新托福阅读的修辞目的题答题技巧
修辞目的题（Rhetorical Purpose Questions），为新托福必考题型，每篇文章考0-2题。它主要考查考生们理解作者为什么在文章中某个特别位置或者以某种特殊的方式陈述一个特别的信息。1）有时要求学生确定一个段落与另一个段落之间的联系
2）有时考查作者提到某一条特别信息的原因（句子之间的联系）
注意：此类题目不考查整篇文章的结构，只考查句子或段落之间的逻辑关系。
问题形式
?The author discusses X in paragraph 2 in order to…
?Why does the author mention X?
?The author uses X as an example of…
答题技巧
修辞目的题一般可分为以下几类：
一、例子题
例子题一般占修辞目的题出题机会的70%, 题干中出现的X本身是段落中出现的一个例子，这类题是送分题，因为要么是先举例子后总结/概述（理论），要么是先概述（理论）后举例子，所以答案多半是绕过例子看概述，而概述的改写就是本题答案的所在，也就是说哪个选项的内容和概述的内容一致就是答案。因此考生也可以用排除法选答案。
如：OG Title: ARTISANS AND INDUSTRIALIZATION
Paragraph 4
4. In paragraph 4, the author includes the quotation from a mill worker in order to
○ support the idea that it was difficult for workers to adjust to working in factories
○to show that workers sometimes quit because of the loud noise made by factory machinery
○ argue that clocks did not have a useful function in factories
○emphasize that factories were most successful when workers revealed their complaints 分析：此题中我们根据mill worker定位，能定位到原文对应内容为One mill worker who finally quit complained revealingly about “obedience to the ding-dong of the bell——just as though we are so many living machines”, 绕过例子读理论，理论“The first generation to experience these changes did not adopt the new attitudes easily. The factory clock became the symbol of the new work rules.”在例子前面，根据此理论，我们即可以选出正确答案为第一个选项。
如：OG Title: AGGRESSION
Paragraph 5
7. Freud describes people as steam engines in order to make the point that people
○ deliberately build up their aggression to make themselves stronger
○ usually release aggression in explosive ways
○ must vent their aggression to prevent it from building up
○ typically lose their aggression if they do not express it.
分析：此题中，我们根据Freud和steam engines定位，原文对应内容为The Freudian perspective, in a sense, see us as “steam engines.”绕过此例子，定位理论，原文中理论“By holding in rather than venting “steam,” we set the stage for future explosions.”在例子之后，根据此例子，我们可以确定答案为第三个选项。
如：OG Title: EARLY CINEMA
Paragraph 2
2. The author discusses phonograph parlors in paragraph 2 in order to
○explain Edison’s financial success
○ describe the model used to design Kinetoscope parlors
○ contrast their popularity to that of Kinetoscope parlors
○ illustrate how much more technologically advanced Kinetoscope parlors were
分析：此题中，我们根据phonograph parlors定位，原文对应内容为These Kinetoscope arcades were modeled on phonograph parlors, which had proven successful for Edison several years earlier. In the phonograph parlors, customers listened to recordings through individual ear tubes, moving from one machine to the next to hear different recorded speeches or pieces of music. 绕过此例子，定位理论，原文中理论“The kinetoscopes functioned in a similar way…”在例子之后，根据此例子，我们可以确定答案为第二个选项。
二、逻辑关系词或者态度类词汇的题
在段落中，将某一个逻辑关系词X划出，题干中why does the author mention X?
这种题目的解法主要是关注逻辑关系词或态度类词所表达的含义，如：
转折关系：however, but, although, instead, rather than等
因果关系：because, result in, result from, consequently, as a result等
递进关系：furthermore, also, moreover等
态度类词汇：happy, worried, panic, luckily, fortunately, unfortunately, impatient, curious, anxious, ridiculous, hopeless等
如：
Why does the author begin paragraph 4 with unfortunately?
○ It indicates that NASA has been unh appy with all of Hubble’s photograph.
○It shows that NASA’s plan to use stars to orient the Hubble telescope was misguided. ○ It emphasizes the need to have telescopes on the earth.
