A large part of the moisture which rises from the ocean’s surface is condensed to vapor before it has ascended three miles, and in this form it makes its transit over land as fleecy or stratiform clouds—the so-called cumulus and stratus clouds and their many intermediate varieties (see Frontispiece). This lower layer within the convective zone is, therefore, a moist one overlaid by a relatively drier middle layer of the convective zone. That moisture which rises above the lower cloud layer is congealed by adiabatic cooling to fine ice needles visible as the so-called cirrus clouds which float as feathery fronds beneath the convective ceiling (see frontispiece at right upper corner of picture). Thus we have within the convective zone an upper layer more or less charged with water in the form of ice needles. It is the clouds of the lower zone whose moisture in the form of vapor supplies the nourishment of mountain glaciers, and the high cirrus clouds whose congealed moisture, after interesting transformations, is responsible for the continued existence of continental glaciers. As we are to see, there are other noteworthy differences between continental and mountain glaciers, in the manner of their sculpture of the lithosphere, so that long after they have disappeared the characters of each are easily identified in their handiwork. How the lower clouds are forced upward and so compelled to give up their moisture to feed the mountain glaciers, and how the upper clouds are pulled downward to nourish the glaciers of continents, can be best understood after the characteristics of each glacier class have been studied. CHAPTER XXI THE CONTINENTAL GLACIERS OF POLAR REGIONS [Illustration: FIG. 299.—Map of Greenland showing the area of inland-ice and the routes of different explorers.] =The inland ice of Greenland.=—In Greenland and in Antarctica the land is almost or quite buried under a cover of snow and ice—the so-called “inland ice”—which always assumes the surface of a very flat dome or shield. In Greenland there is found a marginal ribbon of land generally from five to twenty-five miles in width (Fig. 299), but in Antarctica all the land, with the exception of a few mountain peaks, is inwrapped in a mantle of ice which is also extended upon the sea in a broad shelf of snow and ice. Neither of these vast glaciers has been explored except in its marginal portion, yet such is the symmetry of the profiles along the routes traversed, and such the flatness and monotony of the snow surface within the margins, that there is little reason to doubt that the profile made along Nansen’s route in southern Greenland would, save only for magnitude, fairly represent a section across the middle of the continent (Fig. 300).