remark that the crust abounds most in the oxides of those metals which have the strongest affinity for oxygen, as the alkalies and alkaline earths; while in the peridotic and lower zones the proportion of these elements is much less, and that of the earths and metals is much greater. The minerals composing the superficial crystalline rocks, as well as water, are generally absent from the meteorites. This is especially noticed in respect to the mineral quartz or silica, so common at the surface. According to these views, the granites must once have been in a melted condition, and the excess of silica present in them have assumed the amorphous form. Many geologists have supposed the silica ought to have crystallized first, if the rock cooled from fusion. It may be that our ideas of the intense heat have been exaggerated; yet the Labrador granites of New Hampshire have recently been shown by us to be situated in sheets over a plain, precisely like the erupted lava of the present day.

We have dwelt upon the present concentric structure of the earth, because it was probably the same with that existing in the igneous period, at that time fused, but now largely solid. The order of the alternations has always been the same. It corresponds also with that observed in furnaces, where the metal sinks to the bottom, and is overlaid by one or more successive layers of slag.

This complex sphere, when molten, with its fiery billows and igneous currents, being situated in a fearfully cold region, could not fail to radiate heat; and, like other melted bodies, become covered with a congealed crust. A pot of melted iron taken out of the fire loses heat, and a crust speedily forms over it, shrinking as it cools; and, if the exterior be broken, the red liquid may be poured out. The same thing may be seen on the dumping-heaps connected with melting-works. Masses of slag, with their entire surface congealed, are placed upon the car and wheeled to the end of the pile; but, when thrown down the slope, they are fractured, and the liquid interior flows out like water. When a stream of lava flows down a slope, the surface and sides of the molten river are soon covered by a thick crust, the result of cooling. This will become so firm that men may walk upon it, as upon ice over lakes in the winter. During one of the eruptions from Vesuvius, when lava covered the town of Resina-the old Herculaneum-some of the inhabitants, driven to the tops of the houses, escaped by walking over the stiffened crust, before the flow had ceased. Whenever the lateral walls of the stream are broken, the lava will flow out and change its course. In this way, a current threatening to engulf a village may be averted and directed elsewhere. This is a practical matter, and has been turned to account in Sicily, in warding off from Catania the threatened calamity rolling down the slopes of Etna.

Our entire experience, therefore, of analogous phenomena, leads us to believe that a crust will be formed, and that the several zones will cool in natural order in later periods. Not till the last melted layer

between the crust and solid nucleus has solidified, will eruptions of lava cease to flow from volcanoes.

ness.

As time progressed this congealed crust would increase in thickBeing unyielding, there would be stamped upon it, as plainly as the form of a pitcher by the moulder, the peculiar flattening of the earth, as determined by the rate of rotation. As soon as the internal fires were concealed, the rotation of the earth would give rise to the alternation of day and night-not, certainly, of the same length as now, since the bulk of the sphere was greater, and with a reduction of size the tendency is to an increase in the rate of rotation. But, with the thick atmosphere, the days must have been dark and gloomy.

At the present day the attraction of the sun and moon produces the phenomena of the tides. As the crust is rigid, only the water upon it can now be moulded into different shapes. But, when the whole earth was pliable, its form must have varied daily, much more symmetrically than at present. As the outer envelope stiffened by cooling, tidal waves would form with great difficulty, and eventually the crust would become too rigid to be affected. Perfect rigidity was not attained during the whole inorganic period. While thin, the crust may have been broken by the attraction, and the liquid oozed out through the crevices, overflowing the surface, and returning at low tide. So great is the power of tidal attraction that a rigid envelope, hundreds of miles in thickness, would be fractured by it. The rents formed were like the faults observed in the strata of the organic periods. More or less fracture probably attended every tidal attraction, until the ocean covered the surface, and presented a material easily modulated.

AGE OF CHEMICAL CHANGES.-Following the age of igneous fluidity there succeeded another of great interest. It opens with the surface dry, rough, and slaggy; the interior in intense fusion, and the atmosphere containing all the water of the ocean with numerous volatile compounds. Before its close an ocean is formed, most of the gases have left the atmosphere, and chemical agencies acted with great intensity, and so universally as to characterize the period. The falling of the primeval rain dissolved acids in the air, and poured upon the elements never exposed to moisture streams of acidulous waters, well fitted to dissolve out large portions of the original crust.

In order to ascertain the character of this primitive rock, we must adopt the method suggested by Sterry Hunt, in his lecture before the Royal Institution of Great Britain, and consider what changes would result if intense heat should now act upon the crust. The water everywhere would be evaporated, leaving behind its saline impurities. All the carbon in living plants, and the immense supplies of coal stored up in the earth, would become converted into carbonic acid; the siliceous parts, fused with limestones and other rocks, would make silicates of lime, magnesia, etc., and expel the carbonic acid. The sulphur would form sulphurous acid with oxygen, changing eventually

into sulphuric acid as the temperature moderated. Inasmuch as seasalt, water and silica, when heated together in a confined space, form hydrochloric acid and sodium silicate, it is probable that in these early times the saline residues were decomposed, and the chlorine set free to combine with the hydrogen, and thus manufacture hydrochloric acid on a large scale. The solid bases, therefore-lime, magnesia, soda, potash, and the metals-would be combined into a great slag, and various minerals would crystallize out from it while cooling. By loss of heat the slag would contract irregularly, and there would be inequalities upon the surface, hills and valleys without system or order.

