GRE阅读高频机经原文：冰川沉积物

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GRE阅读高频机经原文：冰川沉积物

　　14. 冰川沉积物

　　Tills are commonly classi?ed according to the perceived process of deposition. However, it is increasingly recognised that this classi?cation, which is mainly based on macroscopic ?eld data, has severe limitations. At the same time the concept of the deforming glacier bed has become more realistic as a framework for discussing tills and their properties, and this (tectonic) concept is irreconcilable with the existing (depositional) till classi?cation scheme. Over the last 20 years large thin sections have been used to study tills, which has provided new insights into the textural and structural properties of tills. These results have revolutionised till sedimentology as they show that, in the main, subglacial tills possess deformational characteristics. Depositional properties are rare.

　　Based on this new insight the process of subglacial till formation is discussed in terms of glacier/ice sheet basal velocity, clay, water and carbonate content and the variability of these properties in space and time. The end result of this discussion is: till, the deforming glacier bed. To distinguish subglacial till from depositional sediments the term ‘tectomict’ is proposed. Within the single framework of subglacial till as the deforming glacier bed, many textural, structural and geomorphological features of till beds can be more clearly and coherently explained and understood.

　　GRE阅读高频机经原文及答案：云和气候变化预测

　　15. 云和气候变化预测 1996年04月北美 D9604

　　As of the late 1980’s, neither theorists nor large-scale computer climate models could accurately predict whether cloud systems would help or hurt a warming globe. Some studies suggested that a four percent increase in stratocumulus clouds over the ocean could compensate for a doubling in atmospheric carbon dioxide, preventing a potentially disastrous planetwide temperature increase. On the other hand, an increase in cirrus clouds could increase global warming.

　　That clouds represented the weakest element in climate models was illustrated by a study of fourteen such models. Comparing climate forecasts for a world with double the current amount of carbon dioxide, researchers found that the models agreed quite well if clouds were not included. But when clouds were incorporated, a wide range of forecasts was produced. With such discrepancies plaguing the models, scientists could not easily predict how quickly the world’s climate would change, nor could they tell which regions would face dustier droughts or deadlier monsoons.

　　15.1. The author of the passage is primarily concerned with

　　(A) confirming a theory

　　(B) supporting a statement

　　(D) predicting future discoveries

　　(E) reconciling discrepant findings

　　15.2. It can be inferred that one reason the fourteen models described in the passage failed to agree was that

　　(A) they failed to incorporate the most up-to-date information about the effect of clouds on climate

　　(B) they were based on faulty information about factors other than clouds that affect climate

　　(D) their originators disagreed about the kinds of forecasts the models should provide

　　(E) their originators disagreed about the factors other than clouds that should be included in the models

　　15.3. It can be inferred that the primary purpose of the models included in the study discussed in the second paragraph of the passage was to

　　(A) predict future changes in the world’s climate

　　(B) predict the effects of cloud systems on the world’s climate

　　(D) assess the percentage of the Earth’s surface covered by cloud systems

　　(E) estimate by how much the amount of carbon dioxide in the Earth’s atmosphere will increase

　　15.4. The information in the passage suggests that scientists would have to answer which of the following questions in order to predict the effect of clouds on the warming of the globe?

　　(A) What kinds of cloud systems will form over the Earth?

　　(B) How can cloud systems be encouraged to form over the ocean?

　　(D) What proportion of cloud systems are currently composed of cirrus of clouds?

　　(E) What proportion of the clouds in the atmosphere form over land masses?

　　答案：B C A A

　　GRE阅读高频机经原文及答案：Supernova

　　16. Supernova D 89-12

　　Astronomers observe a supernova that is dimmer than expected because of dust screening，If a supernova (the explosion of a massive star) triggered star formation from dense clouds of gas and dust, and if the most massive star to be formed from the cloud evolved into a supernova and triggered a new round of star formation, and so on, then a chain of star-forming regions would result. If many such chains were created in a differentially rotating galaxy, the distribution of stars would resemble the observed distribute in a spiral galaxy.

　　This line of reasoning underlies an exciting new theory of spiral-galaxy structure. A computer simulation based on this theory has reproduced the appearance of many spiral galaxies without assuming an underlying density wave, the hallmark of the most widely accepted theory of the large-scale structure of spiral galaxies. That theory maintains that a density wave of spiral form sweeps through the central plane of a galaxy, compressing clouds of gas and dust, which collapse into stars that form a spiral pattern.

　　If a supernova (the explosion of a massive star) triggered star formation from dense clouds of gas and dust, and if the most massive star to be formed from the cloud evolved into a supernova and triggered a new round of star formation, and so on, then a chain of star-forming regions would result. If many such chains were created in a differentially rotating galaxy, the distribution of stars would resemble the observed distribution in a spiral galaxy (spiral galaxy: n.(=spiral nebula)螺旋星云,旋涡星云).

　　This line of reasoning underlies an exciting new theory of spiral-galaxy structure. A computer simulation based on this theory has reproduced the appearance of many spiral galaxies without assuming an underlying density wave, the hallmark (a distinguishing characteristic, trait, or feature “the dramatic flourishes which are the hallmark of the trial lawyer Marion K. Sanders”) of the most widely accepted theory of the large-scale structure of spiral galaxies. That theory maintains that a density wave of spiral form sweeps through the central plane of a galaxy, compressing clouds of gas and dust, which collapse into stars that form a spiral pattern.

