The Age of the RocksAuthor(s): Arthur HolmesSource: The Scientific Monthly, Vol. 35, No. 2 (Aug., 1932), pp. 107-108Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/15275 .

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THE TIME SCALE 107

straight profile and jutting chin at 11.08 and 34 seconds P. m., or thereabouts (mid-Pleistocene times). It was nearly 11.59 before he began to tame wild animals, to plant, to hoe the garden, and to become domesticated.

I think that at the end of the Sec- ondary Era, the earth was becoming tired of reptiles, irritated with their scales, and fed up with their eggs- especially, perhaps, dinosaur eggs. Then came the lively lemurs, the pop-eyed, hopping tarsiers, the amusing monkey tree-dwellers, and the mighty brachiat-

ing anthropoids. One of these last, en- dowed with a better brain and with a super-primate initiative, sought a richer and more abundant life upon the ground. From hiim, in the fulness of time, there developed man, with his stable foot, his sensitive prehensile hand, and his great receptive brain. Some men are born to sit aloft in observatories investigating the fulness of the universe. Others are born to sit on the top of flagpoles, con- templating the emptiness of their stomachs. None need be ashamed to have sprung from an ape.

THE MEASUREMENT OF GEOLOGICAL TIME By Dr. ALFRED C. LANE

PBARSON PROFESSOR OF GEOLOGY AND MINERALOGY, TUFTS COLLEGE

(Abstract) GEOLOGY and astronomy have common

interests. Astronomy depends for its observation of great distances on a base line-the diameter of the orbit of the earth. The knowledge of this quantity is based on the size of the earth, which, in turni, rests on some small base line de- termined by the geodetic survey. The astronomer 's light year rests on measure- ments of a short distance on our earth.

In the measurement of time the same is true. Measnrements of great periods of time are founded on a base line of short duration. The Harvard collection of plates, which this building has beeni constrnieted to, house, reached further

back in time than any other. De Sitter once said: " Theories change every month, but a good observation lasts."'

The geologist, like the astronomer, measures a great period of time by ob- servations made over a very short time. A group of 50,000 observations has just been completed to determine the exact speed of the decay of radium. Helium content has been measured by Paneth and Urry in quantities as small as one hundredth of a cubic centimeter per ton, in rocks and meteorites. Such measures give us a time scale, and help both geologists and astronomers to fix events in the past and in the future from a relatively short base line.

THE AGE OF THE ROCKS By Professor ARTHUR HOLMES

DURHAM UNIVERSITY

(Abstract) THE elements uranium and thorium

may be regarded as natural clocks, tick- ing out helium at regular intervals, and providing, in their lead content, a record

of the amount of, disintegration that has gone on, and hence,of the age of the cor- responding strata.

Determinations of lead content have led to estimates of sixty million years

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108 THE SCIENTIFIC MONTHLY

for certain Tertiary rocks, and as much as 1,460 million years for some Pre- Cambrian rocks. More recently it has been found that pitchblende from Great Bear Lake has also an age of 1,460 mil- lion years, and that a specimen from Manitoba is 1,800 million years old. It is noteworthy that the three greatest recorded ages occur in North America, at points lying roughly on a north-and- south line.

From these results the lower limit of 1,800 to 2,000 million years may be given for the age of the rocks; and the. earth must be at least as old as this.

The new time scale makes it possible to give the approximate lengths of su - cessive geological periods. It also en- ables us to correlate past happenings on different parts of the surface of the globe, and a beginning has been made in piecing together a chronological his- tory of the past.

In addition to estimates based on the amount of lead left from radioactive dis- integration (a method inapplicable to

rocks that have a high lead content of their own), the method of estimating the age of rocks by the amount of oceluded helium (the remains of products of radioactive disintegration) has also wide applications. As some of the helium is, bound to be lost, this method always gives lower values for the age than the method based on the lead ratio.

The helium method is applicable to basaltic rocks, and has, for instance, been applied to specimens from the geolo- gically and historically interesting Whin Sill, in England, and to the Tertiary dykes associated with the volcano of Mull, in Scotland. The ages derived from analyses of the helium content of igneous rocks range from 28 million to 500 million years.

The work just referred to is still in its initial stages. It is hoped that the results of further researches of the same kind will lead to important knowledge in the chronology of the earth's surface, with their bearing on the age of the sur- face of the earth, and of the earth itself.

STABILITY OF THE SOLAR SYSTEM By Dr. ERNEST W. B3ROWN

PROFESSOR OP MATHEMATICS, YALE UNIVERSITY

(Abstract) AT the end of the eighteenth century,

the French astronomer Laplace proved that a system of planets moving around the sun in definite orbits might suffer many serious disturbances, but still would always hold together. Laplace's statement involved certain approxima- tions which, though legitimate at the time, can now no longer be accepted. Geologists place the birth of our solar system about two thousand million years back, and the approximations of Laplace are justifiable over a period of fifty mil- lion years at the very best.

The observational problem of tracing

back the paths of the planets over such enormous intervals of time is very diffi- cult. It can be proved definitely that the present observational uncertainties in the planetary orbits are so large that the astronomer can neither predict where a planet will be two thousand million. years hence, nor trace back the planet's orbit with any degree of accuracy over such an interval of time. Attempts have been made to discuss the stability by general mathematical methods, but since it is impossible to take all necessary fac- tors into account, one can have little con- fidence in the result obtained from an application of these methods to our solar system.

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