[1]

CHAPTER I.
MODES OF FORMATION AND GENERAL CLASSIFICATION OF ROCKS, AQUEOUS AND IGNEOUS.

 IN old days, those who thought upon the subject at all were content to accept the world as it is, believing that from the beginning to the present day it had always been much as we now find it, and that, till the end of all things shall arrive, it will, with but slight modifications, remain the same.

 But, by and by, when Geology began to arrive at the dignity of a science, it was found that the world had passed through many changes; that the time was when the present continents and islands were not, for the strata and volcanic products of which both are formed were themselves sediments derived from the waste of yet older lands now partly lost to our knowledge, or of newer accretions of volcanic matter erupted from below. Thus it happens that what is now land has often been sea, and where the sea now rolls has often been land; and that there was a time before existing continents and islands had their places on the earth, before our present rivers began to flow, and before all the lakes of the world, as we now know them, had begun to be.

 Geology may therefore be defined as the science which investigates the history of the earth, or the successive changes which have taken place in the

[2 Definition of Geology.]

inorganic and organic kingdoms of nature, together with the causes of these changes, as far as they can be traced by observations on the structure and mode of occurrence of the mineral and organic bodies that form or are found in and upon the crust of the earth.

 To place the events of this complicated history in clear chronological succession is the chief business of the geologist; and in doing so he unites the present with past geological epochs, and discovers that the physical world, as it now exists, is the result of all the past changes that have taken place in it. If, therefore, our knowledge were sufficient to admit of the construction of a complete system of physical geography, it would he but a full description of a geological epoch—namely, that of to-day; and a complete account of any old geological epoch, would be a perfect description of the physical geography of the world at that time.

 To us, the chief dwellers on the Earth, the whole subject is of the greatest interest, and it is therefore my intention to endeavour to show in a simple manner—taking our own island as an example—whence the materials that form the present surface of the earth have been derived, why one part of a country consists of rugged mountains, and another part of high tablelands or of low plains; why the rivers run in their present channels; how the lakes that diversify the surface first came into being. In the course of this inquiry I shall have occasion to show that Britain has been joined to and severed again and again from the continent, and how some of the animals that inhabited, or still inhabit it, including its human races, came to occupy the areas where they live.

 Assuming that I am partly addressing those who have not previously studied geological subjects in detail,

[Classification of Rocks. 3]

 it is needful that I should first enter on some rudimentary points, so as to make the remainder intelligible to all. Therefore I begin with an account of the nature of rocks; because it is impossible to understand the causes that produced the various kinds of scenery of our country, and to account for the classification of its mountains and plains, without first explaining the nature of the rocks which compose them.

 To this will be added a concise account of the British strata in serial order, that the reader may understand something of the nature and history of the various stratified formations which, together with igneous rocks, form our island.

 In doing this I will endeavour to get and to give some idea of the scenery of our region during the successive geological epochs, so as to give the reader some glimpses of those older stages of physical geography, each of which in its time, had man been there to see it, would have seemed as enduring as that passing phase of the Earth's history in the midst of which we live.

 All rocks, in the broadest sense, are divided into two great classes—AQUEOUS and IGNEOUS; and there is a sub-class, which mostly consists of aqueous, but sometimes of igneous rocks that have been altered, and which in their characters often approach and even by insensible gradations pass into some of those rocks that are termed igneous, though in many respects very different from ordinary volcanic products such as lavas. In this chapter I shall, however, confine myself to a general description of the two great classes of rocks, those of aqueous or watery origin, and to those easily recognised as of igneous origin, which are products of subterranean heat.

[4 Constitution of Rocks.]

 By far the larger proportion of the surface rocks of the world have been formed by the agency of water, chiefly as a fluid, but partly as ice. Such rocks are made of sediments, and these sediments have been, and still are, chiefly the result of the action of atmospheric agencies, aided by chemical solutions, and of gravitation, aided by moving water. But by what special processes were they formed?

 Air and water, but especially the latter, act both chemically and mechanically on the crust of the earth. Many minerals in rocks, such as felspars, hornblendic minerals, mica, &c., are composed of silicates of alumina and soda, potash, lime and magnesia. These are often associated with free silica. This is especially the case with some igneous rocks; and many of the stratified rocks consist in great part of substances of the same nature variously intermixed. Others consist of carbonate and sulphate of lime, &c., more or less pure. Of these, the carbonate of lime rocks, or common limestones, by far predominate; and they are sometimes nearly pure, forming immense areas of country, and sometimes mechanically intermingling, in every percentage, with other substances. All rain as it falls absorbs part of the carbonic acid in the air; and the water percolating through the rocks unites with and carries away in solution portions of the soda, potash, lime, or magnesia that enter into the composition of the minerals in rocks, and this promotes their disintegration. They crumble, and are in a condition to be borne to lower levels, and finally to the sea, by the mechanical agency of running water, or partly in solution.

