But some eruptions of granitic and other substances, ejected from the interior, never reach the surface at all. In such cases the clefts and crevices--longitudinal or oblique--are filled, but the fissures in the crust do not themselves extend to the surface. Fig. 16 represents an eruption of granite through a ma.s.s of sedimentary rock--the granite ejected from the centre fills all the clefts and fractures, but it has not been sufficiently powerful to force its way to the surface.

[Ill.u.s.tration: Fig. 16.--Eruption of granite.]

On the surface of the earth, then, which would be at first smooth and unbroken, there were formed, from the very beginning, swelling eminences, hollows, foldings, corrugations, and crevices, which would materially alter its original aspect; its arid and burning surface bristled with rugged protuberances, or was traversed by enormous fissures and cracks. Nevertheless, as the globe continued to cool, a time arrived when its temperature became insufficient to maintain, in a state of vapour, the vast ma.s.ses of water which floated in the atmosphere. These vapours would pa.s.s into the liquid state, and then the first rain fell upon the earth. Let us here remark that these were veritable rains of boiling water; for in consequence of the very considerable pressure of the atmosphere, water would be condensed and become liquid at a temperature much above 100 Centigrade (212 Fahr.)

[Ill.u.s.tration: VII.--Condensation and rainfall on the primitive globe.]

The first drop of water, which fell upon the still heated terrestrial sphere, marked a new period in its evolution--a period the mechanical and chemical effects of which it is important to a.n.a.lyse. The contact of the condensed water with the consolidated surface of the globe opens up a series of modifications of which science may undertake the examination with a degree of confidence, or at least with more positive elements of appreciation than any we possess for the period of chaos; some of the features of which we have attempted to represent, leaving of necessity much to the imagination, and for the reader to interpret after his own fashion.

The first water which fell, in the liquid state, upon the slightly cooled surface of the earth would be rapidly converted into steam by the elevation of its temperature. Thus, rendered much lighter than the surrounding atmosphere, these vapours would rise to the utmost limits of the atmosphere, where they would become condensed afresh, in consequence of their radiation towards the glacial regions of s.p.a.ce; condensing again, they would re-descend to the earth in a liquid state, to re-ascend as vapour and fall in a state of condensation. But all these changes, in the physical condition of the water, could only be maintained by withdrawing a very considerable amount of heat from the surface of the globe, whose cooling would be greatly hastened by these continual alternations of heat and cold; its heat would thus become gradually dissipated and lost in the regions of celestial s.p.a.ce.

This phenomenon extending itself by degrees to the whole ma.s.s of watery vapour existing in the atmosphere, the waters covered the earth in increasing quant.i.ties; and as the conversion of all liquids into vapour is provocative of a notable disengagement of electricity, a vast quant.i.ty of electric fluid necessarily resulted from the conversion of such large ma.s.ses of water into vapour. Bursts of thunder, and bright flashes of lightning were the necessary accompaniments of this extraordinary struggle of the elements--a state of things which M.

Maurando has attempted to represent on the opposite page (PLATE VII.).

How long did this struggle for supremacy between fire and water, with the incessant noise of thunder, continue? All that can be said in reply is, that a time came when water was triumphant. After having covered vast areas on the surface of the earth, it finally occupied and entirely covered the whole surface; for there is good reason to believe that at a certain epoch, at the commencement, so to speak, of its evolution; the earth was covered by water over its whole extent. The ocean was universal. From this moment our globe entered on a regular series of revolutions, interrupted only by the outbreaks of the internal fires which were concealed beneath its still imperfectly consolidated crust.

"At the early periods in which the materials of the ancient crystalline schists were acc.u.mulated, it cannot be doubted that the chemical processes which generated silicates were much more active than in more recent times. The heat of the earth's crust was probably then far greater than at present, while a high temperature prevailed at comparatively small depths, and thermal waters abounded. A denser atmosphere, charged with carbonic acid gas, must also have contributed to maintain, at the earth's surface, a greater degree of heat, though one not incompatible with the existence of organic life.

"These conditions must have favoured many chemical processes, which in later times have nearly ceased to operate. Hence we find that subsequently to the eozoic times, silicated rocks of clearly marked chemical origin are comparatively rare."[32]

[32] "Address to the American a.s.sociation for the Advancement of Science," by Thomas Sterry Hunt, LL.D., p. 56. 1871.

In order to comprehend the complex action, now mechanical, now chemical, which the waters, still in a heated state, exercised on the solid crust, let us consider what were the components of this crust. The rocks which formed its first _stratum_--the framework of the earth, the foundation upon which all others repose--may be presumed to have been a compound which, in varying proportions, forms granite and gneiss, and has latterly been designated by geologists Laurentian.