○ It indicates that high expectation were not initially met.
分析：此题即考查态度类词Unfortunately所表达的含义。结合原文内容中的“initially delayed in”,说明作者用Unfortunately一词是为了表达对Hubble telescope最初使用时并未成功的遗憾，因此我们可以确定答案即为第四个选项。
如：OG Title: THE ORIGINS OF CETACEANS
Paragraph 5
9. Why does the author use the word luckily in mentioning that the Ambulocetus natans
fossils included hind legs?
○ Fossil legs of early whales are a rare find.
○ The legs provided important information about the evolution of cetaceans.
○ The discovery allowed scientists to reconstruct a complete skeleton of the whale.
○ Until that time, only the front legs of early whales had been discovered.
分析：此题即考查态度类词luckily所表达的含义。结合原文内容中的“a good proportion of the hind legs”以及“The legs were strong and ended in long feet very much like those of a modern pinniped. The legs were certainly both functional both on land and at sea”, 说
明作者用luckily一词是为了表达对fossil包含了较好比例的后腿的欣喜，而此后腿又提供了很多信息，因此我们可以确定答案即为第二个选项。
老托福阅读真题及答案解析 托福从听、说、读、写四方面进行英语能力全面考核。托福频道为大家提供了这四个方面的资料，希望对大家有所帮助。 Aviculturists, people who raise birds for commercial sale, have not yet learned how to simulate the natural incubation of parrot eggs in the wild. They continue to look for better ways to increase egg production and to improve chick survival rates. When parrots incubate their eggs in the wild, the temperature and humidity of the nest are controlled naturally. Heat is transferred from the bird's skin to the top portion of the eggshell, leaving the sides and bottom of the egg at a cooler temperature. This temperature gradient may be vital to successful hatching. Nest construction can contribute to this temperature gradient. Nests of loosely arranged sticks, rocks, or dirt are cooler in temperature at the bottom where the egg contacts the nesting material. Such nests also act as humidity regulators by allowing rain to drain into the bottom sections of the nest so that the eggs are not in direct contact with the water. As the water that collects in the bottom of the nest evaporates, the water vapor rises and is heated by the incubating bird, which adds significant humidity to the incubation environment. In artificial incubation programs, aviculturists remove eggs from the nests of parrots and incubate them under laboratory conditions. Most commercial incubators heat the eggs fairly evenly from top to bottom, thus ignoring the bird's method of natural incubation, and perhaps reducing the viability and survivability of the hatching chicks. When incubators are not used, aviculturists sometimes suspend wooden boxes outdoors to use as nests in which to place eggs. In areas where weather can become cold after eggs are laid, it is very important to maintain a deep foundation of nesting material to act as insulator against the cold bottom of the box. If eggs rest against the wooden bottom in extremely cold weather conditions, they can become chilled to a point where the embryo can no longer survive. Similarly, these boxes should be protected from direct sunlight to avoid high temperatures that are also fatal to the growing embryo. Nesting material should be added in sufficient amounts to avoid both extreme temperature situations mentioned above and assure that the eggs have a soft, secure place to rest. 1. What is the main idea of the passage ? (A) Nesting material varies according to the parrots' environment. (B) Humidity is an important factor in incubating parrots' eggs. (C) Aviculturists have constructed the ideal nest box for parrots. (D) Wild parrots' nests provide information useful for artificial incubation. 2. The word "They" in line 2 refers to
