掌握了gre阅读里的长难句,到了实战演习的时候了。gre阅读练习每日一篇帮助gre考生循序渐进地进行练习和总结。希望gre考生在进行gre阅读练习时,也按着考试时候的时间规定自己的练习,这样才能有效果。
Geologists have long known that the Earths mantle is heterogeneous, but its spatial arrangement remains unresolvedis the mantle essentially layered or irregularly heterogeneous? The best evidence for the layered mantle thesis is the well-established fact that volcanic rocks found on oceanic islands, islands believed to result from mantle plumes arising from the lower mantle, are composed of material fundamentally different from that of the midocean ridge system, whose source, most geologists contend, is the upper mantle.
Some geologists, however, on the basis of observations concerning mantle xenoliths, argue that the mantle is not layered, but that heterogeneity is created by fluids rich in incompatible elements percolating upward and transforming portions of the upper mantle irregularly, according to the vagaries of the fluids pathways. We believe, perhaps unimaginatively, that this debate can be resolved through further study, and that the underexplored midocean ridge system is the key.
17. Which of the following best expresses the main idea of the passage?
Current theories regarding the structure of the Earths mantle cannot account for new discoveries regarding the composition of mantle xenoliths.
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