What is this gneiss, this granite, speaking of it with reference to its mineralogical character? It is a combination of silicates, with a base of alumina, potash, soda, and sometimes lime--_quartz_, _felspar_, and _mica_ form, by their simple aggregation, _granite_--it is thus a ternary combination, or composed of three minerals.

_Quartz_, the most abundant of all minerals, is silica more or less pure and often crystallised. _Felspar_ is a crystalline or crystallised mineral, composed of _silicate_ of alumina, potash, soda, or lime; potash-felspar is called _orthoclase_, soda-felspar _albite_, lime-felspar _anorthite_. _Mica_ is a silicate of alumina and potash, containing magnesia and oxide of iron; it takes its name from the Latin _micare_, to shine or glitter.

_Granite_ (from the Italian _grano_, being granular in its structure) is, then, a compound rock, formed of felspar, quartz, and mica, and the three const.i.tuent minerals are more or less crystalline. _Gneiss_ is a schistose variety of granite, and composed of the same minerals; the only difference between the two rocks (whatever may be their difference of origin) being that the const.i.tuent minerals, instead of being confusedly aggregated, as in granite, a.s.sume a foliated texture in gneiss. This foliated structure leads sometimes to gneiss being called _stratified granite_. "The term gneiss originated with the Freiberg miners, who from ancient times have used it to designate the rock in which their veins of silver-ore were found."[33]

[33] Cotta's "Rocks Cla.s.sified and Described," by P. H. Lawrence, p.

232.

The felspar, which enters into the composition of granite, is a mineral that is easily decomposed by water, either cold or boiling, or by the water of springs rich in carbonic acid. The chemical action of carbonic acid and water, and the action (at once chemical and mechanical) of the hot water in the primitive seas, powerfully modified the granitic rocks which lay beneath them. The warm rains which fell upon the mountain-peaks and granitic pinnacles, the torrents of rain which fell upon the slopes or in the valleys, dissolved the several alkaline silicates which const.i.tute felspar and mica, and swept them away to form elsewhere strata of clay and sand; thus were the first modifications in the primitive rocks produced by the united action of air and water, and thus were the first sedimentary rocks deposited from the oceanic waters.

The argillaceous deposits produced by this decomposition of the felspathic and micaceous rocks would partic.i.p.ate in the still heated temperature of the globe--would be again subjected to long continued heat; and when they became cool again, they would a.s.sume, by a kind of semi-crystallisation, that parallel structure which is called foliation.

All foliated rocks, then, are metamorphic, and the result of a metamorphic action to which sedimentary strata (and even some eruptive rocks) have been subjected subsequently to their deposition and consolidation, and which has produced a re-arrangement of their component mineral particles, and frequently, if not always, of their chemical elements also.

In this manner would the first beds of crystalline _schist_, such as mica-schist, be formed, probably out of sandy and clayey muds, or arenaceous and argillaceous shales.

At the end of this first phase of its existence, the terrestrial globe was, then, covered, over nearly its whole surface, with hot and muddy water, forming extensive but shallow seas. A few islands, raising their granitic peaks here and there, would form a sort of archipelago, surrounded by seas filled with earthy matter in suspension. During a long series of ages the solid crust of the globe went on increasing in thickness, as the process of solidification of the underlying liquid matter nearest to the surface proceeded. This state of tranquillity could not last long. The solid portion of the globe had not yet attained sufficient consistency to resist the pressure of the gases and boiling liquids which it covered and compressed with its elastic crust. The waves of this internal sea triumphed, more than once, over the feeble resistances which were opposed to it, making enormous dislocations and breaches in the ground--immense upheavals of the solid crust raising the beds of the seas far above their previous levels--and thus mountains arose out of the ocean, not now exclusively granitic, but composed, besides, of those schistose rocks which have been deposited under water, after long suspension in the muddy seas.

On the other hand the Earth, as it continued to cool, would also contract; and this process of contraction, as we have already explained, was another cause of dislocation at the surface, producing either considerable ruptures or simple fissures in the continuity of the crust.

These fissures would be filled, at a subsequent period, by jets of the molten matter occupying the interior of the globe--by _eruptive granite_, that is to say--or by various mineral compounds; they also opened a pa.s.sage to those torrents of heated water charged with mineral salts, with silica, the bicarbonates of lime and magnesia, which, mingling with the waters of the vast primitive ocean, were deposited at the bottom of the seas, thus helping to increase the ma.s.s of the mineral substances composing the solid portion of the globe.