托福阅读修辞目的题实例解析（七） 在托福阅读的十大题型中，修辞目的题(rhetorical purpose questions)是新托福阅读中难度较大的题型之一。这种题型对于国内考生来说较为陌生，在考场上时间紧张的情况下考生往往不知该在文中什么地方找答案。那么，对于这样一种类型的题目我们究竟该从何入手呢?在以下的一些例题中，我们就为大家进行详细的分析。 托福阅读修辞目的题例题 例题7： Paragraph 2: If an impact is large enough, it can disturb the environment of the entire Earth and cause an ecological catastrophe. The best-documented such impact took place 65 million years ago at the end of the Cretaceous period of geological history. This break in Earth’s history is marked by a mass extinction, when as many as half the species on the planet became extinct. While there are a dozen or more mass extinctions in the geological record, the Cretaceous mass extinction has always intrigued paleontologists because it marks the end of the age of the dinosaurs. For tens of millions of years, those great creatures had flourished. Then, suddenly, they disappeared. Q:In paragraph 2, why does the author include the information that dinosaurs had flourished for tens of millions of years and then suddenly disappeared? A:To support the claim that the mass extinction at the end of the Cretaceous is the best-documented of the dozen or so mass extinctions in the geological record To explain why as many as half of the species on Earth at the time are believed to have become extinct at the end of the Cretaceous
TOEFL iBT阅读技巧—关键词法解题 关键词在阅读理解中的重要作用： key words—结构（structure） —推断（inference） 1.key words—结构（structure） 针对题型：大意概括题 解题步骤：1.找结构关键词（but, and, as ,while, so, to……）; 2.分析句子结构; (并列，递进，从属，因果……) 3.结合文本大意选出正确选项。 例1 TPO34 Islamic Art and the Book Papermaking spread quickly to Egypt – and eventually to Sicily and Spain – but it was several centuries before paper supplanted parchment for copies of the Koran, probably because of the conservative nature of religious art and its practitioners. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A) It was several centuries before papermaking techniques spread to faraway areas where parchment was popular and used widely in art. B) Although papermaking came to Egypt quickly, it took much longer for paper to be used when copying the Koran, probably because of the conservative nature of religious art. C) Papermaking spread beyond Egypt, Sicily, and Spain, but it was not widely used by artists for centuries, probably because of the conservative nature of art in those countries. D) Paper replaced parchment in copies of the Koran, probably at the request of conservative practitioners in areas like Egypt, Sicily, and Spain. [解析] 关键词：but, because of. But 连接的是两个句子的转折，而because of 是
ET老师另辟蹊径谈“托福阅读Prose Summary 解题技巧” 一、绝密档案——剖析Prose Summary的前世今生 Prose summary, TOEFL王国中的头号通缉犯，自2008年随IBT入主考试界以来，就是无数江湖人士的心头噩梦。究其为何，原因有二： 其一：Prose Summary 题型天生不讨巧，从来都是在一场战役（一篇文章）行将结束时出现，而且还针对全文出题。很多大侠往往准备不足，加之略有紧张，到最后往往文章中写了些啥全部忘得一干二净。而面对这种题目，定位相对来讲往往也就意味着从头把文章再看一遍，耗时又费力，极为不招人待见。 其二：从Prose Summary的出题标准来看，Prose Summary是希望各位大侠选出符合文章内容的三个选项，当然，这个选项，并非说只要是符合文章的，正确的就行。在江湖英雄帖（OG）中，对于Prose Summary的描述是这样的：“These items measure your ability to understand and recognize the major ideas and the relative importance of information in a passage.”，各位看官不妨注意一下major ideas这个词，翻译成我们汉人的话来说，就是主要观点或者是重要信息。但是，很多大侠，入行不久，功夫不深，往往是细节和主观点傻傻分不清楚。 综上，Prose Summary是各位大侠在战斗中所遇到的终极BOSS，耗时又费力。然而，俗话说，擒贼先擒王，更不要说这个BOSS是个外强中干的菜鸟，稍加注意，不但能逢凶化吉，反而在战场上作用巨大，为何如此，且听ET细细道来。 二、庖丁解牛——prose summary 破题五法 1. 秘技1——巧用黑体字 相信，还有不少大侠还没注意到过prose summary选项前的那一行黑体字吧，这行黑体字，作用又是什么呢？我们不妨来继续看看，英雄帖怎么说？ 现在，拿出自己手里的任何一本托福模拟题，或者是任何的一套TPO，我们都会在题目上的Direction看到这样的一行字：An introductory sentence for a brief summary of the passage is provided below。用汉人的话说，brief summary，也就是全文的简单总结。换而言之，文章大概写了什么内容，出题人已经给你总结出来了。加上，这篇文章，让你选择出来的又是符合文章的重要信息，所以，有了它，我们就对全文的内容，已经略知一二了。、2. 秘技2——巧用黑体字题眼定选项 刚才我们谈到，黑体字是对全文的概括总结。加上我们在之前的时候，就已经给大家