　　16.1. The primary purpose of the passage is to

　　(A) describe what results when a supernova triggers the creation of chains of star-forming regions

　　(B) propose a modification in the most widely accepted theory of spiral-galaxy structure

　　(D) describe a new theory of spiral-galaxy structure and contrast it with the most widely accepted theory

　　(E) describe a new theory of spiral-galaxy structure and discuss a reason why it is inferior to the most widely accepted theory

　　16.2. The passage implies that, according to the new theory of spiral-galaxy structure, a spiral galaxy can be created by supernovas when the supernovas are

　　(A) producing an underlying density wave

　　(B) affected by a density wave of spiral form

　　(D) located in the central plane of a galaxy

　　(E) located in a differentially rotating galaxy

　　16.3. Which of the following, if true, would most discredit the new theory as described in the passage?

　　(A)The exact mechanism by which a star becomes a supernova is not yet completely known and may even differ for different stars.

　　(B) Chains of star-forming regions like those postulated in the new theory have been observed in the vicinity of dense clouds of gas and dust.

　　(D) Computer simulations of supernovas provide a poor picture of what occurs just before a supernova explosion.

　　(E) A density wave cannot compress clouds of gas and dust to a density high enough to create a star.

　　答案：DEC

　　GRE阅读高频机经原文及答案：鸟叫研究

　　17. 鸟叫研究新题

　　Over the years, biologists have suggested two main pathways by which sexual selection may have shaped the evolution of male birdsong. In the first, male competition and intrasexual selection produce relatively short, simple songs used mainly in territorial behavior. In the second, female choice and intersexual selection produce longer, more complicated songs used mainly in mate attraction; like such visual ornamentation as the peacock’s tail, elaborate vocal characteristics increase the male’s chances of being chosen as a mate, and he thus enjoys more reproductive success than his less ostentatious rivals. The two pathways are not mutually exclusive, and we can expect to find examples that reflect their interaction. Teasing them apart has been an important challenge to evolutionary biologists.

　　Early research confirmed the role of intrasexual selection. In a variety of experiments in the field, males responded aggressively to recorded songs by exhibiting territorial behavior near the speakers. The breakthrough for research into intersexual selection came in the development of a new technique for investigating female response in the laboratory. When female cowbirds raised in isolation in sound-proof chambers were exposed to recordings of male song, they responded by exhibiting mating behavior. By quantifying the responses, researchers were able to determine what particular features of the song were most important. In further experiments on song sparrows, researchers found that when exposed to a single song type repeated several times or to a repertoire of different song types, females responded more to the latter. The beauty of the experimental design is that it effectively rules out confounding variables; acoustic isolation assures that the female can respond only to the song structure itself.

　　If intersexual selection operates as theorized, males with more complicated songs should not only attract females more readily but should also enjoy greater reproductive success. At first, however, researchers doing fieldwork with song sparrows found no correlation between larger repertoires and early mating, which has been shown to be one indicator of reproductive success; further, common measures of male quality used to predict reproductive success, such as weight, size, age, and territory, also failed to correlate with song complexity.

　　The confirmation researchers had been seeking was finally achieved in studies involving two varieties of warblers. Unlike the song sparrow, which repeats one of its several song types in bouts before switching to another, the warbler continuously composes much longer and more variable songs without repetition. For the first time, researchers found a significant correlation between repertoire size and early mating, and they discovered further that repertoire size had a more significant effect than any other measure of male quality on the number of young produced. The evidence suggests that warblers use their extremely elaborate songs primarily to attract females, clearly confirming the effect of intersexual selection on the evolution of birdsong.

　　17.1. The passage is primarily concerned with

　　(A) showing that intrasexual selection has a greater effect on birdsong than does intersexual selection

　　(B) contrasting the role of song complexity in several species of birds

　　(C) describing research confirming the suspected relationship between intersexual selection and the complexity of birdsong

　　(D) demonstrating the superiority of laboratory work over field studies in evolutionary biology

　　(E) illustrating the effectiveness of a particular approach to experimental design in evolutionary biology

　　17.2. The author mentions the peacock’s tail in line 8 most probably in order to

　　(A) cite an exception to the theory of the relationship between intrasexual selection and male competition

　　(B) illustrate the importance of both of the pathways that shaped the evolution of birdsong

　　(D) give an example of a feature that may have evolved through intersexual selection by female choice

　　(E) refute a commonly held assumption about the role of song in mate attraction

　　17.3. According to the passage, which of the following is specifically related to intrasexual selection?

　　(A) Female choice

　　(B) Territorial behavior

　　(D) Large song repertoires

　　(E) Visual ornamentation

　　17.4. Which of the following, if true, would most clearly demonstrate the interaction mentioned in lines 11-13?

　　(A) Female larks respond similarly both to short, simple songs and to longer, more complicated songs.

　　(B) Male canaries use visual ornamentation as well as elaborate song repertoires for mate attraction.

　　(D) Male jays use songs to compete among themselves and to attract females.

　　(E) Male robins with elaborate visual ornamentation have as much reproductive success as rivals with elaborate vocal characteristics.

　　17.5. The passage indicates that researchers raised female cowbirds in acoustic isolation in order to

　　(A) eliminate confounding variables

　　(B) approximate field conditions

　　(D) quantify repertoire complexity

　　(E) prevent early mating

　　17.6. According to the passage, the song sparrow is unlike the warbler in that the song sparrow

　　(A) uses songs mainly in territorial behavior

　　(B) continuously composes long and complex songs

　　(D) repeats one song type before switching to another

　　(E) responds aggressively to recorded songs

　　17.7. The passage suggests that the song sparrow experiments mentioned in lines 37-43 failed to confirm the role of intersexual selection because

　　(A) females were allowed to respond only to the song structure

　　(B) song sparrows are unlike other species of birds

　　(D) the experiments included the songs of only a small number of different song sparrows

　　(E) the experiments duplicated some of the limitations of previous field studies

　　答案：CDBDADC

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GRE阅读高频机经原文：冰川沉积物