3. This middle zone is made up of different kinds of rock that require different degrees of heat to melt them. So some parts of this zone would melt and form pockets of liquid rock, while other parts were still unmelted. 4. These masses of liquid rock would also tend to melt their own way upward, especially when given a lift by gases; for gases would be given off, also, in this heating and melting process, and tend to work their way toward the surface, carrying with them the liquid rock. 5. Now the greater the pressure under which a thing is kept the more difficult it becomes for it to flow; the less the pressure the more easily it flows and the longer it remains in the fluid state. So as it rose fluid rock would require less heat to keep it fluid and would have more heat left over for melting its way up. Then, being joined by other fluid travelers, the entire mass would finally come to a crack in the earth. Finally, you see, it would be only a matter of five miles or so of comparatively clear track up to the land of the fresh air and the blue sky where the rest of us live and where the volcanologists (the men who make a special study of volcanoes) would be waiting to give it welcome! THE VOLCANOES AND THE SEA If you will locate with red ink the volcanoes on the world map you will notice that volcanoes, like mountains, seem fond of the sea. Moreover, while a large proportion of mountain chains are near sea water, and some even dip their feet into it, volcanoes bob up right in the seas themselves. Not only do the land volcanoes make a great circle of fire 22,000 miles long around the rim of the Pacific, but within this immense amphitheater are the islands of our story books "scattered in pleiads" over the ocean. These islands are simply the tops of sea volcanoes. Of all the active volcanoes, the great majority are on islands or along the borders of continents. [Illustration: ON THE FIRING-LINES OF THE VOLCANOES] THE MOUNTAINS AND THE SEA Last of all in this story of the secrets of the hills, let us speak of the big brothers of the family--the mountains. You remember in the story of how the continents came up out of the sea about wise old Xenophanes of Colophon, who figured out that the mountains must at one time have been under the sea and why he thought so, don't you? (page 13). Now get your geography and come here a moment; I want to show you something else. Turn to the map of North America. Where are the great mountain chains? Nearly all along the borders of the sea. Now look at the map of South America, and where are the mountains? Along the borders of the sea. Then take Europe, Asia, Africa, Australia, and you see the same thing. Usually the main mountain chains are along the sea border or they stand near the borders of what was once a sea; as in case of the Rocky Mountains. [Illustration: _From Norton's "Elements of Geology." By permission of Ginn and Company_ A BABY MOUNTAIN THAT STOPPED TO REST