Part 7 (1/2)
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 fas.h.i.+on.
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 s.h.i.+ne 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.