The Problem With Lead

In 1912 Imperial College offered Holmes a position as a demonstrator in geology, and in July 1914 the 23-year-old geologist married Margaret Howe. Holmes kept busy lecturing and researching the pet-rographical material he brought back from Mozambique. When World War I broke out in August, the military declared Holmes unfit for military service because of his recurring bouts of malaria. His contributions toward the war effort included making scaled topography maps for naval intelligence and researching alternative sources of potash, an ingredient of fertilizer formerly supplied to Great Britain by Germany.

To convince his contemporaries of the usefulness of radiometric dating, he composed The Age of the Earth (1913), a review of the historical methods for estimating ages of geological materials that also presented all the current related evidence and contrasted the results obtained by different techniques. In this book he pointed out problems with the other approaches and defended his own estimation of 1,600 million years based on uranium/lead measurements.

The possibility that "ordinary" lead, in existence since the formation of the Earth, was already present in the rock samples before any radioactive decaying took place was troublesome. Another difficulty in using lead measurements was that in addition to uranium, the radioactive element thorium also decayed to lead. To overcome this Robert Lawson, a friend from childhood who worked at the Radium Institute of Vienna, determined the atomic masses of the three lead isotopes, enabling one to adjust age calculations accordingly based on the proportions of each type.

Holmes thought he resolved a means to determine the age of rocks with accuracy, but he did not know yet that uranium also had another isotope. Uranium 238 makes up 99 percent of the total uranium, but uranium 235 decays at a faster rate, and Holmes unknowingly included its end product as part of the ordinary lead. This isotope hitch prevented skeptics from recognizing the promise of this new dating technique.

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