ and _trappean breccia_. At other times, however, the ejectamenta give evidence of having been arranged by the action of water, the materials having been sifted and spread out in more or less regular layers. What were formerly rude breccias and agglomerates of angular stones now become _trappean conglomerates_--the stones having been rounded and water-worn--while the fine ingredients, the grit, and sand, and mud, form the rock called _trap tuff_. Fragmental rocks are often quite indurated--the matrix being as hard as the included stones. But as a rule they are less hard than crystalline igneous rocks, and in many cases are loose and crumbling. When a fragmental rock is composed chiefly of rocks belonging to the acidic group, we say it is _felspathic_. When augitic and hornblendic materials predominate, then other terms are used; as, for example, _dolerite tuff_, _greenstone tuff_.

STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.

34. The student can hardly learn much about the mineralogical composition of rocks, without at the same time acquiring some knowledge of the manner of their occurrence in nature. We have already briefly described certain sedimentary rocks, such as conglomerate, sandstone, and shale, and have in some measure touched upon their structure as rock-masses. These rocks, as we have seen, are arranged in more or less thick layers or _beds_, which are piled one on the top of the other. Rocks which are so arranged are said to be _stratified_, and are termed _strata_. We may also use the word _stratum_ as an occasional substitute for _bed_. The planes of _bedding_ or _stratification_ are sometimes very close together, in other cases they are wide apart. When the separate beds are very thin, as in the case of shale, it is most usual to term them _laminae_, and to speak of the _lamination_ of a shale, as distinguished from the _bedding_ of a sandstone. Planes of bedding are generally more strongly marked than planes of lamination. The laminae frequently cohere, while beds seldom do. In the above figure, which represents a vertical cutting or _section_ through horizontal strata, the planes of lamination are shewn at _l, l, l_, and those of stratification at _s, s, s_. There are hardly any limits to the thickness of a bed--it may range from an inch up to many feet or yards, while _laminae_ vary in thickness from an inch downwards.

35. Hitherto we have been considering the _laminae_ and _strata_ as lying in an approximately horizontal plane. Sometimes, however, the layers of deposition in a single stratum are inclined at various angles to themselves, as in the following figure. This structure is called _false bedding_; the layers or laminae not coinciding with the planes of stratification. It owes its origin to shifting currents, such as the ebb and flow of the tide, and very often characterises deposits which have been formed in shallow water. (Hillocks of drifting sand frequently shew a similar structure, but their false bedding is, as a rule, much more pronounced.)

36. _Mud-cracks and Rain-prints._--The surfaces of some beds occasionally exhibit markings closely resembling those seen upon a flat sandy beach after the retreat of the tide--hence they are called _ripple-marks_ or _current-marks_. They are, of course, due to the gentle current action which pushes along the grains of sand, and hence, such marks may be formed wherever a current sweeps over the bottom of the sea with energy just sufficient for the purpose. But since the necessary conditions for the formation of _ripple-mark_ occur most abundantly in shallow water, its frequent appearance in a series of strata may often be taken as evidence, so far, for the shallow-water origin of the beds. Besides ripple-marks we may also detect occasionally on the surfaces of certain strata _mud-cracks_ and _rain-prints_. These occur most commonly in fine-grained beds, as in flagstones, argillaceous sandstones, shales, &c. The _mud-cracks_ resemble those upon a mud-flat which are caused by the desiccation and consequent shrinkage of the mud when exposed to the sun. The old cracks have been subsequently filled up again by a deposition of mud or sand, usually of harder consistency than the rock traversed by the cracks. Hence, when the bed that overlies the mud-cracks is removed, we find a cast of these projecting from its under surface, or frequently the cast remains in its mould, and forms a series of curious ridges ramifying over the whole surface of the old mud-flat. _Rain-prints_ are the small pits caused by the impact of large drops. They are usually deeper at one side than the other, from which we can infer the direction of the wind at the time the rain-drops fell. Like the mud-cracks, they are most commonly met with in fine-grained beds, and have been preserved in a similar manner. Some geologists have also been able to detect _wave-marks_, 'faint outlinings of curved form on a sandstone layer, like the outline left by a wave along the limit where it dies out upon a beach.'

37. _Succession of Strata._--The succession of strata is often very diversified. Thus, we may observe in one and the same section numberless alternating beds of sandstone and shale from an inch or so up to several feet each in thickness, with seams of coal, fireclay, ironstone, and limestone interstratified among them. In other cases, again, the succession is simpler, and some deep quarries shew only one bed, as is the case with certain limestones, fine-grained sandstones (liver-rock), and many volcanic rocks. Some limestones, indeed, shew small trace of bedding throughout a vertical thickness of hundreds of feet.

38. _Beds, their Extent, &c._--Beds of rock are not only of very different thicknesses, but they are also of very variable extent. Some may thin gradually away, or 'die out' suddenly, in a few feet or yards, while others may extend over many square miles. Beds of limestone, for example, can often be traced for leagues in several directions; and if this be the case with certain single beds, it is still more true of groups of strata. Thus the coal-bearing strata belonging to what is called the Carboniferous period cover large areas in Wales, England, Scotland, and Ireland, not less, probably, than 6000 square miles; and strata belonging to the same great period spread over considerable tracts on the Continent, and a very extensive area in North America. It holds generally true that beds of fine-grained materials are not only of more equal thickness throughout, but have also a wider extension than coarser-grained rocks. Fine sandstones, for example, extend over a wider area, and preserve a more equable thickness throughout than conglomerates, while limestones and coals are more continuous than either.

