The four outer planets are mostly gaseous; the four inner ones have hardened crusts. Let us scan for a moment the various surface features of the third one in order from the sun,--the Earth, our own globe, and consider the unique and marvelous manner in which its various features have come to be. In the course of millions of years, the earth, through the pull of gravitation, became gradually more and more dense. Different elements in its gases (which according to the later theory were squeezed out by pressure), combined to form rock, water and atmosphere. Hydrogen united with oxygen formed water; nitrogen, oxygen and small quantities of carbonic acid gas formed the atmosphere; oxygen mixed with silicon made quartz, the earth's first rock. This rock, called granite, rose to the top of the molten liquid material and formed a thin film of crust, for it was lighter than the metallic substance which sank to form the earth's core. This thin, rocky film of crust which covered the hot liquid matter was at first so hot that the gases which had formed the water could not stay on its surface but rose in the air as steam. After the temperature became lower and the rocks cooled, rain poured down in sheets of water, remained on the crust and formed a warm sea. For a long time this film was but a slight obstruction to the raging fiery forces confined beneath it and again and again these forces tore the crust asunder and made the surface of the earth a scene of wild commotion. But each time the cooling gained a slight mastery over the heat, and by cooling the earth's film was continually shrinking thus forming a thicker and more solid protective crust. After an heroic struggle between these mighty forces the crust was at last strong enough even to sink down in places to fit the inner core; the sea then ran into these hollows and into the wrinkles formed from the shrinking, and the first ridges or islands appeared above the surface of the water. These ridges or islands were then attacked by the elements, eroded by the weather and the rains, and the granite which formed them was crumbled into quartz, feldspar and mica. The torrents of rain formed rivers, and these, seeking a level, carried the debris down to the shores of the ridges,--the finer sediment, the feldspar and mica, being washed far out from the rocky land where it settled to the bottom of the sea and in time formed which under pressure turned to clay, while the clay under greater pressure, turned to slate. The quartz of the granite, ground to fine particles, stayed near the shore and formed sandy beaches, which if covered under great pressure, became sandstone. Thus were some of our various rocks formed from the original rock of granite, and no matter where one sees sandstone or claybanks, even if on top of a mountain, such a place was once under water, for that is the only place where such sedimentary material could form.