Some authors think the salt would be volatilized, and form a zone at the base of the atmosphere. The papers of Hunt, Forbes, Wurtz, Winchell, and others, show that authors cannot yet agree upon the details of those wonderful changes. The sources of our information are meagre, and the opportunity for diverse views is easy, where such immense periods of time are concerned, so that this discordance is not strange. We cannot regard Dr. Hunt's illustration as perfect, since the earth may never have been a fused mass of equal density throughout, the concentric zones having been essentially segregated in the nebulous period.

The atmosphere may possibly have been arranged in zones. Containing the present gases encircling the crust, the carbonic acid derived from coal and the carbonates, the sulphurous and hydrochloric acids, water converted into steam, and possibly volatilizable compounds, it would constitute an atmosphere of extraordinary density and insalubrity, perhaps six or seven times heavier than at present. We may suppose that the law of diffusion of gases is subordinate to that of gravitation; whence there would result four zones, viz., sulphuric and hydrochloric acids at the base, surmounted first by carbonic acid, and then by a mixture of nitrogen and oxygen; and, lastly, by steam. This dense gaseous covering would prevent much of the radiation of heat from the earth, and produce a universal tropical climate.

As the steam lies nearest the cooling influences of space, it would be the first to be affected by radiation. Drops of water would aggregate and descend, which would be vaporized again explosively, when brought in contact with hot surfaces. The cooling influence increasing its power, the number of falling drops increases, but they continuously return to the outer envelope, till the crust is sufficiently thick and cool to retain them. Thus, at the beginning of this age, there was a terrible conflict between the clouds and the earth, the former pouring down streams of water, which the latter refuse to receive; but the clouds eventually gain the mastery, and the earth sullenly evolves simmering masses of vapor from a hot-water bath.

Imagine, now, the earth capable of holding the falling drops. The water will descend in torrents, for there is to be a transference of the entire ocean from the upper atmospheric zone to the solid earth, where it

properly belongs; the waters above are to be separated by the "firmament" from the seas beneath. Next, we may observe chemical reactions. The condensed steam, in falling through the lower zones, would dissolve the sulphuric and hydrochloric gases, and convert the rain into powerful acids. When these fall upon the slaggy crust, the excrescences will not only be removed, to be deposited as sediment in the hollows, but a large percentage of the surface will enter into solution, giving rise, not to an acid ocean, but one containing sulphates and chlorides. The more soluble silicates would be converted into chlorides, leaving upon the slaggy floor piles of silica. The sulphates may have been largely of the heavier metals, not excluding the others.

Prof. Wurtz thinks the first ocean would be characterized by the predominance of sulphates. Granting this, we can understand the conversion of the sulphates into sulphurets in subsequent periods, as well as into gypsum. Aqueous deposits of sulphurets of copper, iron, lead, antimony, etc., are common in Eozoic and Paleozoic strata. The action of carbonic acid must not be overlooked. The liquid acids may have disintegrated the silicates of the alkalies and alkaline earths; but the compounds of silica, with alumina and iron, are not so easily decomposed. As soon as the carbonic acid could act upon feldspar compounds, we should have the potash and soda dissolved out as carbon

ates, leaving behind heaps of kaolin clays, such as now form, for the same reason, from the decomposition of granite. This reaction is one peculiar to dry land, and would therefore be subsequent in time to the changes already mentioned. Now, the potassium and sodium carbonates, when brought into contact with calcium chlorides, change their composition, and there result calcium carbonates and sodium and potassium chlorides. These carbonates, being insoluble, will be precipitated to the bottom, and thus will be formed the primitive travertines and limestones, while the sodium chlorides remain in solution to this day, save what has been converted into beds of rock-salt.

With the removal of the bulk of the acids and possible volatile compounds from the atmosphere, only carbonic acid would remain to render it impure at the close of the era of chemical changes. In later periods this part of the atmosphere has also been removed. The world is not yet ready for life, as there must be further chemical and mechanical changes.

THE FORMATION OF SEDIMENTS.-The next era brings into play a phase of action destined to be the chief agent of change in the world— the erosion of existing ledges to form new rocks. The era opens with a continuation of the atmospheric decompositions, whereby we find silica and alumina remaining in irregular heaps of sand and clay, and the accumulation of calcareous deposits beneath the ocean.

The formation of thick deposits of inorganic limestone is extremely interesting. Scientists have been wont to ignore altogether the existence of any deposit of this character, since microscopic researches into the structure of many of the calcareous masses exposed at the surface indicate an organic origin. So many shells and coral fragments aid in building up fossiliferous limestone that its mode of growth is very clear. But, after one has spent months in searching vainly for traces of organisms among the marble layers of Western Vermont, or the auroral limestones of Eastern Pennsylvania, he is tempted to suspect that some of the Silurian limestones even were chemical deposits, though wanting the concentric structure of stalagmite and travertine. But, barring these, and the calcareous dikes in the Laurentian of Northern New York, and in the Silurian beds of Northern Vermont, all the phenomena are best explained by the presence of an inorganic limestone before the origin of life. Whence came the materials for the stony habitations of marine animals? There must have existed great masses of the crude material, stored up in the rocks and in the waters of the sea, to provide with coverings all the testacea of every age, and to furnish the thousands of feet thickness of the Eozoic, Paleozoic, and Mesozoic limestones. This primitive source of supply is now concealed, but much of its material has been used over and over again.

We have suggested how three of the principal rock-materials have been formed the quartz, clay, and limestone. We have them yet as rude piles of rubbish, neither arranged in layers nor possessing any