 Frost is also a powerful disintegrator. Water percolates into hollows, joints, and cracks; it freezes and expands, and thus helps to rend and break up the rocky

[Disintegration of Rocks. 5]

 and earthy masses. Some of its most obviously powerful effects are seen in the regions of glaciers and drift ice. In warm latitudes glaciers are found only at those great elevations on mountain ranges that rise above the limits of perpetual snow. On the Himalaya, the loftiest peaks of which are about 31,000 feet high, the greater glaciers descend to the level of about 14,000 feet; in the Alps, in the lower glacier of Grindelwald, to about 3,300; and in the Glacier du Bois to 3,350 feet above the sea. In the north of Norway, Greenland, and the southern part of South America, and in the Antarctic continent of Victoria Land, the large glaciers descend to the sea-level. In the two last-named regions, towards the poles, surfaces of vast extent are covered by ice in the form of universally diffused glaciers.

 A glacier in temperate regions is chiefly supplied by the drainage of the snow that falls on those parts of the mountains which rise above the limits of perpetual snow; and its size is commensurate to the height of the mountains and the extent of area drained. Pressure of the yearly accumulating snow, and in less degree the summer's heat and the winter's cold, or, indeed, the summer day's thaw and the nightly frost, gradually change snow into ice, which experience proves, acts as a whole, like a plastic body, and glaciers progress down valleys at slow rates, proportionate to the steepness of their inclination, the volume of ice, and the season of the year-moving faster in summer and autumn, and slower in winter. The effect of this motion in these icy masses is to grind, polish, scratch, and groove the rocky valleys over which the glaciers pass, removing asperities, and giving portions of the rocky floor rounded. and mammillated forms, termed roches moutonnées. A necessary result of this action is the

[6 Transportation of Sediments.]

 production of much fine floury sediment. Ice-filled valleys are thus deepened and widened, and much sediment is formed, and brought within reach of the transporting power of rivers. Great blocks of stone and finer debris that fall from the hills on the surfaces of glaciers, are carried steadily onward in long lines till they reach the ends of these ice-rivers, where they form terminal moraines, and often, as fast as the mounds accumulate, these are proportionally wasted by the streams that flow from the ends of the glaciers.

 In cold climates, where special glaciers descend to the sea, bergs break off often laden with blocks and finer sediments, and floating seaward they deposit their freights where they chance to melt. The breaking up of the ice-foot on sea-coasts, and of river ice, also transports large quantities of matter and scatters it abroad.

 The quantity of material degraded and spread in the sea by these united means is immense, and consists of mud, sand, gravel, and rounded, subangular, and angular blocks, often polished, grooved, and scratched; and from the irregular mode of its accumulation, and the frequent grounding and scraping of icebergs along the sea-bottom, the whole of this matter, if exposed, would present one of the rudest forms of stratification.

 But the chief agent in the transportation of sediments from higher to lower levels is running water. Great thunderstorms, water-spouts, atd sudden thaws in snow-covered lands, frequently produce startling effects, stripping large areas bare of soil, and hurrying to lower levels vast masses of earth, shingle, and boulders.

 Every one who has looked at large rivers knows that they are rarely pure and clear. The cause of this is obvious. All rain, especially if long continued, exercises

[Rivers. 7]

 a powerful mechanical effect on the surface of the earth, carrying much sediment into watercourses, which unite to form brooks, rivulets, and finally, if the country be large, great rivers. Soft surface soil is thus easily carried away even in low countries, and in hilly and mountainous regions sands, coarse rounded gravels, arid boulders, won from the adjoining rocks, are harried onward; and thus it happens, that great valleys and ravines have often been formed in all parts of the world by running water, and by the long-continued attrition of stones driven onward by torrents over rocky surfaces. As the accumulated waters of rivers reach low lands, their power of transporting coarse sediment decreases, and finally, in great rivers, like the Rhine, the Nile, the Amazons, the Mississippi, and the mighty rivers of China, India, and Northern Asia, all but the finest sediment is deposited long before they reach the sea.

