Questions 1-9
With the onset of the Industrial Revolution, knowing the age of rocks became a
necessary prerequisite to finding industrial minerals, such as coal, iron, and the
other materials that fueled and sustained the great Western industrialization of the eighteenth
Line and nineteenth centuries. It was in the mining regions where engineers, who needed a better system for organizing the various types of rock scattered across Earth's surface,
first grappled with scientific approaches to understanding the age of various rocks—and
the age of Earth. They realized that if the various rock units could he dated by their
relative ages, correlations among even widely separated rocks could be established and
from this, some order recognized. The pioneering European geologists first believed that identifying a rock's type would
give them a strong clue to the age of the rock formation and that one of the most powerful
clues came from the hardness of a given rock. Specific rock types were thus assumed to
have formed at characteristically different rimes, the softest rocks having formed the most
recently. This crude type of dating was first used to understand the way mountains were formed, In the mid-1700's it was thought that there were three distinct types of mountains
in Europe, each formed by a different type of rock and each created at a different time.
According to this theory, the oldest were the Alps, which had interior cores composed
of very hard, crystalline rocks (such as granite, schist, or basalt). These mountains were
called Primitive. Sitting on the flanks of the Primitive mountains were younger, smaller, Secondary mountains composed of layered sedimentary rocks such as limestone.
They were often rich with fossils and intermediate in hardness. The youngest Tertiary
mountains were composed of softer mudstones and sandstone. Rock type, hardness,
and size thus established mountain type, and rock type also became a proxy for age.
However, study soon exposed the fallacy of these early notions. It was discovered that
25 some of the very high mountains were composed of the softest sediments and that even
hard volcanic rock was sometimes found in very low mountains. By the early 1800's, it
was understood that rock type was of no help in establishing age. 【題組】5. The word 'They" in line 21 refers to
(A) crystalline rocks
(B) the flanks
(C) the Primitive mountains
(D) layered sedimentary rocks
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