These eruptions of granitic or metallic matter--these vast discharges of mineral waters through the fractured surface--would be of frequent occurrence during the primitive epoch we are contemplating. It should not, therefore, be a matter for surprise to find the more ancient rocks almost always fractured, reduced in dimensions by faults and contortions, and often traversed by veins containing metals or their oxides, such as the oxides of copper and tin; or their sulphides, such as those of lead, of antimony, or of iron--which are now the object of the miner's art.

PRIMARY EPOCH.

After the terrible tempests of the primitive period--after these great disturbances of the mineral kingdom--Nature would seem to have gathered herself together, in sublime silence, in order to proceed to the grand mystery of the creation of living beings.

During the primitive epoch the temperature of the earth was too high to admit the appearance of life on its surface. The darkness of thickest night shrouded this cradle of the world; the atmosphere probably was so charged with vapours of various kinds, that the sun's rays were powerless to pierce its opacity. Upon this heated surface, and in this perpetual night, organic life could not manifest itself. No plant, no animal, then, could exist upon the silent earth. In the seas of this epoch, therefore, only unfossiliferous strata were deposited.

Nevertheless, our planet continued to be subjected to a gradual refrigeration on the one hand, and, on the other, continuous rains were purifying its atmosphere. From this time, then, the sun's rays, being less obscured, could reach its surface, and, under their beneficent influence, life was not slow in disclosing itself. "Without light," said the ill.u.s.trious Lavoisier, "Nature was without life; it was dead and inanimate. A benevolent G.o.d, in bestowing light, has spread on the surface of the earth organisation, sentiment, and thought." We begin, accordingly, to see upon the earth--the temperature of which was nearly that of our equatorial zone--a few plants and a few animals make their appearance. These first generations of life will be replaced by others of a higher organisation, until at the last stage of the creation, man, endowed with the supreme attribute which we call intelligence, will appear upon the earth. "The word _progress_, which we think peculiar to humanity, and even to modern times," said Albert Gaudry, in a lecture on the animals of the ancient world, delivered in 1863, "was p.r.o.nounced by the Deity on the day when he created the first living organism."

Did plants precede animals? We know not; but such would appear to have been the order of creation. It is certain that in the sediment of the oldest seas, and in the vestiges which remain to us of the earliest ages of organic life on the globe, that is to say, in the argillaceous schists, we find both plants and animals of advanced organisation. But, on the other hand, during the greater part of the primary epoch--especially during the Carboniferous age--the plants are particularly numerous, and terrestrial animals scarcely show themselves; this would lead us to the conclusion that plants preceded animals. It may be remarked, besides, that from their cellular nature, and their looser tissues composed of elements readily affected by the air, the first plants could be easily destroyed without leaving any material vestiges; from which it may be concluded, that, in those primitive times, an immense number of plants existed, no traces of which now remain to us.

We have stated that, during the earlier ages of our globe, the waters covered a great part of its surface; and it is in them that we find the first appearance of life. When the waters had become sufficiently cool to allow of the existence of organised beings, creation was developed, and advanced with great energy; for it manifested itself by the appearance of numerous and very different species of animals and plants.

One of the most ancient groups of organic remains are the Brachiopoda, a group of Mollusca, particularly typified by the genus Lingula, a species of which still exist in the present seas; the Trilobites (Fig. 17), a family of Crustaceans, especially characteristic of this period; then come Productas, Terebratulae, and Orthocerat.i.tes--other genera of Mollusca. The Corals, which appeared at an early period, seem to have lived in all ages, and survive to the present day.

[Ill.u.s.tration: Fig. 17.--Paradoxides Bohemicus--Bohemia.]

Contemporaneously with these animals, plants of inferior organisation have left their impressions upon the schists; these are Algae (aquatic plants, Fig. 28). As the continents enlarged, plants of a higher type made their appearance--the Equisetaceae, herbaceous Ferns, and other plants. These we shall have occasion to specify when noticing the periods which const.i.tute the Primary Epoch, and which consists of the following periods: the Carboniferous, the Old Red Sandstone, and Devonian, the Silurian, and the Cambrian.

CAMBRIAN PERIOD.

The researches of geologists have discovered but scanty traces of organic remains in the rocks which form the base of this system in England. _Arenicolites_, or worm-tracks and burrows, have been found in Shropshire, by Mr. Salter, to occur in countless numbers through a mile of thickness in the Longmynd rocks; and others were discovered by the late Dr. Kinahan in Wicklow. In Ireland, in the picturesque tract of Bray Head, on the south and east coasts of Dublin, we find, in slaty beds of the same age as the Longmynd rocks, a peculiar zoophyte, which has been named by Edward Forbes _Oldhamia_, after its discoverer, Dr.