 On a smaller scale the same kind of phenomena are obvious in such English rivers as the Thames, the Severn, the Ouse that flows through York, and the Clyde and the Tay, in Scotland. Every river, in fact, carries sediment and impurities of various kinds in suspension or held in solution, and this matter, having been derived from the waste of the lands through which rivers flow, is carried to lower levels. Thus it happens that when rivers empty themselves into lakes—or, what is far more frequently the case, into the sea—the sediments which they hold in suspension are deposited at the bottom, and, constantly increasing, they gradually form accumulations of more or less thickness, generally arranged in beds, or, as geologists usually term them, in strata. Suppose a river flowing into the sea. It carries sediment in suspension, and a layer will fall over a part of the sea-bottom, the coarser and heavier

[8 Waste of Sea Cliffs.]

 particles near the shore, while the finer and lighter matter will often be carried out by the current and deposited further off. Then another layer of sediment may be deposited on the top, and another, and another, until, in the course of time, a vast accumulation of strata may be produced.

 In this manner deltas are formed, and wide bays and arms of the sea have been thus filled up. As they fill, the marshes spread further and further, and, by overflows of the river bearing sediment, the alluvial flats rise higher and higher, till, as in cases like those of the Ganges and the Nile, kingdoms have been founded on mere loose detritus. A little reflection, too, will show that all lakes, be they ever so large, may, with sufficient time, get filled by this process with debris and become plains. Some of the old rocks of Britain are formed of sediments originally deposited in estuaries by rivers as large as the Mississippi or the Ganges, others were formed in lakes fresh or salt, bearing witness to ancient extinct physical geographies; and many a modern flat surface in Britain and in Switzerland, often covered by peat and traversed by a brook or a river, is only a lake—hollow filled with river-borne gravel, sand, and mud, overgrown by a marshy or peaty vegetation.

 Again, if we examine sea-cliffs that rise direct from the shore, we find that the disintegrating effect of the weather produces frequent débâcles great or small on the faces of the cliffs, thus supplying material for the formation of shingle, which in gales the strong breakers driving against the cliff forms a 'powerful artillery with which the ocean assails the bulwarks of the land,' and aids in the work of destruction. On the east and south of England, where the strata largely consist of boulder-clay, Eocene clays, chalk, and oolitic sands,

[Pebbles and Sand. 9]

 clays, and limestones, the waste of the softer strata has been in many places calculated at about two yards a year. Where the strata are harder, as on the west coast in Devon, Cornwall, and Wales, the waste is often so slow as to be generally ignored by ordinary observers. But the form of the coast proves it. Hard rocks resisting waste because of their hardness are apt to form headlands, while softer or more friable strata, wasting more rapidly, often occupy the recesses of coves and bays. The removal of the fallen detritus by the restless waters makes room for further slips of debris from above, and thus it happens that all sea-cliffs are in a state of constant recession, comparatively quick when made of clay or other soft strata, and when the rocks are harder, perhaps very slowly, but still sensibly to the observant eye, so that in time, be they ever so hard, they get worn more and more backwards. The material derived from this waste when sea-cliffs are truly rocky, generally forms, in the first instance, shingle at their bases, as, for example, with the pebbles of flint formed by waste of the chalk which contains them. These, being attacked by the waves, are rolled incessantly backwards and forwards, as everyone who has walked much by the sea must have noticed; for, when a large wave breaks upon the shore, it carries the shingle forward, rolling the fragments one over the other, and in the same way they recede with the retreating wave with a rattling sound. As in the running water of torrents, so this long-continued marine action has the effect of grinding angular fragments into rounded pebbles; and, in the course of time, large quantities of loose gravel have thus been formed. Such material when consolidated becomes a conglomerate.

 If, also, we examine with a lens the sand of the sea

[10 Distribution of Sediments.]

 shore, we shall find that it is formed of innumerable grains of quartz, and these grains are generally not angular, but more or less rounded: their edges having been worn off by the action of waves and tides moving them backwards and forwards upon each other, till they became grains, like water-worn pebbles in shape, only much smaller. Such material when consolidated forms sandstone.

 Finer-grained and more muddy deposits, in like manner, are generally formed of the minutest grains of sand, mixed with aluminous substances originally derived from the waste perhaps of feispathic rocks. Such material, when soft, forms clay; when consolidated, marl shale and slate.

