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  3. Rocks which have solidified directly from molten materials are called igneous rocks. Igneous rocks are commonly referred to as p

Rocks which have solidified directly from molten materials are called igneous rocks. Igneous rocks are commonly referred to as p

admin2011-04-26  51
问题     Rocks which have solidified directly from molten materials are called igneous rocks. Igneous rocks are commonly referred to as primary rocks because they are the original source of material found in sedimentaries and metamorphics. Igneous rocks compose the greater part of the earth’s crust, but they are generally covered at the surface by a relatively thin layer of sedimentary or metamorphic rocks. Igneous rocks are distinguished by the following characteristics: (1) they contain no fossils; (2) they have no regular arrangement of layers; and (3) they are nearly always made up of crystals.
Sedimentary rocks are composed largely of minute fragments derived from the disintegration of existing rocks and in some instances from the remains of animals.  As sediments are transported, individual fragments are assorted according to size. Distinct layers of such sediments as gravels, sand, and clay build up, as they are deposited by water and occasionally wind. These sediments vary in size with the material and the power of the eroding agent. Sedimentary materials are laid clown in layers called strata.
    When sediments harden into sedimentary rocks, the names applied to them change to indicate the change in physical state. Thus, small stones and gravel cemented together are known as conglomerates; cemented sand becomes sandstone; and hardened clay becomes shale. In addition to these, other sedimentary rocks such as limestone frequently result from the deposition of dissolved material. The ingredient parts are normally precipitated by organic substances, such as shells of clams or hard skeletons of other marine life.
    Both igneous and sedimentary rocks may be changed by pressure, heat, solution, or cementing action. When individual grains from existing rocks tend to deform and interlock, they are called metamorphic rocks.  For example,  granite,  an igneous rock,  may be metamorphosed into a gneiss or a schist. Limestone, a sedimentary rock, when subjected to heat and pressure may become marble, a metamorphic rock. Shale under pressure becomes slate.
In a granite quarry, we may expect to find ______.
选项 A、strata
B、limestone
C、metamorphic rock
D、no trace of fossils
答案D
解析 在花岗岩矿中，我们也许找不到化石的遗迹。作者在最后一段指出，花岗岩属于火成岩；还在第一段指出，可以根据三个特征识别火成岩：第一，它们不包含化石；第二，它们的岩层排列不规则；第三，它们几乎总是由水晶构成的。
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