Of the foregoing types of glaciers, the ice-caps far exceed all others both in size and importance, while the valley glaciers out-rank, in the same respects, the other types; but since the valley glaciers are the most familiar type, the general phenomena of glaciers will be discussed with primary reference to them. THE GENERAL PHENOMENA OF GLACIERS.[125] =Dimensions.=—Glaciers which occupy valleys leading down from snow-fields sometimes reach the upper parts of the valleys only, sometimes extend through them, and sometimes push out on the plain beyond. In length they range from a fraction of a mile to many miles, and though their width is usually much less than their length, the reverse is sometimes the case (Figs. 233, 234, and 235). Their thickness is usually measured by hundreds of feet rather than by denominations of other orders, but the variation is great, and exact measurements are almost wholly wanting. The minimum thickness is that necessary to cause movement, and this varies with the slope, the temperature, and other conditions. There is also much variation in the thickness in different parts of a glacier. As a rule, it is thinnest in its terminal portion, and thickest at some point intermediate between this and its source, but nearer the latter than the former. Cliff and reconstructed glaciers are comparable in size to the smaller valley glaciers. Piedmont glaciers may attain greater size. [Illustration: +Fig.+ 235.—A glacier in the Cascades near Cascade Pass, Wash. A glacier intermediate between a cliff glacier and a valley glacier. (Willis, U. S. Geol. Surv.)] An ice-cap is theoretically thickest at its center and thins away to its borders, but its actual dimensions are influenced by the topography on which it is developed. The Greenland ice-cap is known to rise about 9000 feet above the sea, and it probably reaches considerably higher than this in the unexplored center of its broad dome. The height of the land surface beneath is unknown, but it is unlikely that it averages half this amount, and hence the ice is probably 5000 feet or more thick in the center. There is reason to think that it is much thicker in Antarctica.