The Tertiary formations, in many parts of Europe, of more or less extent, are covered by an acc.u.mulation of heterogeneous deposits, filling up the valleys, and composed of very various materials, consisting mostly of fragments of the neighbouring rocks. The erosions which we remark at the bottoms of the hills, and which have greatly enlarged already existing valleys; the mounds of gravel acc.u.mulated at one point, and which is formed of rolled materials, that is to say, of fragments of rocks worn smooth and round by continual friction during a long period, in which they have been transported from one point to another--all these signs indicate that these denudations of the soil, these displacements and transport of very heavy bodies to great distances, are due to the violent and sudden action of large currents of water. An immense wave has been thrown suddenly on the surface of the earth, making great ravages in its pa.s.sage, furrowing the earth and driving before it debris of all sorts in its disorderly course.

Geologists give the name of _diluvium_ to a formation thus removed and scattered, which, from its heterogeneous nature, brings under our eyes, as it were, the rapid pa.s.sage of an impetuous torrent--a phenomenon which is commonly designated as a _deluge_.

To what cause are we to attribute these sudden and apparently temporary invasions of the earth's surface by rapid currents of water? In all probability to the upheaval of some vast extent of dry land, to the formation of some mountain or mountain-range in the neighbourhood of the sea, or even in the bed of the sea itself. The land suddenly elevated by an upward movement of the terrestrial crust, or by the formation of ridges and furrows at the surface, has, by its reaction, violently agitated the waters, that is to say, the more mobile portion of the globe. By this new impulse the waters have been thrown with great violence over the earth, inundating the plains and valleys, and for the moment covering the soil with their furious waves, mingled with the earth, sand, and mud, of which the devastated districts have been denuded by their abrupt invasion. The phenomenon has been sudden but brief, like the upheaval of the mountain or chain of mountains, which is presumed to have been the cause of it; but it was often repeated: witness the valleys which occur in every country, especially those in the neighbourhood of Lyons and of the Durance. These strata indicate as many successive deposits. Besides this, the displacement of blocks of minerals from their normal position is proof, now perfectly recognisable, of this great phenomenon.

There have been, doubtless, during the epochs anterior to the Quaternary period of which we write, many deluges such as we are considering.

Mountains and chains of mountains, through all the ages we have been describing, were formed by upheaval of the crust into ridges, where it was too elastic or too thick to be fractured. Each of these subterranean commotions would be provocative of momentary irruptions of the waves.

But the visible testimony to this phenomenon--the living proofs of this denudation, of this tearing away of the soil, are found nowhere so strikingly as in the beds superimposed, far and near, upon the Tertiary formations, and which bear the geological name of _diluvium_. This term was long employed to designate what is now better known as the "boulder"

formation, a glacial deposit which is abundant in Europe north of the 50th, and in America north of the 40th, parallel, and re-appearing again in the southern hemisphere; but altogether absent in tropical regions.

It consists of sand and clay, sometimes stratified, mixed with rounded and angular fragments of rock, generally derived from the same district; and their origin has generally been ascribed to a series of diluvial waves raised by hurricanes, earthquakes, or the sudden upheaval of land from the bed of the sea, which had swept over continents, carrying with them vast ma.s.ses of mud and heavy stones, and forcing these stones over rocky surfaces so as to polish and impress them with furrows and striae.

Other circ.u.mstances occurred, however, to establish a connection between this formation and the glacial drift. The size and number of the erratic blocks increase as we travel towards the Arctic regions; some intimate a.s.sociation exists, therefore, between this formation and the acc.u.mulations of ice and snow which characterise the approaching glacial period.

As we have already stated at the beginning of this chapter, there is very distinct evidence of two successive deluges in our hemisphere during the Quaternary epoch. The two may be distinguished as the _European Deluge_ and the _Asiatic_. The two European deluges occurred prior to the appearance of man; the Asiatic deluge happened after that event; and the human race, then in the early days of its existence, certainly suffered from this cataclysm. In the present chapter we confine ourselves to the two cataclysms which overwhelmed Europe in the Quaternary epoch.