 In this manner very large amounts of mechanical sediments are forming and have been formed. The daily sifting action of breakers, intensified during long-continued heavy gales, the forcible ejection of muddy waters, sometimes hundreds of miles out to sea, from the mouths of great rivers like the Amazons, the power of tidal and great ocean currents such as the Gulf Stream, all contribute to scatter sediments abroad, and by their rapid or more gradual subsidence, the bottoms of vast submarine areas are being covered by mechanical sediments, which must of necessity often be of great thickness, and in which various kinds of strata may alternate with each other.

 With sufficient time all land would, by these processes of waste, be eventually degraded beneath the sea (as was suggested by the naturalist Ray), were it not that the loss is compensated by disturbance and elevation of land, always slowly taking place over portions of the continents and islands of the world. Large areas are also slowly depressed beneath the sea; but to

[Elevation and Depression of Land. 11]

 maintain the average balance of sea and continent, the amount of land elevated must exceed that depressed, or be equal to the amount of that depressed by gradual submergence, added to that destroyed by degradation.

 The evidences of past elevation and depression are simple. 1st. A large proportion of the rocks in many mountain ranges, however high above the sea, contain marine fossils, generally of extinct species. Such strata are in great part highly disturbed, broken, contorted, often pierced by igneous intrusions, and largely denuded. 2nd. On all continents and on many large islands raised beaches occur, and also superficial accumulations of loose strata, lying on the older rocks, and yielding shells, in great part, or altogether identical with those that now inhabit neighbouring seas; and these organic remains occur in such a manner, that it is plain they lived and died on the spots where they lie, ere those parts of the sea-bottom were elevated. In Britain, such beds are found more than 1,000 feet above the sea; and in South America, 1,300 feet on the western side of the Andes. 3rd. Experience shows that certain volcanic regions subject to earthquakes are often areas of elevation. The earthquake of 1835 in Chili is an instance when a large tract of the coast of South America was suddenly raised from four to twelve feet, and part of the sea-bottom converted into land; and it is probable that similar causes have conduced to raise by degrees the shelly strata above alluded to, to the height of 1,300 feet above the level of the sea. The chain of the Andes is volcanic, and the elevating forces and earthquakes of SouthWestern America are connected with this circumstance. The Mediterranean volcanic region (though marked by many oscillatory movements)

[12 Consolidation of Strata.]

 is also as a whole one of elevation. The same is true of the volcanic islands of the Pacific, and also of Java, which contains many active volcanoes, and around the shores of which there are old coral reefs 140 feet above the level of the sea. Under other circumstances a great number of coral reefs of the kind called atolls and barrier reefs, yield, according to Darwin, perfect evidence of depression of land. In the Pacific an area more than 4,000 miles in length is now undergoing this kind of submergence. The same takes place in the Laccadive and Maldive archipelagos in the Indian Ocean. All these islands are non-volcanic. Where volcanoes occur the land is generally rising.

 During such depressions strata may accumulate to an immense thickness under favourable conditions of supply, and time being also allowed for consolidation, when these are again unheaved they will, both as regards quantity and structure, be more apt to resist destruction than smaller masses of (probably) softer strata that were formed during periods of minor oscillations of sea and land.

 Strata are consolidated (petrified) chiefly by pressure and chemical decomposition and recomposition. Some formations are many thousands of feet in thickness. In a set of strata 10,000 feet thick, the superincumbent weight on the lowest bed would be about 12,333 lbs. per square inch; but beside this, more intense pressures have taken place throughout all but the very latest geological epochs. This kind of pressure has been brought about by contraction of the crust of the earth due to radiation of the proper heat of our globe into space, the result being, that over broad areas rocky masses have been much contorted and compressed, and thus mountain ranges have been upheaved. In some

[Stratified Rocks. 13]

 rocks the particles are partly cemented by oxides of iron, in others by carbonate of lime. Minor beds of limestone are often formed on land from calcareous springs. Marine strata, formed of limestone, in the Adriatic, were found by Marsihi to be consolidated a foot beneath the surface. A great many rocks contain more or less carbonate of lime, and along with this, or alone, many others contain silicates of soda or potash. These are soluble in carbonic acid, and entering into new combinations the whole becomes petrified. During these processes shells, echini, corals, bones, teeth, and scales of fish and of marine mammals, &c., are imbedded and cased in stone, and in a less degree terrestrial plants and animals are floated into lakes and estuaries, and occasionally out to sea, where those parts that escape decay and predaceous fish may become fossilised.

