The walls of various chambers differ from each other according to the minerals that compose them. Some are creamy white limestone arches, some are walled with black gypsum, some are hung with great curtains of stalagmites, solid but suggesting the lightness and grace of folds of crêpe. Under such hangings the floor is built up in stalactites. The mineral-laden water, the constant drip of which has produced a hanging, icicle-like stalagmite, has built up the stalactite to meet it. Probably nothing is more beautiful than the flower-like crystals that bloom all over the walls of a chamber called "Mary's Bower." The floor, even, sparkles with jewels that have fallen from the wonderful and delicate flower clusters built from deposits of the lime-laden water which goes on building and replacing the bits that fall. "Martha's Vineyard" is decorated with nodules, like bunches of grapes, that glisten as if the dew were on them. The white gypsum in some caves makes the walls look as if they were carved out of snow. Still others have clear, transparent crystals that make them gleam in the torches' light as if the walls were encrusted with diamonds. The cave region of Indiana is also famous. The great Wyandotte Cave in Crawford County is the most noted of many similar caverns. In some of the chambers, bats are found clinging to the ceiling, heads downward, like swarms of bees. The caverns of Luray, in Virginia, are complex and wonderful in their structure, and famous for the beautiful stalactites and stalagmites they contain. But there is no cave in this country so wonderful and so grand in its dimensions as the Mammoth Cave in Kentucky. LAND-BUILDING BY RIVERS Once a year, when the rainy season comes in the mountainous country south of Egypt, the old Nile floods its banks and spreads its slimy waters over the land, covering the low plains to the very edge of the Sahara Desert. The people know it is coming, and are prepared for this flood. We should think such an overflow of our nearest river a monstrous calamity, but the Egyptians bless the river which blesses them. They know that without the Nile's overflow their country would be added to the Desert of Sahara. In a short time after the overflow, the river reaches its highest point and begins to ebb. Canals lying parallel to its course are filled with water which is saved for use in the hot, dry summer. As the flood goes down, a deposit of slimy mud lies as a rich fertilizer on the land. It is this and the water which the earth has absorbed that make Egypt one of the most fertile agricultural countries in the world.