ter the snows reach to the plains; in summer they retreat to the snow-line,-to that particular line where the snowfall of every year is exactly balanced by the consumption, and above which is the region of eternal snows.

2. But, if a residue remains annually above the snowline, the mountains must be loaded with a burden which increases every year. Supposing, at a particular point above the line referred to, a layer of three feet each year to be annually added to the mass; this deposit, accumulating even through the brief period of the Christian era, would produce an elevation of nearly 6000 feet. It is manifest that no accumulation of this kind takes place: the quantity of snow on the mountains is not increasing in this way. By some means or other the sun is prevented from lifting the ocean out of its basin, and piling its waters permanently upon the hills.

3. How then is this annually increasing load taken off the shoulders of the mountains? The snows sometimes detach themselves, and rush down the slopes in avalanches, melting to water in the warmer air below. But the violent rush of the avalanche is not their only motion; they also creep by almost insensible degrees down the slopes. As layer, moreover, heaps itself upon layer, the deeper portions of the mass become squeezed and consolidated; the air, first entrapped in the meshes of the snow, is squeezed out, and the compressed mass approximates more and more to the character of ice. You know how the particles of a snowball will adhere, and you know how hard you can make the ball if you are mischievously inclined. The snowball is incipient ice; increase your pressure and you actually convert it into ice. Even after it has attained a compactness which would entitle it to be called ice, it is still capable of yielding more or less, as the snow yields, to pressure. When, therefore, a sufficient depth of the

substance collects upon the earth's surface, the lower portions are squeezed out by the pressure of the upper ones, and if the snow rests upon a slope, it will yield principally in the direction of the slope and move downwards.

4. This motion is incessantly going on along the slopes of every snow-laden mountain; in the Himalayas, in the Andes, in the Alps; but in addition to this motion, which depends upon the power of the substance itself to yield to pressure, there is also a sliding motion over the inclined bed. The consolidated snow moves bodily over the mountain slope, grinding off the asperities of the rocks, and polishing their hard surfaces. The under surface of the mighty polisher is also scarred and furrowed by the rocks over which it has passed; but as the compacted snow descends, it enters a warmer region, is more copiously melted, and sometimes, before the base of its slope is reached, it is wholly cut off by fusion. Sometimes, however, large and deep valleys receive the icy masses thus sent down; in these valleys it is further consolidated, and through them it moves at a slow, but measurable pace, imitating in all its motions those of a river. The ice is thus carried far beyond the limits of perpetual snow, until at length the consumption below equals the supply above, and at this point the glacier ceases. From the snow-line downwards in summer, we have ice; above the snow-line, both summer and winter, we have, on the surface, snow. The portion below the snow-line is called a glacier, that above the snow-line is called the névé. The névé, then, is the feeder of the glacier.

5. Several valleys, thus united, may unite in a single valley, the tributary glaciers welding themselves together to form a trunk glacier. Both the main valley and its tributaries are often winding, and the tributaries must change their direction to form the trunk. The width of

the valley, also, often changes; the glacier is forced through narrow gorges, widening after it has passed them. The centre of the glacier moves more quickly than the

sides, and the surface more quickly than the bottom. The point of swiftest motion follows the same law as that observed in the flow of rivers, changing from one side of the centre to the other, as the bend of the valley changes.

a

Most of the great glaciers in the Alps have, in summer, central velocity of two feet a day. There are points on the Mer-de-Glace which have a daily motion of thirty inches in summer, and in winter have been found to move at half this rate.

Questions on the lesson:-What is meant by the snow-line? What every year takes place below this line? Why? What takes place above it? What might be expected to result from the accumulation of snow above the snow-line? How much must it have amounted to since the beginning of the Christian era? What is the first means by which the mountains are relieved of their load? As the snow creeps down what happens to the lower layers? What is first pressed out? What does the mass resemble? Where is this downward movement in progress? What is done to the rocks by the descending mass? As it enters a warmer region what occurs? In what respects does a glacier resemble a river? At what rate does the glacier move in summer? In winter?

Alps, the highest mountains in Europe.

Himalayas ("abode of snow "), mountains in the north of India.

Mer-de-Glace, a famous glacier at the foot of Mt. Blanc.

THE PASSENGER PIGEON.

1. The passenger pigeon, or, as it is usually named in America, the wild pigeon, moves with extreme rapidity. It propels itself by quickly repeated flaps of its wings, bringing them more or less near to its body according to the degree of speed which is required. Their great power of flight enables the pigeons to survey and pass over an

astonishing extent of country in a very short time. This is proved by facts well known in America. Thus pigeons have been killed in the neighbourhood of New York with their crops full of rice, which they must have collected in the fields of Georgia and Carolina, these districts being the nearest in which they could possibly have procured a supply of that kind of food. As their power of digestion is so great that they will decompose food entirely in twelve hours, they must in this case have travelled between three hundred and four hundred miles in six hours, which shows their speed to be on an average about one mile in a minute. A velocity such as this would enable one of these birds, were it so inclined, to visit the European continent in less than three days.

2. Audubon describes the multitudes of wild pigeons in the American woods as great beyond all computation. In the autumn of 1813, not far from the Ohio river, he observed them flying in greater numbers than he thought he had ever seen before. Feeling an inclination to count the flocks that might pass within the reach of his eye in one hour, he seated himself on an eminence and began to mark with his pencil, making a dot for every flock that passed. In a short time he found the task he had undertaken to be impracticable, as the birds poured in in countless multitudes. Counting the dots he had already put down, he found that one hundred and sixty-three had been made in twenty-one minutes. Audubon travelled on and still met more the farther he proceeded. The air was literally filled with pigeons; the light of noonday was obscured as by an eclipse; and the continued buzz of wings, he says, had a tendency to lull his senses to repose.

3. As soon as the pigeons discover a sufficiency of food to entice them to alight, they fly round in circles, reviewing the country below. During their movements on such