托福阅读真题3 PASSAGE 3 The Native Americans of northern California were highly skilled at basketry, using the reeds,grasses, barks, and roots they found around them to fashion articles of all sorts and sizes- not only trays, containers, and cooking pots, but hats, boats, fish traps, baby carriers, and ceremonialobjects. Of all these experts, none excelled the Pomo — a group who lived on or near the coast during the 1800's, and whose descendants continue to live in parts of the same region to this day. They made baskets three feet in diameter and others no bigger than a thimble. The Pomo people were masters of decoration. Some of their baskets were completely covered with shell
pendants; others with feathers that made the baskets' surfaces as soft as the breasts of birds. Moreover, the Pomo people made use of more weaving techniques than did their neighbors. Most groups made all their basketwork by twining —the twisting of a flexible horizontal material, called a weft, around stiffer vertical strands of material, the warp. Others depended primarily on coiling — a process in which a continuous coil of stiff material is held in the desired shape with tight wrapping of flexible strands. Only the Pomo people used both processes with equal ease and frequency. In addition, they made use of four distinct variations on the basic twining process, often employing more than one of them in a single article. Although a wide variety of materials was available, the Pomo people used only a few. The warp was always made of willow, and the most commonly used weft was sedge root,
托福阅读解题技巧-句子插入题（一） 托福阅读不同于传统阅读，其中包含了”句子插入题”这类特殊题型，这也是托福阅读中相对难得分的题型。此类题型中，题干中会给出一个新的句子，然后会在原文段落中给出四个黑框，考察新句子在文章中最合适的位置。针对于此类题型的解答，有很多实用的方法，今天我们要介绍的是在剩余时间不多的情况下，最简单最便捷的解决方法-语义指代匹配法。 在上下句的关联中，代词是最为紧密的一类词。当作者再次谈及上句相关话题时，为了避免重复，遵循语言学的最简单化原理，作者会用代词指代上文内容，常见指示代词this, that, these, those, 人称代词they, she, he, 或it ，还有their, such, another等。对于此类题型的具体解题步骤如下： ①确认指代对象：根据指代词找到其所指代的对象； ②定位指代对象：通过浏览扫描段落信息，确定指代对象位置； ③代入验证：把插入句代入含有指代对象所在句的后面进行验证即可。 接下来，我们通过具体实例来论证使用语义指代匹配法。 案例： Paragraph 1: Before 1815 manufacturing in the United States had been done in homes or shops by skilled artisans.■As master craft workers, they imparted the knowledge of their trades to apprentices and journeymen. ■In addition, women often worked in their homes part-time, making finished articles from raw material supplied by merchant capitalists. ■After 1815 this older form of manufacturing began to give way to factories with machinery tended by unskilled or semiskilled laborers. ■Cheap transportation networks, the rise of cities, and the availability of capital and credit all stimulated the shift to factory production. 11. Look at the four squares[■]that indicate where the following sentence can be added to the passage. This new form of manufacturing depended on the movement of goods to distant locations and a centralized source of laborers.