 If we examine the stratified rocks that form the land, we very soon discover that a large proportion of them are arranged in thin layers or thicker bands or beds of shale, sandstone, conglomerate, and limestone, more or less pure; for shales are sometimes sandy, sandstones sometimes shaly, and most conglomerates have a sandy and sometimes a shaly or marly base in which the
FIG. 1.

fig 1

pebbles are embedded, while limestones occur of every degree of impurity. These must have been formed in a manner analogous to that which I have just described, proving that such beds have been deposited as sediments from water. Take, for instance, a possible cliff
[14 Stratified Rocks.]

 by the sea-shore, and we shall perhaps find that it is made of strata, which may be horizontal, as in fig. 1,
FIG. 2.

fig 2

or inclined, as in fig. 2, or even bent and contorted into every conceivable variety of form, as in fig. 3. If, as in the diagram, fig. 1, we take a particular bed, No. 1, we may find that it consists of strata of lime
FIG. 3.

fig 3

stone lying one upon the top of another. Bed No. 2 may be of shale, arranged in thin layers, more regularly than in No. 1. No. 3 may consist of pebbly materials, arranged in ruder layers, for, the material being coarse, the bedding may be irregular, or even quite indistinct. Then in No. 4, the next and highest deposit, we may have a mass of sandstone, arranged in definite beds. The whole of these various strata in the aggregate form one cliff. Rocks, more or less of these kinds, compose the bulk of the strata of the British Islands; and it must be remembered that these were originally loose stratified sediments, piled on each other often to enormous thicknesses, and subsequentiy consolidated by pressure and chemical action. In some

 [Strata and Fossils. 15]

 cases after consolidation, they have been so much altered by heat and other agents of metamorphism, as to have lost almost all signs of their original stratification, while sometimes they are almost undisturbed, except by mere upheaval above the sea: in other cases the beds have been violently contorted, in the manner shown in diagram No. 3.

 Next comes the question: Under what special conditions were given areas of these rocks formed?

 Some formations, such as great part of the Silurian rocks of Wales and its neighbourhood, consist essentially of deposits that were originally marine mud and sand, accumulated bed upon bed, intercalated here and there with strata of limestone, the whole being many thousands of feet in thickness. These have since been hardened into rock. Others, like the Old Red Sandstone, were originally spread out in alternating beds of mud, sand, and stony banks, all coloured red by precipitation of peroxide of iron. Others, like the Liassic and Oolitic deposits, were formed of alternating strata of clay, sand, and limestone; while others, like the greater masses of the Carboniferous Limestone and the Chalk, were formed almost wholly of carbonate of lime.

 When we examine such rocks in detail, we often find that they contain fossils of various kinds—shells, corals, sea-urchins, crustaceans, such as crabs and trilobites, the bones, teeth, and scales of fishes, &c., land plants, and more rarely the bones of terrestrial animals. For instance, in the bed of sandstone, No. 4 (fig. 1), we might find that there are remains of seashells; occasionally—but more rarely—similar bodies might occur in the conglomerate, No. 3; frequently they might lie between the thin layers of shale in

[16 Limestones.]

 No. 2; and it is equally common to find large quantities of shells, corals, sea-urchins, eucrinites, and various other forms of life in such limestones as No. 1, which, in many cases, are almost wholly composed of entire or broken shells and other marine organic remains.

 Marine and lake sediments form soils on and in which the creatures live that inhabit the bottom of the waters, and it is easy to understand how numerous shells and other organic bodies happen thus to have been buried in muddy, sandy, or conglomeratic mechanical sediments, the component grains of which, large or small, have been borne from the land into water, there by force of gravitation to arrange themselves as strata. By the life and death of shells in these fossilised sediments, it is also easy to understand why they are so often more or less calcareous. The question, however, arises, how it happens that strata of pure or nearly pure carbonate of lime or limestone have been formed.

