Geoscience Reference
In-Depth Information
can fathom a few hundred years, even a few thousand, but we can-
not comprehend the passage of millions and billions of years. A meta-
phor that well captures the different character of geologic time,
again from McPhee, uses the old English yard, the distance from the
King's nose to the tip of his extended finger, as the equivalent of
geologic time. Apply to the regal digit one light stroke of a nail file,
and the equivalent of human history disappears. Comprehension of
geologic time must be accorded its rightful place as one of the great
achievements of human induction. Its importance to an analysis of
meteorite impact theory is that with time enough—with "time out
of mind"— earth history can be fully explained with no need to
appeal to catastrophes. To do so is to betray the key success of geol-
ogy: recognition that within the vast length of geologic time, every-
thing could be accomplished.
Because it is so foreign to our human time scale, the concept of
deep time not surprisingly took several centuries to develop. Leo-
nardo da Vinci was among the first to realize that the fossil shells
found high in the mountains were the remains of animals that had
once lived deep in the sea. Had he not been such a great painter, we
would likely remember Leonardo as the outstanding scientist of his
day. Another great advance came in the middle of the seventeenth
century, when a Dane named Nicolaus Steno compared fossils en-
cased in rock with the shark's teeth that sailors had brought him for
study. He could see that the two were identical and reasoned that
the teeth had somehow become enclosed in the rock; the teeth had
existed before the rock had fully formed, therefore the teeth were
older. This point seems elementary now but in its day was revolu-
tionary, for it led Steno to realize that some materials of the earth
were older than others and therefore that the earth, like a person,
has a history that can be interpreted and understood.
Until two centuries ago, science was required to be consistent
with the Book of Genesis. In the seventeenth century, James Ussher,
archbishop of Armagle, working backward from the beginning of that
topic, allowing due time for the events described, calculated that
the earth had been formed about 6,000 years before. Since all of
the earth's history had to be fitted into such a short period, in this
view, geologic processes must be rapid and catastrophe must be the
rule. (Creationists today argue that the earth can be no more than
10,000 years old, even though the Sumerians were so advanced as to
have a written language 6,000 years ago.)
But by the 1780s some had come to find catastrophism unten-
able, for it did not agree with the slow, inexorable processes of ero-