2019托福阅读：模拟试题及答案解析（6） 【托福】 Although only 1 person in 20 in the Colonial period lived in a city, the cities had a disproportionate influence on the development of North America. They were at the cutting edge of social change. It was in the cities that the elements that can be associated with modern capitalism first appeared — the use of money and commercial paper in place of barter, open competition in place of social deference and hierarchy, with an attendant rise in social disorder, and the appearance of factories using coat or water power in place of independent craftspeople working with hand tools. "The cities predicted the future," wrote historian Gary. B. Nash, "even though they were but overgrown villages compared to the great urban centers of Europe, the Middle East and China." Except for Boston, whose population stabilized at about 16,000 in 1760, cities grew by exponential leaps through the eighteenth century. In the fifteen years prior to the outbreak of the War for independence in 1775, more than 200,000 immigrants arrived on North American shores. This meant that a population the size of Boston was arriving every year, and most of it flowed into the port cities in the Northeast. Philadelphia's population nearly doubted in those years, reaching about 30,000 in 1774, New York grew at almost the same rate, reaching about 25,000 by 1775. The quality of the hinterland dictated the pace of growth of the cities. The land surrounding Boston had always been poor farm country, and by the mid-eighteenth century it
托福阅读真题3 PASSAGE 3 The Native Americans of northern California were highly skilled at basketry, using the reeds,grasses, barks, and roots they found around them to fashion articles of all sorts and sizes- not only trays, containers, and cooking pots, but hats, boats, fish traps, baby carriers, and ceremonialobjects. Of all these experts, none excelled the Pomo — a group who lived on or near the coast during the 1800's, and whose descendants continue to live in parts of the same region to this day. They made baskets three feet in diameter and others no bigger than a thimble. The Pomo people were masters of decoration. Some of their baskets were completely covered with shell pendants; others with feathers that made the baskets' surfaces as soft as the breasts of birds. Moreover, the Pomo people made use of more weaving techniques than did their neighbors. Most groups made all their basketwork by twining — the twisting of a flexible horizontal material, called a weft, around stiffer vertical strands of material, the warp. Others depended primarily on coiling — a process in which a continuous coil of stiff material is held in the desired shape with tight wrapping of flexible strands. Only the Pomo people used both processes with equal ease and frequency. In addition, they made use of four distinct variations on the basic twining process, often employing more than one of them in a single article. Although a wide variety of materials was available, the Pomo people used only a few. The warp was always made of willow, and the most commonly used weft was sedge root, a woody fiber that could easily be separated into strands no thicker than a thread. For color, the Pomo people used the bark of redbud for their twined work and dyed bullrush root for black in
