Each delta, then, is made of materials worn from the surface of the land, and brought down by the river. And yet vast though some of these deltas are, they do not show all the materials which have been so worn away. A great deal is carried far out and deposited on the sea-bottom; for the sea is the great basin into which the spoils of the land are continually borne. Having now followed the course taken by the water which falls on the land as rain, we come to that taken by snow. On the tops of some of the highest mountains in Britain snow lies for great part of the year. On some of them, indeed, there are shady clefts wherein you may meet with deep snow-wreaths even in the heat of summer. But in other parts of Europe, where the mountains are more lofty, the peaks and higher shoulders of the hills gleam white all the year with unmelted snow. Let us see why it is that perpetual snow should occur in such regions, and what part this snow plays in the general machinery of the world. You have learned that the higher parts of the atmosphere are extremely cold. You know also that in the far north and the far south, around those two opposite parts of the earth’s surface called the Poles, the climate is extremely cold—so cold as to give rise to dreary expanses of ice and snow, where sea and land are frozen, and where the heat of summer is not enough to thaw all the ice and drive away all the snow. Between these two polar tracts of cold, wherever mountains are lofty enough to get into the high parts of the atmosphere where the temperature is usually below the freezing-point, the vapor condensed from the air falls upon them, not as rain, but as snow. Their heads and upper heights are thus covered with perpetual snow. In such high mountainous regions the heat of the summer always melts the snow from the lower hills, though it leaves the higher parts still covered. From year to year it is noticed that there is a line or limit below which the ground gets freed of its snow, and above which the snow remains. This limit is called the snow-line, or the limit of perpetual snow. Its height varies in different parts of the world. It is highest in the warmer regions on either side of the equator, where it reaches to 15,000 feet above the sea. In the cold polar tracts, on the other hand, it approaches the sea-level. In other words, while in the polar tracts the climate is so cold that perpetual snow is found even close to the sea-level, the equatorial regions are so warm that you must climb many thousand feet before you can reach the cold layers of the air where snow can remain all the year.