The History of Geology
By noting the relationships of different
rock units, Nicolaus Steno in 1669 described two basic geologic
principles. The first, "The Law of Original Horizontality",
stated that sedimentary rocks are layed down in a horizontal manner. The
second, "The Law of Superposition," stated that in an undeformed
sequence of sedimentary rocks, younger rock units were deposited on top of
older rock units. To envision this latter principle think of the layers of
paint on a wall. The oldest layer was put on first and is at the bottom,
while the newest layer is at the top. Steno recognition of fossils as
traces of ancient life helped him with his discoveries.
An additional concept was introduced by
James Hutton in 1795, and later emphasized by Charles Lyell in the early
1800s.
- "The Principle of Uniformitarianism"
is the idea that natural geologic processes have been uniform in
frequency and magnitude throughout time. The principle is basically
stating that "the present is the key to the past."
Steno's principles allowed workers in
the 1600's and early 1700's to begin to recognize rock successions.
However, because rocks were locally described by the color, texture,
or even smell, comparisons between rock sequences of different areas
were often not possible. It was the use of fossils that provided the
opportunity for workers to correlate between geographically distinct
areas. This contribution was possible because fossils are widely found
in areas of the earth's crust.
For the next major contribution to the
geologic time scale we turn to William "Strata" Smith, a
canal builder and geologist from England.
- "The Principle of Faunal
Succession" was validated in 1815 by Smith when he produced a
geologic map of England demonstrating the principle. This principle
simply stated that fossils have a very definite order in which they
are found in rocks and therefore any specific time period can be
recognized by its fossil content. Once established, this principle
allowed geologists to identify rocks of the same age in widely
separated regions. This enabled the building of the geologic time
scale.
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