Evening star., 1936-01-11

Virginia has rocks 8oo.ooOooO years
old-the oldest yet found IN Eastern
North America.
The age of a mineral specimen COL
lected so years ago in Amherst County,
as determined by chemical analysis in
the laboratory of the United States Geo.
logical Survey and checked by another
analysis at the University Of Vienna, has
just been announced by the National Re
search Councils Committee on the
Measurement of Geologic Time.
The rock is estimated to date back to
the middle pre-Cambrian age of the
geologists-about 300 100000 years before
the first life on earth which left any
fossil traces. Over the bare rocks of this
part of Virginia at that period there may
have crawled very low order of worm
like creatures whose soft bodies entirely
disappeared.
The particular specimen from which
this estimate was made was sample
of the mineral known as allanite-a
blackish, glaSSlike rock composed chiefly
of silica, but which also contains slightly
more than one part in a hundred of
thorium, one Of the chief radioactive
elements. Thorium, by the process of
radioactive diSintegration, breaks down
at a definite rate into a peculiar form
Of lead Since the rate of this change is
known, the age of a thorium- bearing
rock can be determined by the ratio of
the amount Of this lead to the amount
Of the parent material. The process IS
very slow, so that this method of time
measurement is hardly applicable unless
the process has been in operation for at
least four or five million years.
The chemical problem of analyZing
atlantic is extremely difficult and the
task was undertaken for the committee
only because of the extreme scarcity of
any kind of radioactive minerals in any
part of the earth. The amount of tho
rum is extremely small in the first place.
In addition, there is a bare trace of
uranium, the beSt-known of all the radio
active elements which breaks down into
another kind of lead, and small amount
of common lead which does not result
from the disintegration process. The
work was done by DR.. John P. Marble,
research associate of the committee, at
the Geological Survey laboratories. The
specimen was one collected in the late
70s for Prof. j. w. Mallet of the Uni
versity of Virginia and presented by him
to the United States National Museum.
A fresh specimen recently has been COL
lected from the same locality. This will
be analyzed as check on the correctness
of the lead ratio found in the National
Museum sample.
A part of the same sample was analyzed
by another method, considered more
difficult with this particular mineral, by
experts at the University of Vienna as a
check on the work in Washington. The
age determinations turned out to be
essentially the same, within the limits
Of experimental error.
Few rock samples are available repre-
senting this particular period in the
earth's history. Very much the same
lead ratios, and hence the same age,
have been found in the analysis of rocks
from the Tanganyika Territory of East
Africa and from Ceylon. Specimens Of
similar age also have been found near
Easton, Pa. Most of the strata in Eastern
North America, however, are little more
than half as old, representing periods
when life already was well advanced on
earth.
The Amherst County atlantic dates
from the earth's middle ages"
About as great span of time separated
it from the actual beginning Of things
so far as this can be calculated by the
lead ratio method-as from the present.
The world had been in existence for at
least billion years.
Even for North America It Is not truly
ancient. compared With some other