The times. [volume] (Washington [D.C.]) 1897-1901, 1900-07-02
TEE AGE OF THE EARTH
Methods Employed by Scientists in
Computing II.
Prof. C. n. Gilbert, of the Geological
Survey. Discusses the SnbJect In
Attempt to Reach IL Conclusion
The Rotation of the Globe.

What is the age of the earth? This is
a question that scientists for years have
been trying to answer. Some have an
swered it according to their own assump-
tons and calculations, and their results
have shown discrepancies ranging from
millions to tens Of thousands of millions
Prof. G. X. Gilbert of the United States
| Geological Survey, who discussed the sub
| ject at the Conference OF the American
| Association for the Advancement of Science
| at Columbia University, New York, during
| the past week, did not answer the question.
That vain expectation may not be
aroused he said, Il admit at the outset
that l have not solved the problem ard
shall submit you no estimate"
What ne did do, however, was to show
how scientists are approaching and must
in the future approach the problem, what
are the means by which they can hope
age Of the earth.
In attempting to measure the age of the
earth, scientists make use of either con-
tinuous processes or rhythms, just as Is
done ordinarily in measuring time.
The clepsydra, a more moderc form Of
which is the hour glass with sand sub
stituted for water, and the taper, which
Is still used In monastic yigiis in the form
Of the graduated candle. are examples Of
the application of continuous processes 10
measuring time. The pendulum, on the
other hand, is an example of the use Of
rhythm in time measurements.
The Motion of the Earth.
While the motion of the earth is con-
tinuous, as compared with any other ce-
lestial body it is rhythmic Compared with
fixed star, it gives us the siderial day,
with the sun, the solar day, nearly four
minutes longer, with the moon. the lunar
day, longer still by forty-nine minutes.
The siderial day is reckoned only by as
tronomers. The lunar day finds express
sion in the tides. Besides the day. there
are two other natural time limits, the
month. or rhythmic period OF the moon re-
garded as a luminary, and the year, or
rhythm of the earth's orbit. The genera
ton and the dynasty are two time meas
ures of waning Importance.
In computing geologic time, scientists
make use Of variety or continuous proc
esses.
First may be mentioned river deposits.
Throughout the ages the rivers have beef
deposting layers of sediment at their
mouths. In course of time these deposits
have been converted into rock. and in cal
culating the time necessary to produce the
various layers, biologists have to take &C
count of their volume and thickness. The
amount of the deposits equals the amount
of material washed away. and by calcuiat-
ing the amount of material carried away
by rivers in a given period OF time, geol-
ogists can estimate the amount Of time
represented by the different layers of rock.
In some such way as this. from the accr-
mutation of sand in beaches. Andrews esti-
mated the age OF Lake Michigan.
Another System of Measuring.
Another system of measuring geologic
time is founded on the principle of the
evolution OF life. Fossils are the records
in this case, and account is taken of the
amount of change from one full record to
the next.
The study of the processes pertaining to
the earth as a planet opens up other ways
of approaching the problem of geologic
time.
las the rotation OF the earth causes
the tides" Prof. Gilbert says, land
~
act as a brake and check the speed of ro-
tation. Therefore the earth has in the
PAST spun faster than now, and its rate
OF spinning at any remote point may be |
Assuming that the whole globe |
is solid and rigid, and that geologic record |
could not begin until that condition had |
been attained, there could not have been |
great checking of rotation since CONSOLE |
dation. For If there had been it would |
have resulted in the gathering OF the |
oceans about the poles and the barring OF |
the land near the equator, condition very /
different from what actually obtains. This |
line of reasoning yields an obscure outer |
limit to the age of the earth.
"0n the assumption that the globe lacks |
of perfect rigidity G. H. OAR
win has traced back the history of the |
earth and the moon to an epoch when the !
two bodies were united. their separation ,
having been followed by the gradual en-
argement OF the moons orbit and the
gradual retardatiOn of the earth's rota-
lion: and this line of enquiry has also
Yielded an obscure outer limit to the an
iquity of the earth as a habltable globe"
An Elaborate Computation.
One of the most elaborate OF all COM
)utations starts with the assumption that
At the initial epoch, when the outer part
IF the earth was consolidated from a
liquid condition, the whole body of the
planet had approximately the same tenn
,erature; and that as the surface after
ward cooled by outward radiation, there
was flow of heat to the surface by
from below. The rate of this
lev has diminished from that epoch to
he present time according to a definite
aw, and the present rate, being known
2y observation, affords a measure Of the
.ge of the crust. The strength of this
lies in its definiteness and
simplicity; its weakness in the fact that
t postulates Il knowledge of certain prop
rites of rock when subjected to tempera-
u1es far greater than have ever been
Investigated eXperimentally.
Another calculation is based on the as
sumption that loss Of heat from the sun
s accompanied with a loss OF volume
The sun, on this basis, is shrinking at