39. When a bed is followed for any distance it is frequently found to thin away, and give place to another occupying the same plane or _horizon_. Thus a shale will be replaced by a sandstone, a sandstone by a conglomerate, and _vice versa_. Sometimes also we may find a shale, as we trace it in some particular direction, gradually becoming charged with calcareous matter, so as by and by to pass, as it were, into limestone. Every bed must, of course, end somewhere, either by thus gradually passing into another, or by thinning out so as to allow beds which immediately overlie and underlie it to come together. Not unfrequently, however, a bed will stop abruptly, as in fig. 3.

40. _Sequence of Beds._--It requires little reflection to see that the division plane between two beds may represent a very long period of time. Let the following diagram represent a section of strata, _s_ being beds of grit, and _a_, _b_, _c_, beds of sandstone and shale. It is evident that the beds s must have been formed before the strata _b_ were deposited above them. At x, the beds _a_ and _b_ come together, and were attention to be confined to that part of the section, the observer might be led to infer that no great space of time elapsed between the deposition of these two beds. Yet we see that an interval sufficient to allow of the formation of the beds _s_ must really have intervened. It is now well known that in many cases planes of bedding represent 'breaks in the succession' of strata--'breaks' which are often the equivalents of considerable thicknesses of strata. In one place, for example, we may have an apparently complete sequence of beds, as _a_, _b_, _c_, which a more extended knowledge of the same beds, as these are developed in some other locality, enables us to supplement, as _a_, _s_, _b_, _c_.

41. _Joints._--Besides _planes_ or _lines of bedding_, there are certain other division planes or _joints_ by which rocks are intersected. The former, as we have seen, are congenital; the latter are subsequent. Joints cut right across the bedding, and are often variously combined, one set of joint planes traversing the rock in one direction, and another set or sets intersecting these at various angles. Thus, in many cases the rocks are so divided as easily to separate into more or less irregular fragments of various sizes. Besides these confused joints there are usually other more regular division planes, which intersect the strata in some definite directions, and run parallel to each other, often over a wide area: these are called _master-joints_. Two sets of master-joints may intersect the same strata, and when such is the case, the rock may be quarried in cuboidal blocks, the size of which will vary, of course, according as the two sets of joints are near or wide apart. Joints may either gape or be quite close; so close, indeed, as in many cases to be invisible to the naked eye. Certain igneous rocks frequently shew division planes which meet each other in such a way as to form a series of polygonal prisms. The basalt of Staffa and Giants' Causeway are familiar examples of this structure. Jointing is due to the gradual consolidation of the strata, and hence, in a series of strata, we may find the separate beds, according to their composition, very variously affected, some being much more abundantly jointed than others. Master-joints which traverse a wide district in some definite direction probably owe their origin to tension, the strata having been subjected to some strain by the underground forces.

42. _Cleavage._--Fine-grained rocks, more especially those which are argillaceous, occasionally shew another kind of structure, which is called _cleavage_. Common clay-slate is a type of the structure. This rock splits up into innumerable thin laminae or plates, the surface of which may either be somewhat rough, or as smooth nearly as glass. The cleavage planes, however, need not be parallel with the planes of bedding; in most cases, indeed, they cut right across these, and continue parallel to each other often over a very wide region. The original bedding is sometimes entirely obliterated, and in most cases of well-defined cleavage is always more or less obscure.

In the preceding diagram, the general phenomena of _bedding_, _jointing_, and _cleavage_ are represented. The lines of bedding are shewn at S, S; another set of division-planes (joints) is observed at J, J, intersecting the former at right angles--A, B, C being the exposed faces of joints. The lines of cleavage are seen at D, D, cutting across the planes of bedding and jointing.

43. _Foliation_ is another kind of superinduced structure. In a foliated rock the mineral ingredients have been crystallised and arranged in layers along either the planes of original bedding or those of cleavage. Mica-schist and gneiss are typical examples.

44. _Concretions._--In many rocks a concretionary structure may be observed. Some sandstones and shales appear as if made up of spheroidal masses, the mineral composition of the spheroids not differing apparently from that of the unchanged rock. So in some kinds of limestone, as in _dolomite_, the concretionary structure is often highly developed, the rock resembling now irregular heaps of turnips with finger-and-toe disease, again, piles of cannon-balls, or bunches of grapes, and agglomerations of musket-shot. A spheroidal structure is occasionally met with amongst some igneous rocks. This is well seen in the case of rocks having the basaltic structure, in which the pillars, being jointed transversely, decompose along their division planes, so as to form irregular globular masses. In many cases, certain mineral matter which was originally diffused through a rock has segregated so as to form nodules and irregular layers. Examples of this are _chert_ nodules in limestone; _flint_ nodules in chalk; _clay-ironstone_ balls in shale, &c.