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People eslewhere agree on what a mountain is—or do __1__ they? The fact is that definitions vary. Everyone admits for example, that Everest is a mountain, the lowest of __2__ them all, an altitude of almost 30,000 feet. But what about__3__ Snowdon, the loftiest peak in Wales? It raises a mere 3000__4__ feet, yet it is also called a mountain. Comparison—a little matter of relativity—is the key. To the average person living on North America's Great Plains, Vermont's Green Mountains look lofty indeed, but to anyone from the Rocky Mountains, the Green Mountains seem something more__5__ than hills. Geographers generally agree that, to be a mountain topographically, a landmass might reach an altitude of 3000__6__ feet above the level of the sea. Mount Everest, for instance, is 30,000 feet above sea level, but not only 15,000 feet above the__7__ neighboring Tibetan plateau. Geologists restrict the definition even more, maintaining that a mountain is a mountain with__8__ virtue of its geological structure. Some rugged highlands are not really mountains, when some flat, low-lying rock surfaces__9__ are ture mountains. They are low now because of centuries of erosion. There are even mountains under the sea—the Mid-Atlantic Ridge, for example. Like all ture mountains,they are originally formed by large-scale movements of the__10__ earth's crust.