2个实例详解托福阅读修辞目的题从题干找线索解题技巧 托福阅读中既有不需要思考太多直接就能从原文找到答案的题目，也有需要考生自己 主动思考分析才能得出结论的题目。今天小编给大家带来了2个实例详解托福阅读修辞目的题从题干找线索解题技巧希望能够帮助到大家，下面小编就和大家分享，来欣 赏一下吧。 2个实例详解托福阅读修辞目的题从题干找线索解题技巧 托福阅读修辞目的题出题形式简介 托福阅读修辞目的题的格式是这个样子的：“why does author mention ”注意这里的why不是定位文中的because来找答案，而是应采取这样一种思维即：题干作为一个细节，其存在的价值是为了支撑前面出现的观点，所以答案在题干细节出现的位置前 面去找，也就是找到观点 (注意避开其他同样支撑观点的细节) 修辞目的题解题思路实例分析 实例1： Paragraph 6: But neither the human imitativeinstinct nor a penchant for fantasy by itself leads to an autonomous theater.Therefore, additional explanations are needed. One necessary condition seems tobe a somewhat detached view of human problems. For example, one sign of thiscondition is the appearance of the comic vision, since comedy requires sufficientdetachment to view some deviations from social norms as ridiculous rather thanas serious threats to the welfare of the entire group. Another condition thatcontributes to the development of autonomous theater is the emergence of theaesthetic sense. 2. Why does the author mention “comedy”? A. To give an example of early types of theater. B. To explain how theater helps a society respond to threats to its welfare. C. To help explain why detachment is needed for the development of theater. D. To show how theatrical performers become detached from other members of society. 本题中的comedy虽为一个小词，看似不起眼，但是它所存在的句子，句首为for example，这就构成了明显的举例关系，所以答案还是在前面找这个例子所证明的观点。第一句中的neither直接否定了两个观点，后面出现的therefore带起的句子是个
对于目标明确，清晰认识自己薄弱的考生一定很想知道托福阅读summary题型该怎么解。接下来上海环球托福为考生解答。 首先，做阅读就要了解掌握文章的主旨是什么，新托福阅读SUMMARY题作为文章最后一种题型，考察的是对文章主要段落的主旨概括能力，而非全篇文章主旨。直白一点说，就是只需要关注文章局部段落，而不需要在意全局。这不同于老托福阅读里面每篇文章都有一道“What is the passage mainly about”这样的全文主旨题。 接着，考生要如何掌握主旨，来完成summary的题目。阅读英语段落的时候，应该是“匀速阅读”还是“变速阅读”呢？当然是“变速阅读”！这样考生才能做到阅读时的详略得当。但是，“变速阅读”是“先快后慢”还是“先慢后快”呢？这就要从英语的思维方式说起。 汉语族人的思维方式是“螺旋式”的，喜欢画龙点睛，我们称之为“Save the best for the last”。而英语族人的思维方式是“直线式”的，喜欢直入主题，我们称之为“Say what you want to say, then say why”。阅读英语段落的要领：变速阅读，先慢后快，精读首句，浏览全段。据统计，70%-80%的英语段落都是首句是主题句。所以，以后阅读英语段落，应该先精读首句（把握首句的“主题”和“方向”），再浏览全段（把握全段的“关系词”），才能实现快速阅读。而很多同学以前阅读的时候，要么就是倾向于到段落结尾找主题句（这主要是受了汉语思维方式的影响），要么就干脆把有限的精力平均分配到一段话内部的每个单词。 说了那么多，上海环球托福为你提出掌握主旨的方法： 1、常用法——分析段落结构找主题句 通过段落结构来概括主旨，常见的段落结构有如下六大类：（1）总分；（2）总分总（由总分演变而成）；（3）分总；（4）分总分（由分总演变而成）；（5）对比；（6）并列。其中，前四种段落结构都存在主题句和支持句之分。而后两种结构中可能不存在主题句，其段落内部会分成若干个势均力敌的部分，只不过对比结构段落的两部分之间方向相反，而并列结构段落的各部分之间方向相同。一、总分结构 总分结构的主题句一般为首句（如果首句结束后，第二句话存在转折关系，那么主题句就是第二句）。最常见的总分段落的标志就是第二句话前面出现“for example/instance”等举例关系词。如果总分结构段落最后再加一个总结句，就是总分总结构。这种段落结构约占70%。这个方法最大好处是大部分段落都存在主题句，所以能够广泛应用该方法概括段落主旨。这就是“常用法”的由来。但是，该方法的缺点是，如果段落结构比较复杂，又存在大量生词，可能导致我们难以找到主题句。这时候“简易法——重复出现是主旨”就会帮上大忙了。 2、简易法——重复出现是主旨 之所以有第二个方法是因为第一个方法有缺点，常用法的缺点是，如果段落结构比较复杂，又存在大量生词，可能导致我们难以找到主题句。这时候“简易法——重复出现是主旨”就会帮上大忙了。如果一段话在不断重复某一概念，就说明这段话在围绕这个概念展开，这个概念就是这段话的主旨。这就为我们快速把握主旨提供了极大的方便。因此，这种方法称为“简易法”。 在一段话内部重复出现可以分为三种类型。第一种是“原词”重复，但是这也是作者极力避
托福阅读解题有没有技巧怎么办 托福阅读解题有没有技巧?提升硬实力才是高分关键力。今天给大家来托福阅读解题有没有技巧，希望能够帮助到大家，下面就和大家分享，来欣赏一下吧。 托福阅读解题有没有技巧?提升硬实力才是高分关键 托福阅读对词汇量要求很严格 从某种意义上来讲，你积累的英语词汇量的多少，影响着你在托福考试中取得的成绩。如果说你具备的词汇量达不到最基本的要求，即使你的语言能力再强也过不了托福难关。想要训练阅读能力首先就要从词汇量上下手，只有掌握足够多的词汇后，你才具备他警告阅读能力的最低标准。 遇到生词，即使不知道什么意思，也可以根据上下文语境弄清它们之间的关系，运用逻辑推理的方法作出正确的答案。其次，还可以分析选项，进行对比，往往会发现选项中通常会出现两个意思相反的选项，此时再仔细重读原文就可以排除错误选项了。 托福阅读要积累英语*量增加知识面 读书破万卷，下笔如有神，放在托福阅读备考中依然适用。考生利用业余时间丰富自己的知识面，对各国或各地区等相关文
化及常识有一定了解后，在参加托福考试时，当阅读到自己有所了解的东西后，解题试题时就会比较有自信和感觉，不会因为自己的不了解或是不清楚，对*要说的东西特别盲目，答题的准确率自然而然的也就会有所上升。 总而言之，托福阅读虽然也需要一定技巧，但每位同学需要掌握的技巧都是不同的，适合别人的不一定适合自己。比起一味训练解题技巧，提升如词汇量和阅读量方面的硬实力，其实才是真正通往阅读高分的必经之路。 托福阅读100个长难句精选汇总深度解析：Contrary to the arguments... 托福阅读100个长难句实例分析 原句案例： Contrary to the arguments of some that much of the pacific was settled by Polynesians accidentally marooned after being lost and adrift, it seems reasonable that this feat was accomplished by deliberate colonization expeditions that set out fully stocked with food and domesticated plants and animals. 词汇讲解： maroon作动词，表示“被困于荒无人烟且无法脱身的地方”，比如：The car broke down leaving usmarooned in the middle of