 Though the materials of shale (once mud), sandstone (once loose sand), and conglomerate (once loose pebbles), have been carried from the land into the sea, and there arranged as strata, and though limestones have, in great part, been also mechanically arranged, yet it comparatively rarely happens that quantities of fine unmixed calcareous sediment have been carried in a tangible form by rivers to the sea, though it has sometimes been directly derived from the waste of sea-cliffs and mixed with other marine sediments. When, therefore, it so happens that we get a mass of limestone consisting entirely of shells, corals, and other remains, which are the skeletons of creatures that lived in the sea, in estuaries, or in lakes, the conclusion is forced upon us that, be the limestone ever so thick, it has been

[Limestones Organic. 17]

 formed entirely by the life and death of animals that lived in water. In many a formation—for instance, in some of the masses great and small of the Carboniferous Limestone—the eye tells us that they are formed perhaps entirely of rings of encrinites or stone-lilies, or of shells and corals, of various kinds, or of all these mixed together; and in many other cases where the limestone is homogeneous, the microscope reveals that it is made of foraminifera, or of exceedingly small particles of other organic remains. Even when these fragments are indistinguishable to the naked eye, reflection tells us that such marine limestone deposits must have been built up from the debris of life, for there is no reason to believe that vast formations of limestone, extending over hundreds of square miles, are now, or ever have been precipitated in the open ocean by inorganic chemical processes acting on mere chemical solutions. It sometimes happens, indeed, that gradual accumulations of such beds of limestone have attained thousands of feet of vertical thickness in what belongs to recent times in a geological sense, as for example in the great coral reefs of the Pacific Ocean, and, in less known degree, in the calcareous and foraminiferous mud of that ocean and of the Atlantic.

 But where does the carbonate of lime come from by which these animals make their skeletons? If we analyse the waters of springs and rivers, we discover that many of them consist of water that is more or less hard—that is to say, not pure, like rain-water, but containing various salts in a state of chemical solution, the most important of which is generally bicarbonate of lime; for the rain-water that falls upon the land percolates the rocks, and, rising again in springs, carries with it salts of soda, potash, &c., and, if the rocks be

[18 Igneous Rocks.]

 calcareous, large percentages of bicarbonate of lime in solution. The reason of this is, that all rain in descending through the air takes up a certain amount of carbonic acid-one of the constituents, accidental or otherwise, of the air; and this carbonic acid has the power of dissolving the carbonate of lime which enters into the composition of a large proportion of stratified rocks, which sometimes as pure limestone, form great tracts of country. In this way it happens that springs are often charged with lime, in the form of a soluble bicarbonate, which is carried by rivers into lakes and estuaries, and, finding its way to the sea, affords material to shell-fish and other marine animals, through their nutriment, to make their shells and bones. Thus it happens that, by little and little, lime is abstracted from sea-water to form parts of animal, which, dying in deep clear water, frequently produce by their skeletons and shells immense masses of strata of nearly pure limestone, which is consolidated into rock almost as fast as it is formed.

 What is going on now has been going on throughout all known geological time, from that of the deposition of the Laurentian rocks down to the present day.

 Igneous rocks form a much smaller proportion of the surface rocks of most parts of the world, though in given areas, such as Iceland and the Faroe Islands, they largely predominate. To take Britain as an example: in North Wales, a considerable proportion, perhaps a twentieth part, of the rocks of Lower Silurian age are formed of igneous masses. The whole of the rest of Wales, till we come to Pembrokeshire, contains almost none whatever. In Cumberland a very large part of the Lower Silurian rocks are igneous, while a comparatively small proportion of igneous rocks is found

[Igneous Rocks. 19]

 among the Silurian rocks of the mainland of Scotland. Even the large masses of granite there, occupy but small areas when compared with the great extent of ordinary stratified and metamorphic rocks amid which they lie. It is chiefly in the Inner Hebrides that great masses of tertiary basalts occur. Igneous rocks exist even in much smaller proportions in Derbyshire, Northumberland, Devon, and Cornwall, excepting the occasional occurrence of large bosses of granite in the two last-named counties, as for example on Dartmoor, and at Land's End. If, however, we examine all the midland, southern, and eastern parts of England, we shall find hardly any igneous rocks whatever.

 I have now briefly to indicate how we are able to distinguish igneous from aqueous rocks, in countries where there are neither active nor obvious craters of extinct volcanoes, such as those of Auvergne and the Eifel. To do this in detail would occupy a volume.