The first occurred in the north of Europe, where it was produced by the upheaval of the mountains of Norway. Commencing in Scandinavia, the wave spread and carried its ravages into those regions which now const.i.tute Sweden, Norway, European Russia, and the north of Germany, sweeping before it all the loose soil on the surface, and covering the whole of Scandinavia--all the plains and valleys of Northern Europe--with a mantle of transported soil. As the regions in the midst of which this great mountainous upheaval occurred--as the seas surrounding these vast s.p.a.ces were partly frozen and covered with ice, from their elevation and neighbourhood to the pole--the wave which swept these countries carried along with it enormous ma.s.ses of ice. The shock, produced by the collision of these several solid blocks of frozen water, would only contribute to increase the extent and intensity of the ravages occasioned by this violent cataclysm, which is represented in PLATE x.x.x.

The physical proof of this _deluge of the north of Europe_ exists in the acc.u.mulation of unstratified deposits which covers all the plains and low grounds of Northern Europe. On and in this deposit are found numerous blocks which have received the characteristic and significant name of erratic blocks, and which are frequently of considerable size.

These become more characteristic as we ascend to higher lat.i.tudes, as in Norway, Sweden, and Denmark, the southern borders of the Baltic, and in the British Islands generally, in all of which countries deposits of marine fossil sh.e.l.ls occur, which prove the submergence of large areas of Scandinavia, of the British Isles, and other regions during parts of the glacial period. Some of these rocks, characterised as _erratic_, are of very considerable volume; such, for instance, is the granite block which forms the pedestal of the statue of Peter the Great at St.

Petersburg. This block was found in the interior of Russia, where the whole formation is _Permian_, and its presence there can only be explained by supposing it to have been transported by some vast iceberg, carried by a diluvial current. This hypothesis alone enables us to account for another block of granite, weighing about 340 tons, which was found on the sandy plains in the north of Prussia, an immense model of which was made for the Berlin Museum. The last of these erratic blocks deposited in Germany covers the grave of King Gustavus Adolphus, of Sweden, killed at the battle of Lutzen, in 1632. He was interred beneath the rock. Another similar block has been raised in Germany into a monument to the geologist Leopold von Buch.

[Ill.u.s.tration: x.x.x.--Deluge of the North of Europe.]

These erratic blocks which are met with in the plains of Russia, Poland, and Prussia, and in the eastern parts of England, are composed of rocks entirely foreign to the region where they are found. They belong to the primary rocks of Norway; they have been transported to their present sites, protected by a covering of ice, by the waters of the northern deluge. How vast must have been the impulsive force which could carry such enormous ma.s.ses across the Baltic, and so far inland as the places where they have been deposited for the surprise of the geologist or the contemplation of the thoughtful!

The second European deluge is supposed to have been the result of the formation and upheaval of the Alps. It has filled with debris and transported material the valleys of France, Germany, and Italy over a circ.u.mference which has the Alps for its centre. The proofs of a great convulsion at a comparatively recent geological date are numerous. The Alps may be from eighty to 100 miles across, and the probabilities are that their existence is due, as Sir Charles Lyell supposes, to a succession of unequal movements of upheaval and subsidence; that the Alpine region had been exposed for countless ages to the action of rain and rivers, and that the larger valleys were of pre-glacial times, is highly probable. In the eastern part of the chain some of the Primary fossiliferous rocks, as well as Oolitic and Cretaceous rocks, and even Tertiary deposits, are observable; but in the central Alps these disappear, and more recent rocks, in some places even Eocene strata, graduate into metamorphic rocks, in which Oolitic, Cretaceous, and Eocene strata have been altered into granular marble, gneiss, and other metamorphic schists; showing that eruptions continued after the deposit of the Middle Eocene formations. Again, in the Swiss and Savoy Alps, Oolitic and Cretaceous formations have been elevated to the height of 12,000 feet, and Eocene strata 10,000 feet above the level of the sea; while in the Rothal, in the Bernese Alps, occurs a ma.s.s of gneiss 1,000 feet thick between two strata containing Oolitic fossils.

Besides these proofs of recent upheaval, we can trace effects of two different kinds, resulting from the powerful action of ma.s.ses of water violently displaced by this gigantic upheaval. At first broad tracks have been hollowed out by the diluvial waves, which have, at these points, formed deep valleys. Afterwards these valleys have been filled up by materials derived from the mountain and transported into the valley, these materials consisting of rounded pebbles, argillaceous and sandy mud, generally calcareous and ferriferous. This double effect is exhibited, with more or less distinctness, in all the great valleys of the centre and south of France. The valley of the Garonne is, in respect to these phenomena, cla.s.sic ground, as it were.

