EXPANDING HORIZONS 99
In 1770, Hutton built a house
overlooking Salisbury Crags in
Edinburgh, Scotland. Among the
crags he found evidence of volcanic
penetration through sedimentary rock.older, since eons of time would
be needed for chalk mountains to
build up from the remains of marine
fossils, but he did not want to
publish this view without evidence.
Secrets of the rocks
In Scotland, quite a different
approach to the problem of Earth’s
age was being taken by James
Hutton, one of the preeminent
natural philosophers of the Scottish
Enlightenment. Hutton was a
pioneer of geological fieldwork, and
used field evidence to demonstrate
his arguments to the Royal Society
of Edinburgh in 1785.
Hutton was impressed by
the apparent continuity of the
processes by which landscape was
denuded and its debris deposited
into the sea. And yet all these
processes did not lead to loss of the
land surface, as might be expected.
Perhaps thinking of the famous
steam engine built by his friend
James Watt, Hutton saw Earth as
“a material machine moving in all
its parts,” with a new world
constantly reshaped and recycled
from the ruins of the old.
Hutton formulated his Earth-
machine theory before he had found
the supporting evidence, but, in
1787, he found the “unconformities”
he was looking for—breaks in the
continuity of sedimentary rocks.
He saw that much of the land had
once been seabed, where layers
of sediment had been laid down and
compressed. In many places these
layers had been pushed upward, so
that they were above sea level, and
often distorted, so that they were
not horizontal. He repeatedly found
that rock material from the
truncated upper boundary of older
strata was incorporated into the
base of the younger rocks above.
Such unconformities showed
that there had been many episodes
in Earth’s history when the
sequence of erosion, transportation,
and deposition of rock debris had
been repeated, and when rock
strata had been moved by volcanicSee also: Isaac Newton 62–69 ■ Louis Agassiz 128–29 ■ Charles Darwin 142–49 ■ Marie Curie 190–95 ■
Ernest Rutherford 206–13
activity. Today, this is known as
the geological cycle. From this
evidence, Hutton declared that
all continents are formed from
materials derived from previous
continents by the same processes,
and that these processes still
operate today. Famously, he wrote
that “the result, therefore, of this
present enquiry is, that we find
no vestige of a beginning—no
prospect of an end.”
The popularization of Hutton’s
ideas about “deep time” was
primarily due to John Playfair, a
Scottish scientist who published
Hutton’s observations in an
illustrated book, and to British
geologist Charles Lyell, who
transformed Hutton’s ideas into a
system called uniformitarianism.
This held that the laws of nature ❯❯