From such small beginnings has resulted the shattering of great rock masses by the growth of plants upon them. Tree roots that grow in rock crevices exert a power that is irresistible. The roots of smaller plants do the same great work in a quieter way. When a hurricane or a flood tears down the mountain-side, sweeping everything before it, trees, torn out by the roots, drag great masses of rock and soil into the air, and fling them down the slope. Wind and water thus finish the destruction which the humble mosses and lichens began. What seemed an impregnable fortress of granite has crumbled into fragments. Its particles are reduced to dust, or are on the way to this condition. The plant food locked up in granite boulders becomes available to hungry roots. Forests, grain-fields, and meadows cover the work of destructive agencies with a mantle of green. HOW ROCKS ARE MADE The granite shaft is made out of the original substance of the earth's crust. Its minerals are the elements out of which all of the rock masses of the earth are formed, no matter how different they look from granite. Sandstone is made of particles of quartz. Clay and slate are made out of feldspar and mica. Iron ore comes from the hornblende in granite. The mineral particles, reassembled in different proportions, form all of the different rocks that are known. Here in my hand is a piece of pudding-stone. It is made of pebbles of different sizes, each made of different coloured minerals. The pebbles are cemented together with a paste that has hardened into stone. This kind of rock the geologists call _conglomerate_. Pudding-stone is the common name, for the pebbles in the pasty matrix certainly do suggest the currants and the raisins that are sprinkled through a Christmas pudding. Under the seashores there are forming to-day thick beds of sand. The rivers bring the rock material down from the hills, and it is sorted and laid down. The moving water drops the heaviest particles near shore, and carries the finer ones farther out before letting them fall. [Illustration: The town of Cripple Creek, Colorado, which has grown up like magic since 1891, covers the richest gold and silver mines in the world] [Illustration: The level valley is filled up with fine rock flour washed from the sides of the neighboring mountains] The hard water, that comes through limestone rocks, adds lime in solution to the ocean water. All the shellfish of the sea, and the creatures with bony skeletons, take in the bone-building, shell-making lime with their food. Generations of these inhabitants of the sea have died, and their shells and bones have accumulated and been transformed into thick beds of limestone on the ocean floor. This is going on to-day; but the limestone does not accumulate as rapidly as when the ocean teemed with shell-bearing creatures of gigantic size. Of these we shall speak in another chapter.