The Extinction of the Dinosaurs million years ago) and the Paleocene period (65..C55 million years ago) in part by the types and amounts of rocks and fossils they contain or lack. Before the limit of 65 million years ago，marine 2.strata are rich in calcium carbonate due to accumulations of fossils of microscopic algae deposited on the sea floor. Above the 65-million-year limit，sea-floor sediments contain much less calcium carbonate ，and fossils of several families of mollusks are no longer found. In continental sediments ，dinosaur fossils ，though frequent before 65 million years ago ，are totally absent. By 3.contrast，new families of mammals appear，including large mammals for the first time. Scientists wondered for many years about what could have caused the dinosaurs' rapid disappearance at the end of the Cretaceous period，coming up with a great variety of theories and scenarios. For some, it could have been due to unfavorable genetic changes triggered by a dramatic increaseby a factor of 10，100，1，000 in cosmic-ray particles reaching the Earth after a supernova explosion somewhere in the neighborhood of the solar system. For these high-energy particles to affect life，they would have to get through the protective barrier of the Earth's magnetosphere ，the region of the upper atmosphere controlled by Earth's magnetic field. That could have happened if the cloud of particles from the supernova explosion reached the Earth during a period when the magnetosphere was weakened ， something that may happen when the Earth's magnetic field changes direction. And we know that the magnetic north and south poles of the Earth switch on the average twice every million years. However，this is not the only possible explanation for dinosaur destruction . 4.Other theories have raised the possibility of strong climate changes in the tropics (but they then must be explained) . Certainly，if climate changes，the changed distributions of temperature and rainfall modify the conditions that favor one ecosystem over another. The extinction of a particular family，genus，or species may result from a complicated chain of indirect causes and effects. Over thirty years ago ，scientist Carl Sagan quoted one suggestion that the demise of the dinosaurs resulted from the disappearance of a species of fern plant that was important for dinosaur digestion. Other theories involved a worldwide cold wave following the spread of a layer of cold but not very salty water in the world's oceans ，which floated on the surface because，with its low salinity，the water was less dense. 5.Proponents of another theory that remains under consideration today postulate that the extinction of the dinosaurs corresponds to a period of intense volcanic activity. It's not a question of just one or even of a thousand eruptions comparable to the explosion of Krakatoa in 1883，one of the largest volcanic events in modern times ，but rather of a prolonged period of activity. On the Deccan plateau in India，basalt (volcanic) rocks cover more than 500，000 square kilometers (nearly 200，000 square miles)，and correspond to massive lava outflows occurring precisely at the end of the Cretaceous. This sort of outflow could correspond to volcanic activity similar to the activity that drives sea-floor spreading ，with lava emerging from elongated fractures in the crust rather than from craters. 6.The volcanic convulsion that buried the Deccan plateau in lava must also have changed the composition of the atmosphere and severely affected climate. Initially，there must have been strong sudden cooling resulting from the blocking of sunlight by sulfate aerosol veils in the