Evening star., 1952-05-07
Predown Age Of Life
Close to Total Darkness
Conodiun Scientist Reports
On Pre- -Cumbriun Era

Canadian Scientist Reports
On Pre- -Combriun Era
By Thomas R. Henry
Darkness Of prelife covers five-sixths
OF earth's history.
The period from the beginning Of this
planet. now generally estimated at about
three billion years ago. to the dawn age
of life approximately 500 million years
ago is in what comes very close to total
darkness.
There were no living things to keep a
record Of the sweep of time by dying and
leaving imprints of their bodies in the
soft sea ooze to be interpreted by pale
ontologists of the future. 1t is possible.
however. that something may be deter
mined now about this vast, apparently
eventless pre-Cambrian geological era,
according to a report presented to the
American Geophysical Union, whose an
nual meetings end here today, by Dr. j.
Tuzo Wilson of the University of To-
ronto.
1t has been customary in the past to
divide the pre- e-Cambrian rocks into two
main subdivisions. First and the briefer
~.
was the period during which, according
to the evidence of the rocks, there prob
ably was life of some sort on earth
which left no actual remains. There
are tracks in rocks which might have
been made by crawling things in the
ancient sea slime. There are deposits
of iron of type impossible without Ger
tain iron-secreting plants Second is
hon fn hn hn
ginning, during which there is no evil
dence Of life whatsoever.
Oldest Known Rocks.
Probably the largest and most impor-
tant area of pref -Cambian rocks is that
of the so-called Canadian Shield. It
covers most of Northern Canada east Of
the Rockies and extends into the Great
Lakes region of the United States. Con-
tanning what long has been recognized
as the Oldest known rocks on earth, it
has been intensively studied for at least
q generation.
Now with more precise age-determin-
ing methods, DR.. Wilson reports to the
GeophySical Union, the picture appears
far from as simple as that accepted in
the past. The recent studies show, he
says, that the Archean rocks of the
shield can be divided into provinces
which are distinct from each other
both Structurally and in types of stone.
Each, he believes, may be the root Of q
former mountain system. In the pre-
Cambrian, apparently. there were
periods of mountain building, character
lied by great activity of the earth's
crust, just as such periods are known to
have been in the ost-Cambrian.
1t seems possible, according to DR..
Wilson's report. to isolate at least one
of these periods. The oldest rocks in the
Canadian Shield are so-called green
stones These are also the oldest rocks
in Australia and South Africa. They
are of volcanic origin. They usually
contain considerable gold but very little
limestone. ALI evidence of radioactive
decay gives them an age of about bill
lion years. ..,
Determination of Eras.
It can be assumed, DR.. Wilson holds,
i that this green-stone complex through
out the world represents one very wide
spread spasm Of mountain building
which can be considered distinct divi-
: sion Of pre-Cambrian time. Other erss
can be determined by more intensive
study of the ancient rocks, just as they
now are determined for post-Cambrian
time by the kinds of fossils the rocks
contain. ........-. ..>....-,. ..
The assumption perhaps may be
made, Dr. Wilson holds, that when most
of the rocks of a certain re-Cambrian
region are structurally and mineralog-
ically the same and are shown by radio
element analysis to be of about the same
age, they represent distinct period Of
mountain building.
He says: 1.-THE only natural way to
subdivide pre-Cambrian time is by
means of the belts of mountains by
which the continents have grown. In
the Canadian Shield these correspond
to geological provinces. These are not
universal divisions of time like the fos-
sill stages. But in spite of this indefi-
niteneSs it seems better to suggest that
we no longer try to divide pref -Cambrian
time on the basis of universal divi-
sion between young Proterozoic and old
Archean and face the fact that we have
as yet no good universal time scale for
the pre-Cambrian. The existence and
possible significance of belts or prov
inces in the continents each formed
within definite period of time should
also be accepted.
Comparison on Time.
"These views will seem less radical II
we recall that pre-Cambrian time is
from three to six times as long as all
subsequent time, and If we realize that
to grow continents it is only necessary
to produce and rework into sediments
less than one cubic mile of rocks each
year. Volcanic rocks are probably being
produced at that rate today Erosion and
redepoSition of sediments is certainly
proceeding at that rate.
"The idea already has been advanced
that the atmosphere and the oceans
have grown during geologic time. Per-
haps the easiest explanation of the rid
die of the continents is to suggest that
they have grown in stu by the rework-
ing of volcanic material produced from
the depths by the same processes that
we see in operation today, and at about
the same rates. According to this view,
the earth started with a basic crust
without air oceans mr. cOnfipnpts opd