[Illustration: +Fig.+ 220.—Figure illustrating the principle of artesian wells in drift.] CHAPTER V. THE WORK OF SNOW AND ICE. A part of the atmospheric precipitation falls as snow, and this, like the rain, does its appropriate work in degrading the land. Over the larger part of the land surface the snow of the winter does not endure through the succeeding summer, and when it melts it follows the same course as the precipitation which falls as rain; but in cold regions where the fall of snow is heavy some of it remains unmelted and constitutes perennial snow-fields. SNOW- AND ICE-FIELDS. =Snow-fields.=—Mountain heights and polar lands are the most common habitats of snow-fields, though they are not confined to these situations. In North America there are numerous small snow-fields in the western mountains, from Mexico on the south to Alaska on the north, their number and size increasing in the latter direction. In the United States there are few snow-fields south of the parallel of 36° 30′, and most of the many hundreds north of that latitude (excluding Alaska) are small (Pl. XVIII, Fig. 1, Washington, lat. 48° 5′, long. 121° 5′; Fig. 2, lat. 41° 25′, long. 122° 12′. From Glacier Peak and Shasta Special Quadrangles, U. S. Geol. Surv.). Farther north, especially in Alaska, the snow-fields of the western mountains attain much greater size. In Europe snow-fields comparable to those of the northwestern part of the United States and British Columbia occur in the Alps (Fig. 221), the Pyrenees, the Caucasus, and the Scandinavian mountains. In Asia snow-fields occur in the Himalayas and in many of the high mountains farther north, from Turkestan on the southwest nearly to the coast on the northeast. In South America there are snow-fields of small size even in equatorial latitudes, and farther south in the Chilean Andes there are some of considerable size. Small snow-fields occur on the highest peaks of tropical Africa, and in the mountains of New Zealand. For reasons which will appear later, much of every considerable snow-field is really ice. In addition to these limited fields of snow in mountain regions, there are fields of much greater extent covering wide expanses of plain and plateau in the polar regions. The greater part of the island of Greenland is covered with a single field of ice and snow, the size of which is variously estimated at 300,000 to 400,000 square miles (Fig. 222)—an area 400 to 600 times as large as the snow-and-ice-covered area of Switzerland. Numerous islands to the west of North Greenland are also partly covered with snow, the areas of the snow-fields far exceeding those of most mountain regions. In Antarctica there is believed to be a still larger field, the largest of the earth. Its area is not even approximately known, but such data as are at hand indicate that it may have an extent of 3,000,000 or 4,000,000 square miles.