2006年考试之GMAT阅读练习(4)

文章作者 100test 发表时间 2007:03:10 10:21:56
来源 100Test.Com百考试题网


In choosing a method for determining climatic condi-

  tions that existed in the past, paleoclimatologists invoke

  four principal criteria. First, the material-rocks, lakes,

  vegetation, etc.-on which the method relies must be

  (5) widespread enough to provide plenty of information,

  since analysis of material that is rarely encountered will

  not permit correlation with other regions or with other

  periods of geological history. Second, in the process of

  formation, the material must have received an environ-

  (10) mental signal that reflects a change in climate and that

  can be deciphered by modern physical or chemical

  means. Third, at least some of the material must have

  retained the signal unaffected by subsequent changes in

  the environment. Fourth, it must be possible to deter-

  (15) mine the time at which the inferred climatic conditions

  held. This last criterion is more easily met in dating

  marine sediments, because dating of only a small

  number of layers in a marine sequence allows the age of

  other layers to be estimated fairly reliably by extrapola-

  (20) tion and interpolation. By contrast, because sedimenta-

  tion is much less continuous in continental regions, esti-

  mating the age of a continental bed from the known

  ages of beds above and below is more risky.

  One very old method used in the investigation of past

  (25) climatic conditions involves the measurement of water

  levels in ancient lakes. In temperate regions, there are

  enough lakes for correlations between them to give us a

  reliable picture. In arid and semiarid regions, on the

  other hand, the small number of lakes and the great

  (30) distances between them reduce the possibilities for corre-

  lation. Moreover, since lake levels are controlled by rates

  of evaporation as well as by precipitation, the interpreta-

  tion of such levels is ambiguous. For instance, the fact

  that lake levels in the semiarid southwestern United

  (35) States appear to have been higher during the last ice age

  than they are now was at one time attributed to

  increased precipitation. On the basis of snow-line eleva-

  tions, however, it has been concluded that the climate

  then was not necessarily wetter than it is now, but rather



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