Genetics, paleontology, and evolution / edited by Glenn L. Jepsen, Ernst Mayr, George Gaylord Simpson, for the Committee on Common Problems of Genetics, Paleontology, and Systematics, of the National Research Council.

Date:
1949
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics, paleontology, and evolution / edited by Glenn L. Jepsen, Ernst Mayr, George Gaylord Simpson, for the Committee on Common Problems of Genetics, Paleontology, and Systematics, of the National Research Council. Source: Wellcome Collection.

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    time the mineral was formed; and on an unaltered mineral it gives three independent values for the age. A considerable number of determinations of the ages of minerals based on the lead method have now been made. To evaluate their reliability they can be divided into four classes: Class I, in which U, Th, and Pb have been determined by standard gravimetric chemical analyses and the isotopie composition of the lead has been determined by means of the mass spectrograph; Class II, in which U, Th, and Pb have been determined by standard gravimetric methods and the atomic weight of the lead has been determined chemically; Class III, in which U, Th, and Pb have been determined by standard gravimetric methods; and Class IV, in which a few milligrams of mineral have been analyzed for their U, Th, and Pb content by microanalytic methods. About 25 Class I determinations have so far been made, 21 of them by A. O. Nier (18) alone. They represent 18 localities scattered over the globe. The oldest indicate ages of about 2000 million years. As these minerals occur in pegmatites that cut older rocks, and these older rocks themselves record a long history, it follows that the age of the Earth is considerably more than 2000 million years. The youngest mineral determined by Class I measurements is a pitchblende, from Central City, Colorado; its absolute age is 58 million years; and the youngest minerals as determined by the lead method, Class III determinations, are a uraninite from Chihuahua, Mexico, which is 35 million years old, and brannerite, from Idaho, also 35 million years old. One of the tasks of geology is to build up an absolute time scale, to determine the length in years of the major divisions and of the subdivisions of the established geologic time scale. How long was the Ice Age? Was it 600,000 years, as some astronomers think, or 1,350,000 years, as other astronomers think, or 3 million years, as some geologists think? How long was the world's principal coal-forming period, the Carboniferous? And so on, for all the divisions of the geologic time scale. The importance of such absolute time measures to studies of the rates of organic evolution is very great. Two severe limitations have retarded the use of the lead method in building up the absolute geologic time-scale. First, the highly radioactive minerals, uraninite and other minerals rich in uranium and thorium, are restricted almost exclusively to pegmatites. Pegmatites are not common, however, and only a few of them carry radioactive minerals. Secondly, from their nature, most pegmatite masses cannot be accurately dated in the geologic time-scale. To illustrate, pegmatite dikes or veins occur at Branchville near Danbury, Conn., and others occur near Middletown, Conn. The radioactive evidence tells us that the Branchville pegmatite is 350 million years old, and those at Middletown are 260 million years old.