填空题

Every geologist is familiar with the erosion cycle. No sooner has an area of land been raised alive sea-level than it becomes subject to the erosive forces of nature. The rain beats down on the ground and washed (51) the finer particles, sweeping them into rivulets and then into rivers and out to sea. The frost freezes the rain water in cracks of the rocks and breaks (52) even the hardest of the constituents of the earth’s crust. Blocks of rock dislodged at high levels are brought down by the force of gravity. Alternate heating and (53) of bare rock surfaces causes their disintegration. In the dry regions of the world the wind is a powerful force in removing material from one area to another. All this is natural. But nature has also provided certain defensive forces. Bare rock surfaces are in (54) course protected by soil itself dependent initially on the weathering of the rocks. Slowly (55) surely, different types of soil with differing "profiles" evolve the main types depending primarily on the climate. The protective soil covering, once it is formed, is held together by the growth of vegetation. Grass and herbaceous plants, (56) long, branching tenuous roots, hold firmly together the surface particles. The (57) is true with the forest cover. The heaviest tropical down- pours beating on the leave of the giant trees reach the ground only (58) spray, gently watering the surface layers and penetrating along the long passages provided by the roots to the lower levels of the soil. The soil, thus protected by grass, herb, or trees, furnishes a quiet habitat for a myriad varied organisms-earth-worms that importantly modify the soil, bacteria, active in their work of converting (59) leaves and decaying vegetation into humus and food for the growing plants. Chemical action is constantly taking (60) . Soil acids attack mineral particles and salts in solution move from one layer in the soil to another.