单项选择题

Physical Features of the Country The Map The map on the adjoining page represents the portion of the North American continent which is at the present time occupied by the people of the United States. As you look upon the map imagine that you are in the air, looking down upon it as from a balloon, and take notice of what you see. On the east and on the west are the shores of two oceans. That on the east is the Atlantic. The Pacific is on the west. The Lake Country Toward the north is an immense tract of nearly level land, covered with forests, all of which are filled with water and form lakes, some large and others small. This land, though level, is high, so that there is a very considerable though gradual descent from the lakes to the ocean. The lakes are kept constantly full by the rains and by the melting of the snows, and the surplus waters flow off in one vast channel, northward and eastward to the sea. One of the large lakes, though still much higher than the sea, is marked as a low lake, for it is two or three hundred feet below the level of the others, and the water flowing from the upper lakes into it, in descending from one level to the other, passes over a high precipice (悬崖), thus producing an immense fall, which is the celebrated Niagara (尼亚加拉大瀑布). The surplus waters of all the large lakes flow off finally in a northeasterly direction, almost exactly parallel to the coast until they reach the sea. The river thus formed is now known as the St. Lawrence. All this region of the lakes is inhabited — during the summer season, immense numbers of beasts run upon the land, birds fly in the air, and fishes swim in the water. In the winter, the region is buried deep in ice and snow. The birds at that season have all flown. The animals have retired to dens and holes, where some sleep, torpid (蛰伏), till the spring returns, and others digging beneath the frosty ground, gain their livelihood there by digging for roots, or gnawing (咬) the bark of trees, or catching the fish that are still swimming in the shallow waters. The Great Central Valley South of the lake country, and occupying a very large portion of the whole interior of the continent, is a broad though shallow valley, bounded both on the east and on the west by ranges of mountains. The extent of the valley is marked on the map, not only by the mountains which bound it on the east and on the west, but also by the ramifications (支流) of the great river which drains it. These ramifications are seen spreading in every direction, like the branches of a mighty tree, and meeting in the south in one great trunk. This is the great river Mississippi, with its thousand tributaries (支流). The Soil of the Great Valley The soil of the whole valley is extremely fertile. However, it is so broad and so shallow that it would have more the appearance of an extended plain than of a valley. It is what is called an alluvial (冲积的) formation; that is, a very large portion of the territory has been covered with deposits (沉淀物) from the rivers themselves, left after overflows and floods. These deposits have accumulated, in the course of ages, to a great depth, and they form an exceedingly rich and fertile soil. The rivers twist and turn this way and that in meandering (蜿蜒) through these plains. When it rains hard or snows begin to melt, huge water washes the banks, and brings down great masses of earth and great numbers of immense trees into the water. The earth thus washed in is carried down by the flood. After being mingled with a great variety of animal and vegetable remains, the earth is distributed over widely extended districts below. When the water has overflowed the banks, the soil is covered by a new layer of fertility. Formation of Islands in the River The trees float on, too, upon the current (水流，洪流). Some trees might still be dragged by their roots and get lodged along the banks or in the bed of the stream. In this latter case they intercept others coming down, and so create an obstruction, around which sand and sediment (沉淀物) accumulate, until an island is formed. There is another way by which islands are formed. The river is washing continually upon its banks, and sometimes cuts through a narrow neck. A new channel is thus made for a part of the water, while the rest flows on round the circuit in the old course. By this means an island is formed, which may, perhaps, continue for centuries to divide the stream. The lakes are kept constantly full by the rains and by the melting of the snows.