of days before the actual outbreak takes place. First smoke is perceived,

perhaps, and the escape of various noxious gases accompanied by earthquakes Now the eruption may commence and blow away the summit of

Occur.

the mountain, as in the case of the commencement of the catastrophe of A.D. 79, when the whole side of Vesuvius was torn away, and continuous showers of ashes fell for days and nights, burying everything, while the hot lava poured down the sides. Stones and ashes with vapours are hurled into the air. Clouds of steam are formed, and vivid electrical discharges take place in these clouds, while water dashes down, carrying stones ("volcanic bombs"), and reflected lurid flames from within are cast on the steaming clouds, which look like fiery column's. Then the lava issues in a white, hot, steady, irresistible stream, covering everything, and burning up all vegetation. New volcanoes are continually in process of formation, and at Santorin for hun

dreds of

years

volcanic

action

has been

busy in

forming

islands.

These

violent

efforts of

Nature

frequently give rise to earth

quakes,

which

are the most destructive of natural convulsions. The records of late occurrences are fresh in the minds of all readers, and need not be specified. The slow subsidence and gradual upheaval of the land is still going on, but we are frequently startled by the account of a rupture of the ground or the destruction of a portion of a city.

Fig. 710.-Birth of a volcano.

The motion of the earthquake is generally in a direct line, and undulating. Sometimes what are termed vertical shocks arise and destroy solidlybuilt edifices. Mountains have been overturned by earthquake shocks, and trees have been twisted round. Sometimes the ground yawns into enormous fissures. The sea is tossed into great waves and encroaches upon the land, and when the sea recedes the recession of the water is followed by a more terrible invading wave sweeping all before it. Earth tremblings often occur far away from volcanoes, and without any visible connection with volcanic action. There are many aspects of land and water which the student of

geography will remember, but which need not be separately treated of. We must, however, refer to plains, plateaus, and lakes. The mountains also play a most important part in Physical Geography and in "Climatology," as they collect the vapours for rain, and make the valleys fertile, and thick with vegetation. We have spoken of the mountains under GEOLOGY, and the various formations and strata will be found enumerated there, but now we have to do with the mountain chains in their physical aspect as regards their shape and appearance on the globe.

Any elevation rising from a base more than 1,000 feet may fairly be termed a mountain, and solitary mountains are usually volcanic, because eruptive rock does not produce chains of mountains. The origin of mountains is probably due to the contraction and compression of the crust of the earth-not merely the surface, but the whole thickness between us and the

supposed molten interior.. Mountains did not exist from everlasting, for the very good reason that they are (in most cases) composed of stratified rocks. Stratified rocks are sedimentary rocks, and must have been deposited below water, and hardened long before they were thrust up by pressure. Moreover, we find (as has already been explained) shells and remains of marine animals on the higher summits, which prove to demonstration that these mountains are composed of rocks which were laid down under the sea.

Professor Dana was one of the first geologists to advance the theory that contraction and lateral displacement are the causes of the elevation of mountains. A very good illustration of this theory was made by Chamontier, who covered an india-rubber balloon with a thick layer of wax, and when it had hardened sufficiently he pricked a hole in the bladder, which immediately contracted, and the wax at once rose up into tiny similitudes of mountains, showing in a sufficiently clear manner that such protuberances may be

produced by the pressure of the earth's contraction, and in such a mass as our earth the elevations would naturally be very great.

Professor Geikie has shown how, by a very simple experiment, the contortion of mountain strata is effected by pressure. A number of cloths or towels placed flat on a table represent the sedimentary rocks. Place a board with a weight on the top, and the towels will remain flattened. But by holding two boards at the sides and pressing them together (the weighted board still remaining), we shall find the towels crumpled and upheaved like the Jurassic strata shown in the illustration (fig. 712). Professor Heim calls the central masses wrinkles of the earth's crust. So the Alps were pressed up or heaved into the air, the weather-rain, frost, snow, and sunshine-imparting the infinite variety of " Horn," "Needle," and "Peak," so expressively applied in Alpine nomenclature ;-the Matterhorn, Wetterhorn, Weisshorn; the Pic du Midi, Aiguille de Dru, Aiguille Verte, and many other mountains in well-trodden

Switzerland will occur to the reader at

once.

The slopes of mountains-though to the casual observer they may appear very much the same-are very different. We sometimes find a long, easy ascent, -more usually a steepish inclination, perhaps 20°;-in other places, such as on the Matterhorn, an almost perpendicular face. Forty-five degrees rise is very steep, and 53° is the limit of any great mountain's slope. Cliffs and precipices there are, of course; witness the terrible fall from the Matterhorn to the glacier below-thousands of feet with one tremendous leap from the rock to the ice underneath; but mountain slopes are not precipices. As a rule, we find that one side of a mountain chain is steeper than the opposite one. It is harder to climb up from Italy to Switzerland than to ascend in the opposite direction. The Pyrenees are also steeper on the south side. The Scandinavian mountains likewise are steeper in the west. The Himalaya are steepest towards the sea, so are the Ghauts. We here find a difference between the slopes of the NEW and OLD WORLDS. In the former we have the less precipitous mountain slopes towards the east; in the old world they are towards the north, and an inspection of a physical map of the world leads us to the conclusion that the Atlantic and Pacific Oceans are the boundaries of entirely different degrees of slopes. The Pacific and Indian Oceans would appear to border the more precipitous mountain sides; the Atlantic and its connections those less steep.

As a rule, we have the most elevated portions of the earth, mountains, and high tablelands, in equatorial regions; and within the torrid zone every terrestrial climate is to be found, owing to the snows of the high mountains

and the heat of the valleys, which are naturally closely connected with the upheaval of mountain ranges. We have already spoken of the never-ceasing influences of the air and water upon the rocks, and we need say little about valleys. There are valleys of dislocation, denudation, and undulation. The great valley of Western Asia, wherein lie the Caspian and Aral seas, seems to have been caused by the upheaval of the Caucasus and the Persian plateau.

PLAINS are very varied. We have European Heaths and Landes; American Savannahs, Prairies, and Pampas; Asian Steppes, and African Deserts. All of these possess certain features in common, more or less vegetation, and sometimes absolute sterility.

PLATEAUS, or Tablelands, are elevated plains frequently undulating in character. The Plateau of Bolivia is 13,000 feet high, and extends along by the Andes. The tableland of Quito is nearly 10,000 feet high, and borders on the giants Cotopaxi and Chimbarazo.

Rivers and lakes add not only to the wealth of nations by their usefulness, but, by the additional picturesqueness of their appearance, to the beauty of the landscape. The velocity of rivers would be very much increased if it were not for the strong resistance offered by the banks and the stones to the current, and by friction. The Rhine and the Rhone, if thus unimpeded, would flow at a rate considerably over one hundred miles an hour; and our own little stream (the Thames), instead of eddying peacefully and twirling gracefully by Medmenham or Cookham, would rush along at the speed of the train which so often crosses it on its way to the sea.

The slopes of river-beds, like the slopes of mountains, vary very considerably, and the inclination of a river varies at different places; in a distance of seven hundred miles the Amazon only falls twelve feet, and the current flows chiefly by impetus already acquired. A slope of one foot in two hundred precludes all navigation, and at still greater inclines rapids and cataracts are formed-the great falls wearing away the river-bed by degrees; so it is calculated that hundreds of years ago Niagara Fail was much farther down the river, and the cataract is slowly moving up stream. In time, as the rock wears away, the height will disappear as the celebrated "Falls," and will become a rapid within a few miles of the lake.