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  3. Since the dawn of civilization, people have been curious about the age of Earth. In addition, we have not been satisfied in bein

Since the dawn of civilization, people have been curious about the age of Earth. In addition, we have not been satisfied in bein

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问题     Since the dawn of civilization, people have been curious about the age of Earth. In addition, we have not been satisfied in being able to know merely the relative geologic age of a rock or fossil. Human curiosity demands that we know actual age in years.
    Geologists working during the nineteenth century understood rock bodies, they would have to concentrate on natural processes that continue at a constant rate and that also leave some sort of tangible record in the rocks. Evolution is one such process, and geologist Charles Lyell (1797—1875) recognized this. By comparing the amount of evolution exhibited by marine mollusks then, Lyell estimated that 80 million years had elapsed since the beginning of the Tertiary Period. He came astonishingly close to the mark, since it was actually about 65 million years. However, for older sequence of evolutionary development, estimates based on parts in the fossil record. Rates of evolution for many orders of plants and animals were not well understood.
    In another attempt, geologists reasoned that if rates of deposition could be determined for sedimentary rocks, they might be able to estimate the time required for deposition of a given thickness of strata, or rock layers. Similar reasoning suggested that one could estimate total elapsed geologic time by dividing the average thickness of sediment transported annually to the oceans into the total thickness of sedimentary rock that had ever been deposited in the past. Unfortunately, such estimates did not adequately account for past difference in rates of sedimentation or losses to the total section of strata during episodes of erosion. Also, some very ancient sediments were no longer recognizable, having been converted to igneous and metamorphic rocks in the course of mountain building. Estimates of Earth’s total age based on sedimentation rates ranged from as little as million to over a billion year.
    Yet another scheme for approximating Earth’s age had been proposed in 1715 by Sir Edmund Halley (1656—1742), whose name we associate with the famous comet. Halley surmised that the ocean formed soon after the origin of the planet and therefore would be only slightly younger than the age of the solid Earth. He reasoned that the original ocean was not salty and that subsequently salt derived from the weathering of rocks was brought to the sea by streams. Thus, if one knew the total amount of salt dissolved in the ocean and the amount added each year, it might be possible to calculate the ocean’s age. In 1899, Irish geologist John Joly (1857—1933) attempted the calculation. From information provided by gauges placed at the mouths of streams, Joly was able to estimate the annual increment of salt to the oceans. Then, knowing the salinity of ocean water and the approximate volume of water, he calculated the amount of salt already held in solution in the oceans. An estimate of the age of the ocean was obtained by dividing the total salt in the ocean by the rate of salt added each year. Beginning with essentially non-saline oceans, it would have taken about 90 million years of the oceans to reach their present salinity, according to Joly. The figure, however, was off the currently accepted mark of 4.54 billion by a factor of 50, largely because there was no way to account accurately by recycled salt and salt incorporated into clay mineral deposited on the sea floors. Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.  
According to Paragraph 2, Lyell’s strategy for estimation geologic dates was not very accurate for periods before the Tertiary Period party because________.
选项 A、Marine mollusks did not evolve until the Tertiary Period
B、fossil records of the very distant past are incomplete
C、there was not much agreement about how to identify or categorize earlier eras
D、the duration of previous geologic periods was difficult to determine
答案B
解析 根据第2段倒数两句“然而，更早时期的进化顺序只能基于部分化石记录，许多动植物的进化速率也不清楚”可知，原因是化石记录不完整，一些动植物进化速度不清楚，B项符合原文表述。A项“直到第三纪海洋软体动物才开始进化”、C项“关于如何确定和划分更早时期，并没有一致的说法”、D项“之前地质年代的持续时间难以确定”在文中都未被提及。
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