As we leave the little city of Muret, three successive levels will be observed on the left bank of the Garonne. The lowest of the three is that of the valley, properly so called; while the loftiest corresponds to the plateau of Saint-Gaudens. These three levels are distinctly marked in the Toulousean country, which ill.u.s.trates the diluvial phenomena in a remarkable fashion. The city of Toulouse reposes upon a slight eminence of diluvial formation. The flat diluvial plateau contrasts strongly with the rounded hills of Gascony and Languedoc. They are essentially const.i.tuted of a bed of gravel, formed of rounded or oval pebbles, and again covered with sandy and earthy deposits. The pebbles are princ.i.p.ally quartzose, brown or black externally, mixed with portions of hard "Old Red" and New Red Sandstone. The soft earth which accompanies the pebbles and gravel is a mixture of argillaceous sand of a red or yellow colour, caused by the oxide of iron which enters into its composition. In the valley, properly so called, we find the pebbles again a.s.sociated with other minerals which are rare at the higher levels. Some teeth of the Mammoth, and _Rhinoceros tichorhinus_, have been found at several points on the borders of this valley.

The small valleys, tributary to the princ.i.p.al valley, would appear to have been excavated secondarily, partly out of diluvial deposits, and their alluvium, essentially earthy, has been formed at the expense of the Tertiary formation, and even of the diluvium itself. Among other celebrated sites, the diluvial formation is largely developed in Sicily.

The ancient temple of the Parthenon at Athens is built on an eminence formed of diluvial earth.

In the valley of the Rhine, in Alsace, and in many isolated parts of Europe, a particular sort of _diluvium_ forms thick beds; it consists of a yellowish-grey mud, composed of argillaceous matter mixed with carbonate of lime, quartzose and micaceous sand, and oxide of iron. This mud, termed by geologists _loess_, attains in some places considerable thickness. It is recognisable in the neighbourhood of Paris. It rises a little both on the right and left, above the base of the mountains of the Black Forest and of the Vosges; and forms thick beds on the banks of the Rhine.

The fossils contained in diluvial deposits consist, generally, of terrestrial, lacustrine, or fluviatile sh.e.l.ls, for the most part belonging to species still living. In parts of the valley of the Rhine, between Bingen and Basle, the fluviatile loam or loess, now under consideration, is seen forming hills several hundred feet thick, and containing, here and there, throughout that thickness, land and fresh-water sh.e.l.ls; from which it seems necessary to suppose, according to Lyell, first, a time when the loess was slowly acc.u.mulated, then a later period, when large portions of it were removed--and followed by movements of oscillation, consisting, first, of a general depression, and then of a gradual re-elevation of the land.

We have already noticed the caverns in which such extraordinary acc.u.mulations of animal remains were discovered: it will not be out of place to give here a resume of the state of our knowledge concerning _bone-caves_ and _bone-breccias_.

The _bone-caves_ are not simply cavities hollowed out of the rock; they generally consist of numerous chambers or caverns communicating with each other by narrow pa.s.sages (often of considerable length) which can only be traversed by creeping. One in Mexico extends several leagues.

Perhaps the most remarkable in Europe is that of Gailenreuth in Franconia. The Harz mountains contain many fine caverns; among others, those of Scharrfeld and _Baumann's Hohl_, in which many bones of Hyaena, Bears, and Lions have been found together. The _Kirkdale Cave_, so well known from the description given of it by Dr. Buckland, lying about twenty-five miles north-north-east of York, was the burial-place, as we have stated, of at least 300 Hyaenas belonging to individuals of different ages; besides containing some other remains, mostly teeth (those of the Hyaena excepted) belonging to ruminating animals. Buckland states that the bones of all the other animals, those of the Hyaenas not excepted, were gnawed. He also noticed a partial polish and wearing away to a considerable depth of one side of many of the best preserved specimens of teeth and bones, which can only be accounted for by referring the partial destruction to the continual treading of the Hyaenas, and the rubbing of their skin on the side that lay uppermost at the bottom of the den.

From these facts it would appear probable that the Cave at Kirkdale was, "during a long succession of years, inhabited as a den by Hyaenas, and that they dragged into its recesses the other animal bodies, whose remains are found mixed indiscriminately with their own."[103] This conjecture is made almost certain by the discovery made by Dr. Buckland of many coprolites of animals that had fed on bones, as well as traces of the frequent pa.s.sage of these animals to or from the entrance of the cavern or den. A modern naturalist visiting the Cavern of Adelsberg, in Carniola, traversed a series of chambers extending over three leagues in the same direction, and was only stopped in his subterranean discoveries by coming to a lake which occupied its entire breadth.