 In a general way we can distinguish them from strata formed by aqueous deposition because many of them are unstratified, and have other external and internal structures different from those of aqueous deposits. To take examples: If we examine the lavas that flowed from any existing volcano, and have afterwards consolidated, we find that they are frequently vesicular. This vesicular structure is largely due to watery vapour, and partly to gases ejected along with the melted matter, which, expanding in their efforts to escape from the melted lava, form a number of vesicles, just as yeast does in bread, or as we see in some of the slags of iron furnaces, which, indeed, are simply artificial lavas. This peculiar vesicular structure is never found in the case of unaltered stratified rocks. Here, then, experience tells that modern rocks with this

[20 Igneous Rocks.]

 structure were formed by igneous agency, and this in ancient cases is not the less certain though the vesicles have since been filled by the infiltration and deposition of mineral matters in solution, such as carbonate of lime, zeolites and silica. Such igneous rocks are called amygdaloids, and it has not infrequently happened that on the surfaces of old masses of rock, the amygdaloidal kernels, say of carbonate of lime, have been dissolved out by the influence of rainwater bearing carbonic acid, and the surface has regained its original vesicular appearance.

 Experience also tells us that some modern lavas are crystalline—that is to say, in cooling, their constituents, according to their chemical affinities, have crystallised in distinct minerals such as augite, various feispars, &c. When we meet with similar, even though not identical crystalline rocks, such as feispar—porphyries, trachytes, diorites and dolorites, associated with old strata, we are therefore entitled to consider them as having had an igneous origin.

 In modern volcanic regions, such as Iceland, and in tertiary regions dotted with extinct volcanoes of Miocene or later age, where the forms of the craters still remain, the lavas are often columnar; and when we meet with columnar and crystalline rock-masses of Silurian, Carboniferous, or of any other geological age, we may fairly assume that such rocks are of igneous origin. Modem lavas have often a vitreous structure (glassy) such as obsidian, which its ancient analogue pitchstone closely resembles. Others possess a slaggy structure, and are sometimes formed of wavy ribboned layers that indicate a state of viscous flowing, similar to the contorted ribbon-like structure common in iron and other slag s. Iron slag in fact is nothing but

[Igneous Rocks. 21]

artificial lava, formed of the silica and alumina of the iron ore and its flux of lime, melted together and still retaining a percentage of iron. Ancient lavas, such as those of Snowdon, of Lower Silurian age, often still possess a slaggy and ribboned structure. Further, igneous rocks are apt to alter any strata through which they are ejected or over which they flow. Accordingly, in rocks of all ages, and of various composition, felspathic, doloritic (hornblende and feispar), dioritic
FIG. 4.



 (augite and feispar), and various others, as in fig. 4, we frequently find veins (2) that have been injected among the strata, from dykes, as they are termed (1), rising vertically or nearly vertically through the beds from the end of which sometimes an overflow of lava (3) proceeded, that may or may not be columnar. In such cases the stratified rocks are apt to be altered for a few inches or even for several feet at their junction with the igneous rocks. If shales, they may be hardened or baked into a kind of porcellanic substance; if sandstones, turned into quartzrock, something like the sandstone floor of an iron-furnace that has long been exposed to intense heat. Occasionally the strata have been actually softened by heat, and a semi-crystalline structure has been developed.

 From these and many other circumstances, a skilled geologist finds no difficulty in deciding that such and such rocks are of igneous origin, or have been melted

 [22 Igneous Rocks.]

 by heat. The crystalline structure identical with or similar to some modern lavas, the occasional columnar structure, the amorphous earthy look, also common in certain lavas, the slaggy, ribboned, and vesicular structures, the penetration of strata by dykes and veins, and the alteration of the stratified rocks at the lines of contact, all prove the point.

 Modern volcanic ashes are simply fragments, small and large, of lava ground often to powder in the crater by the rise and fall of the steam-driven rocky material. This is finally ejected by the expansive force of steam, and with the liberated vapour, volcanic dust, lapilli and blocks of stone, are sometimes shot thousands of feet into the air mingled with watery vapour, which condensing in the higher atmosphere, falls with the ashes on the sides of the volcanic cone in heavy showers of rain. By the study of modern volcanic ashes, it is, after practice, not difficult to distinguish those of ancient date, even though they have become consolidated into hard stratified rocks. Their occasional tufaceous character, the broken crystals, the imbedded slaggy-looking fragment of rocks and bombs, and sometimes the occurrence of coarse volcanic conglomerates, every fragment of which consists of broken lava, all help in the decision. In fact, tracing back, from modern to ancient volcanoes, step by step through the various formations, the origin of ancient volcanic rocks is clear; and further, it leads to similar conclusions with respect to the igneous origin of bosses of crystalline rocks, such as some granites, syenites, and clioritic masses which, having been melted and cooled deep in the earth, were not ejected, and never saw the light till they were exposed by denudation.