GRE不光有技巧，不光有快速解題法，GRE更加考察大家的邏輯硬實(shí)力，文字理解實(shí)打?qū)嵉纳疃?/span>。良好的GRE備考應(yīng)當(dāng)在課堂上向老師學(xué)習(xí)快速解題法，取同取反邏輯的同時(shí)，課下扎扎實(shí)實(shí)的讀懂每一句話。讀懂，是一切技巧使用的前提。皮之不存，毛將焉附？

Passage 72

Roughly 250 million years ago, in the worst series of mass extinction in Earth’s history, almost all species of life simply vanished. The traditionally held view has been that these extinctions occurred gradually, as changing environmental conditions—global cooling, global warming, dropping sea levels, or some combination of such factors—made life increasingly difficult during the last stages of the Permian period (284-245 million years ago). As Permian life-forms declined, they reached a fatal threshold and species succumbed to mass extinction.

Supporters of the traditionalist view note that species begin disappearing from the fossil record well before the end of the Permian period. Yet, while such a pattern could have resulted from a slowly rising extinction rate, some scientists argue that just because a species disappears from the fossil record doesn’t necessarily mean it became extinct at that point. Moreover, Paul Wignall and others have found sites where marine fossil deposits reached a peak of diversity at the very end of the Permian period. Thus, Wignall believes late-Permian marine life was thriving until it was decimated suddenly by some catastrophe.

Some geoscientists think that this catastrophe involved a meteor striking the earth. Michael Rampino, for instance, claims that a giant meteor crashed into Gondwanaland (the southern portion of the supercontinent Pangea, which covered about 30 percent of the globe in Permian times), setting off intense volcanism. Airborne soot and dirt from the impact blanketed Earth, inhibiting photosynthesis and triggering mass extinction of species. This view is widely disputed by traditionalists, however. Grant Young, for example, holds that the gradual breakup of the Pangean supercontinent triggered widespread climatic change and glaciation, causing the Permian mass extinctions to occur over millions of years. Glaciation, Young claims, has occurred throughout Earth’s history, and there is evidence that it occurred toward the end of the Permian period. Yet Rampino and others question one of the traditionalists’ basic assumption: the periodic occurrence of glaciation throughout Earth’s history. Crucial evidence includes the rock deposits know as tillites. because they resemble the unstratified drift produced by modern glaciers, geologists have long considered tillites to be signposts of glaciation. When trying to determine whether particular sedimentary layers represent tillite deposits, geologists look for stones with faceted shapes, scratched surfaces, and other features that presumably resulted from glacial action. The problem with ascribing all tillites to glaciation, however, is that tillite deposits are widespread in Precambrian sediments. Yet temperatures during the Precambrian (3.8 billion to 540 million years ago) are thought to have been warmer than those in today’s nonglacial world. Thus, Rampino thinks that certain tillites were produced by meteor impacts, not glaciers. The first compelling evidence that meteor impacts might have produced tillites came from Moon rocks that showed the same features as earthly tillites. Since no one believes the Moon ever had glaciers, that leaves but one possible cause: impacts.

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