[103] "Reliquiae Diluvianae," by the Rev. W. Buckland, 1823, p. 19.

The interior walls of the bone-caves are, in general, rounded off, and furrowed, presenting many traces of the erosive action of water, characteristics which frequently escape observation because the walls are covered with the calcareous deposit called _stalact.i.te_ or _stalagmite_--that is, with carbonate of lime, resulting from the deposition left by infiltrating water, through the overlying limestone, into the interior of the cavern. The formation of the stalact.i.te, with which many of the bones were incrusted in the Cave of Gailenreuth, is thus described by Liebig. The limestone over the cavern is covered with a rich soil, in which the vegetable matter is continually decaying. This mould, or humus, being acted on by moisture and air, evolves carbonic acid, which is dissolved by rain. The rain-water thus impregnated, permeating the porous limestone, dissolves a portion of it, and afterwards, when the excess of carbonic acid evaporates in the caverns, parts with the calcareous matter, and forms _stalact.i.te_--the stalact.i.tes being the pendent ma.s.ses of carbonate of lime, which hang in picturesque forms either in continuous sheets, giving the cave and its sides the appearance of being hung with drapery, or like icicles suspended from the roof of the cave, through which the water percolates; while those formed on the surface of the floor form _stalagmite_. These calcareous products ornament the walls of these gloomy caverns in a most brilliant and picturesque manner.

Under a covering of stalagmite, the floor of the cave frequently presents deposits of mud and gravel. It is in excavating this soil that the bones of antediluvian animals, mixed with sh.e.l.ls, fragments of rocks, and rolled pebbles, are discovered. The distribution of these bones in the middle of the gravelly argillaceous mud is as irregular as possible. The skeletons are rarely entire; the bones do not even occur in their natural positions. The bones of small Rodents are found acc.u.mulated in the crania of great Carnivora. The teeth of Bears, Hyaenas, and Rhinoceros are cemented with the jaw-bones of Ruminants. The bones are very often polished and rounded, as if they had been transported from great distances; others are fissured; others, nevertheless, are scarcely altered. Their state of preservation varies with their position in the cave.

The bones most frequently found in caves are those of the Carnivora of the Quaternary epoch: the Bear, Hyaena, the Lion, and Tiger. The animals of the plain, and notably the great Pachyderms--the Mammoth and Rhinoceros--are only very rarely met with, and always in small numbers.

From the cavern of Gailenreuth more than a thousand skeletons have been taken, of which 800 belonged to the large _Ursus spelaeus_, and sixty to the smaller species, with 200 Hyaenas, Wolves, Lions, and Gluttons. A jaw of the Glutton has lately been found by Mr. T. McK. Hughes in a cave in the Mountain Limestone at Plas Heaton, a.s.sociated with Wolf, Bison, Reindeer, Horse, and Cave Bear; proving that the Glutton, which at the present day inhabits Siberia and the inclement northern regions of Europe, inhabited Great Britain during the Pleistocene or Quaternary Period. In the Kirkdale cave the remains, as we have seen, included those of not less than 300 Hyaenas of all ages. Dr. Buckland concludes, from these circ.u.mstances, that the Hyaenas alone made this their den, and that the bones of other animals acc.u.mulated there had been carried thither by them as their prey; it is, however, now admitted that this part of the English geologist's conclusions do not apply to the contents of all bone-caves. In some instances the bones of the Mammals are broken and worn as with a long transport, _rolled_, according to the technical geological expression, and finally cemented in the same mud, together with fragments of the rocks of the neighbourhood. Besides bones of Hyaenas, are found not only the bones of inoffensive herbivora, but remains of Lions and Bears.

We ought to note, in order to make this explanation complete, that some geologists consider that these caves served as a refuge for sick and wounded animals. It is certain that we see, in our own days, some animals, when attacked by sickness, seek refuge in the fissures of rocks, or in the hollows of trunks of trees, where they die; to this natural impulse it may, probably, be ascribed that the skeletons of animals are so rarely found in forests or plains. We may conclude, then, that besides the more general mode in which these caverns were filled with bones, the two other causes which we have enumerated may have been in operation; that is to say, they were the habitual sojourn of carnivorous and destructive animals, and they became the retreat of sick animals on some particular occasions.

What was the origin of these caves? How have these immense excavations been produced? Nearly all these caves occur in limestone rocks, particularly in the Jura.s.sic and Carboniferous formations, which present many vast subterranean caverns. At the same time some fine caves exist in the Silurian formation, such as the _Grotto des Demoiselles_ (Fig.

