GRE考试

【单选题】Rocks which have solidified directly from
molten materials are called igneous rocks.
Igneous rocks are commonly referred to as
Line primary rocks because they are the original
(5) 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
(10) are distinguished by the following characteristics:
(1) they contain no fossils; (2) they have no regu-
lar arrangement of layers; and (3) they are nearly
always made up of crystals.
Sedimentary rocks are composed largely of
(15) 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,
(20) 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
down in layers called stratA、
(25) When sediments harden into sedimentary
rocks, the names applied to them change to indi-
cate the change in physical state. Thus, small
stones and gravel cemented together are known as
conglomerates; cemented sand becomes sand-
(30) stone; and hardened clay becomes shale. In addi-
tion to these, other sedimentary rocks such as
limestone frequently result from the deposition of
dissolved material. The ingredient parts are nor-
mally precipitated by organic substances, such as
(35) 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
(40) 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
(45) marble, a metamorphic rock. Shale under pressure
becomes slate.
The relationship between igneous and sedimentary rocks may best be compared to the relationship betweenA．leaves and compost
B．water and land
C．DNA、and heredity
D．nucleus and cell wall
E、sand and clay

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