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直男的表现特征托福阅读TPO20(试题+答案+译文)第3篇:Fossil Prervation
托福阅读原文
【1】When one considers the many ways by which organisms are completely destroyed after death, it is remarkable that fossils are as common as they are. Attack by scavengers and bacteria, chemical decay, and destruction by erosion and other geologic agencies make the odds against prervation ver
y high. However, the chances of escaping complete destruction are vastly improved if the organism happens to have a mineralized skeleton and dies in a place where it can be quickly buried by diment. Both of the conditions are often found on the ocean floors, where shelled invertebrates (organisms without spines) flourish and are covered by the continuous rain of dimentary particles. Although most fossils are found in marine dimentary rocks, they also are found in terrestrial deposits left by streams and lakes. On occasion, animals and plants have been prerved after becoming immerd in tar or quicksand, trapped in ice or lava flows, or engulfed by rapid falls of volcanic ash.
【2】The term "fossil" often implies petrifaction, literally a transformation into stone. After the death of an organism, the soft tissue is ordinarily consumed by scavengers and bacteria. The empty shell of a snail or clam
may be left behind, and if it is sufficiently durable and resistant to dissolution, it may remain basically unchanged for a long period of time. Indeed, unaltered shells of marine invertebrates are known from deposits over 100 million years old. In many marine creatures, however, the skeleton is compod of a mineral variety of calcium carbonate called aragonite. Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is rel
atively unstable, and in time changes to the more stable calcite.
【3】Many other process may alter the shell of a clam or snail and enhance its chances for prervation. Water containing dissolved silica, calcium carbonate, or iron may circulate through the enclosing diment and be deposited in cavities such as marrow cavities and canals in bone once occupied by blood vesls and nerves. In such cas, the original composition of the bone or shell remains, but the fossil is made harder and more durable. This addition of a chemically precipitated substance into pore spaces is termed "permineralization."
【4】Petrifaction may also involve a simultaneous exchange of the original substance of a dead plant or animal with mineral matter of a different composition. This process is termed " replacement" becau solutions have dissolved the original material and replaced it with an equal volume
of the new substance. Replacement can be a marvelously preci process, so that details of shell ornamentation, tree rings in wood, and delicate structures in bone are accurately prerved.
【5】Another type of fossilization, known as carbonization, occurs when soft tissues are prerved as thin films of carbon. Leaves and tissue of soft-bodied organisms such as jellyfish or worms may accumulate, become buried and compresd, and lo their volatile constituents. The carbon often r
emains behind as a blackened silhouette.榴莲怎么种
【6】Although it is certainly true that the posssion of hard parts enhances the prospect of prervation, organisms having soft tissues and organs are also occasionally prerved. Incts and even small invertebrates have been found prerved in the hardened resins of conifers and certain other trees. X-ray examination of thin slabs of rock sometimes reveals the ghostly outlines of tentacles, digestive tracts, and visual organs of a variety of marine creatures. Soft parts, including skin, hair, and viscera of ice age mammoths, have been prerved in frozen soil or in the oozing tar of oil eps.荷尔蒙是什么气味
炸花生米
【7】The probability that actual remains of soft tissue will be prerved is improved if the organism dies in an environment of rapid deposition and oxygen deprivation. Under such conditions, the destructive effects of
珠穆拉玛峰
bacteria are diminished. The Middle Eocene Mesl Shale (from about 48 million years ago) of Germany accumulated in such an environment. The shale was deposited in an oxygen-deficient lake where lethal gas sometimes bubbled up and killed animals. Their remains accumulated on the floor of the lake and were then covered by clay and silt. Among the superbly prerved Mesl fossil
s are incts with iridescent exoskeletons (hard outer coverings), frogs with skin and blood vesls intact, and even entire small mammals with prerved fur and soft tissue.
托福阅读试题
1.The word "agencies" in the passage (paragraph 1) is clost in meaning to
B.problems.
C.forces.
豆一D.changes.
深的反义词
2.In paragraph 1, what is the author's purpo in providing examples of how organisms are destroyed?
文化的重要作用
A.To emphasize how surprising it is that so many fossils exist.
B.To introduce a new geologic theory of fossil prervation.