194) near Ganges, of Herault. It should be added, in order to complete the explanation of the cave formations, that the greater part of these vast internal excavations have been chiefly caused by subterranean watercourses, which have eroded and washed away a portion of the walls, and in this manner greatly enlarged their original dimensions.

But there are other modes than the above of accounting, in a more satisfactory manner, for the existence of these caves. According to Sir Charles Lyell, there was a time when (as now) limestone rocks were dissolved, and when the carbonate of lime was carried away gradually by springs from the interior of the earth; that another era occurred, when engulfed rivers or occasional floods swept organic and inorganic debris into the subterranean hollows previously formed; finally, there were changes, in which engulfed rivers were turned into new channels, and springs dried up, after which the cave-mud, breccia, gravel, and fossil bones were left in the position in which they are now discovered. "We know," says that eminent geologist,[104] "that in every limestone district the rain-water is _soft_, or free from earthy ingredients, when it falls upon the soil, and when it enters the rocks below; whereas it is _hard_, or charged with carbonate of lime, when it issues again to the surface in springs. The rain derives some of its carbonic acid from the air, but more from the decay of vegetable matter in the soil through which it percolates; and by the excess of this acid, limestone is dissolved, and the water becomes charged with carbonate of lime. The ma.s.s of solid matter silently and unceasingly subtracted in this way from the rocks in every century is considerable, and must in the course of thousands of years be so vast, that the s.p.a.ce it once occupied may well be expressed by a long suite of caverns."

[104] "Elements of Geology," p. 122.

The most celebrated of these bone-caves are those of Gailenreuth, in Franconia; of Nabenstein, and of Brumberg, in the same country; the caves on the banks of the Meuse, near Liege, of which the late Dr.

Schmerling examined forty; of Yorkshire, Devonshire, Somersetshire, and Derbyshire, in England; also several in Sicily, at Palermo, and Syracuse; in France at Herault, in the Cevennes, and Franche Comte; and in the New World, in Kentucky and Virginia.

The _ossiferous breccia_ differs from the bone-caves only in form. The most remarkable of them are seen at Cette, Antibes, and Nice, on the sh.o.r.es of Italy; and in the isles of Corsica, Malta, and Sardinia.

[Ill.u.s.tration: Fig. 194.--Grotto des Demoiselles, Herault.]

Nearly the same bones are found in the _breccia_ which we find in the caves; the chief difference being that fossils of the Ruminants are there in greater abundance. The proportions of bones to the fragments of stone and cement vary considerably in different localities. In the _breccia_ of Cagliari, where the remains of Ruminants are less abundant than at Gibraltar and Nice, the bones, which are those of the small Rodents, are, so to speak, more abundant than the mud in which they are embedded. We find, there, also, three or four species of Birds which belong to Thrushes and Larks. In the _breccia_ at Nice the remains of some great Carnivora are found, among which are recognised two species of Lion and Panther. In the Grotto di San-Ciro, in the Monte Griffone, about six miles from Palermo, in Sicily, Dr. Falconer collected remains of two species of Hippopotamus and bones of _Elephas antiquus_, Bos, Stag, Pig, Bear, Dog, and a large _Felis_, some of which indicated a Pliocene age. Like many others, this cave contains a thick ma.s.s of bone-breccia on its floor, the bones of which have long been known, and were formerly supposed to be those of giants; while Prof. Ferrara suggested that the Elephants' bones were due to the Carthaginian elephants imported into Sicily for purposes of sport.[105]

[105] _Quart. Jour. Geol. Soc._, 1859.

But the _breccia_ is not confined to Europe. We meet with it in all parts of the globe; and recent discoveries in Australia indicate a formation corresponding exactly to the _ossiferous breccia_ of the Mediterranean, in which an ochreous-reddish cement binds together fragments of rocks and bones, among which we find four species of Kangaroos.

[Ill.u.s.tration: Fig. 195.--Beloptera Sepioidea.]

GLACIAL PERIOD.

The two cataclysms, of which we have spoken, surprised Europe at the moment of the development of an important creation. The whole scope of animated Nature, the evolution of animals, was suddenly arrested in that part of our hemisphere over which these gigantic convulsions spread, followed by the brief but sudden submersion of entire continents.