Oldham, Superintendent of the Geological Survey of India. This fossil represents one of the earliest inhabitants of the ocean, which then covered the greater part of the British Isles. "In the hard, purplish, and schistose rocks of Bray Head," says Dr. Kinahan,[34] "as well as other parts of Ireland which are recognised as Cambrian rocks, markings of a very peculiar character are found. They occur in ma.s.ses, and are recognised as hydrozoic animal a.s.semblages. They have regularity of form, abundant, but not universal, occurrence in beds, and permanence of character even when the beds are at a distance from each other, and dissimilar in chemical and physical character." In the course of his investigations, Dr. Kinahan discovered at least four species of Oldhamia, which he has described and figured.

[34] Trans. Roy. Irish Acad., vol. xxiii., p. 556.

The Cambrian rocks consist of the Llanberis slates of Llanberis and Penrhyn in North Wales, which, with their a.s.sociated sandy strata, attain a thickness of about 3,000 feet, and the Barmouth and Harlech Sandstones. In the Longmynd hills of Shropshire these last beds attain a thickness of 6,000 feet; and in some parts of Merionethshire they are of still greater thickness.

Neither in North Wales, nor in the Longmynd, do the Cambrian rocks afford any indications of life, except annelide-tracks and burrows. From this circ.u.mstance, together with general absence of Mollusca in these strata, and the sudden appearance of numerous sh.e.l.ls and trilobites in the succeeding Lingula Flags, a change of conditions seems to have ensued at the close of the Cambrian period.

Believing that the red colour of rocks is frequently connected with their deposition in inland waters, Professor Ramsay conceives it to be possible, that the absence of marine mollusca in the Cambrian rocks may be due to the same cause that produced their absence in the Old Red Sandstone, and that the presence of sun-cracks and rain-pittings in the Longmynd beds is a corroboration of this suggestion.[35]

[35] "On the Red Rocks of England," by A. C. Ramsay. _Quart. Jour.

Geol. Soc._, vol. xxvii., p. 250.

THE SILURIAN PERIOD.

The next period of the Primary Epoch is the _Silurian_, a system of rocks universal in extent, overspreading the whole earth more or less completely, and covering up the rocks of older age. The term "Silurian"

was given by the ill.u.s.trious Murchison to the epoch which now occupies our attention, because the system of rocks formed by the marine sediments, during the period in question, form large tracts of country in Shropshire and Wales, a region formerly peopled by the _Silures_, a Celtic race who fought gloriously against the Romans, under Caractacus or Caradoc, the British king of those tracts. The reader may find the nomenclature strange, as applied to the vast range of rocks which it represents in all parts of the Old and New World, but it indicates, with sufficient exactness, the particular region in our own country in which the system typically prevails--reasons which led to the term being adopted, even at a time when its vast geographical extent was not suspected.

On this subject, and on the principles which have guided geologists in their cla.s.sification of rocks, Professor Sedgwick remarks in one of his papers in the _Quarterly Journal of the Geological Society_: "In every country," he says,[36] "which is not made out by reference to a pre-existing type, our first labour is that of determining the physical groups, and establishing their relations by natural sections. The labour next in order is the determination of the fossils found in the successive physical groups; and, as a matter of fact, the natural groups of fossils are generally found to be nearly co-ordinate with the physical groups--each successive group resulting from certain conditions which have modified the distribution of organic types. In the third place comes the collective arrangement of the groups into systems, or groups of a higher order. The establishment of the Silurian system is an admirable example of this whole process. The groups called Caradoc, Wenlock, Ludlow, &c., were physical groups determined by good natural sections. The successive groups of fossils were determined by the sections; and the sections, as the representatives of physical groups, were hardly at all modified by any consideration of the fossils, for these two distinct views of the natural history of such groups led to co-ordinate results. Then followed the collective view of the whole series, and the establishment of a nomenclature. Not only the whole series (considered as a distinct system), but every subordinate group was defined by a geographical name, referring us to a local type within the limits of Siluria; in this respect adopting the principle of grouping and nomenclature applied by W. Smith to our secondary rocks. At the same time, the older slate rocks of Wales (inferior to the system of Siluria), were called _Cambrian_, and soon afterwards the next great collective group of rocks (superior to the system of Siluria) was called _Devonian_. In this way was established a perfect congruity of language.

It was geographical in principle, and it represented the actual development of all our older rocks, which gave to it its true value and meaning." The period, then, for the purposes of scientific description, may be divided into three sub-periods--the Upper and Lower Silurian, and the Cambrian.

[36] _Quart. Jour. Geol. Soc_., vol. iii., p. 159.

[Ill.u.s.tration: VIII.--Ideal Landscape of the Silurian Period.]