Organic life had scarcely recovered from the violent shock, when a second, and perhaps severer blow a.s.sailed it. The northern and central parts of Europe, the vast countries which extend from Scandinavia to the Mediterranean and the Danube, were visited by a period of sudden and severe cold: the temperature of the polar regions seized them. The plains of Europe, but now ornamented by the luxurious vegetation developed by the heat of a burning climate, the boundless pastures on which herds of great Elephants, the active Horse, the robust Hippopotamus, and great Carnivorous animals grazed and roamed, became covered with a mantle of ice and snow.

To what cause are we to attribute a phenomenon so unforeseen, and exercising itself with such intensity? In the present state of our knowledge no certain explanation of the event can be given. Did the central planet, the sun, which was long supposed to distribute light and heat to the earth, lose during this period its calorific powers? This explanation is insufficient, since at this period the solar heat is not supposed to have greatly influenced the earth's temperature. Were the marine currents, such as the _Gulf Stream_, which carries the Atlantic Ocean towards the north and west of Europe, warming and raising its temperature, suddenly turned in the contrary direction? No such hypothesis is sufficient to explain either the cataclysms or the glacial phenomena; and we need not hesitate to confess our ignorance of this strange, this mysterious, episode in the history of the globe.

There have been attempts, and very ingenious ones too, to explain these phenomena, of which we shall give a brief summary, without committing ourselves to any further opinion, using for that purpose the information contained in M. Ch. Martins' excellent work. "The most violent convulsions of the solid and liquid elements," says this able writer, "appear to have been themselves only the effects due to a cause much more powerful than the mere expansion of the pyrosphere; and it is necessary to recur, in order to explain them, to some new and bolder hypothesis than has yet been hazarded. Some philosophers have belief in an astronomical revolution which may have overtaken our globe in the first age of its formation, and have modified its position in relation to the sun. They admit that the poles have not always been as they are now, and that some terrible shock displaced them, changing at the same time the inclination of the axis of the rotation of the earth." This hypothesis, which is nearly the same as that propounded by the Danish geologist, Klee, has been ably developed by M. de Bouchep.o.r.n. According to this writer, many multiplied shocks, caused by the violent contact of the earth with comets, produced the elevation of mountains, the displacement of seas, and perturbations of climate--phenomena which he ascribes to the sudden disturbance of the parallelism of the axis of rotation. The antediluvian equator, according to him, makes a right angle with the existing equator.

"Quite recently," adds M. Martins, "a learned French mathematician, M.

J. Adhemar, has taken up the same idea; but, dismissing the more problematical elements of the concussion with comets as untenable, he seeks to explain the deluges by the laws of gravitation and celestial mechanics, and his theory has been supported by very competent writers.

It is this: We know that our planet is influenced by two essential movements--one of rotation on its axis, which it accomplishes in twenty-four hours; the other of translation, which it accomplishes in a little more than 365 days. But besides these great and perceptible movements, the earth has a third, and even a fourth movement, with one of which we need not occupy ourselves; it is that designated _nutation_ by astronomers. It changes periodically, but within very restricted limits, the inclination of the terrestrial axis to the plane of the ecliptic by a slight oscillation, the duration of which is only eighteen hours, and its influence upon the relative length of day and night almost inappreciable. The other movement is that on which M. Adhemar's theory is founded.

"We know that the curve described by the earth in its annual revolution round the sun is not a circle, but an ellipse; that is, a slightly elongated circle, sometimes called a circle of two centres, one of which is occupied by the sun. This curve is called the ecliptic. We know, also, that, in its movement of translation, the earth preserves such a position that its axis of rotation is intercepted, at its centre, by the plane of the ecliptic. But in place of being perpendicular, or at right angles with this plane, it crosses it obliquely in such a manner as to form on one side an angle of one-fourth, and on the other an angle of three-fourths of a right angle. This inclination is only altered in an insignificant degree by the movement of _nutation_. I need scarcely add that the earth, in its annual revolution, occupies periodically four princ.i.p.al positions on the ecliptic, which mark the limits of the four seasons. When its centre is at the extremity most remote from the sun, or _aphelion_, it is the summer solstice for the northern hemisphere.

When its centre is at the other extremity, or _perihelion_, the same hemisphere is at the winter solstice. The two intermediate points mark the equinoxes of spring and autumn. The great circle of separation of light and shade pa.s.ses, then, precisely through the poles, the day and night are equal, and the line of intersection of the plane of the equator and that of the ecliptic make part of the vector ray from the centre of the sun to the centre of the earth--what we call the _equinoctial line_.