The next important advance was contributed in 1946 by Arthur Holmes in England and by F. G. Houtermans in Germany. Each of these scientists had seen Nier’s reports before the war and had realized that Nier’s mass-spectrometer analyses of lead made it possible, for the first time, to make rational calculations about the age of the earth. The two scientists independently calculated that age at about 2 to 3 billion years, using the handful of data available to them from Nier’s measurements. It is interesting that today, thousands of analyses later, our planet’s age usually is given as 4.5 billion years. The early estimates were not far off. Development of various methods for measuring the age of minerals followed rapidly, and by 1955 many fundamental studies needed for measuring the age of very old substances were complete. The basic techniques are summarized in Table I. They will be explained later. The new methods produced broad confirmation of the early rough estimates and they also brought a few surprises. Before we go into these discoveries, let us look at some theoretical foundations. Table I BASIC MEASUREMENT METHODS Method Material Time Dated Useful Time Span (years) Carbon-14 Wood, peat, When plant died 1000-50,000 charcoal Bone, shell Slightly before 2000-35,000 animal died Potassium-argon Mica, some When rock last 100,000 and up whole rocks cooled to about 300°C Hornblende When rock last 10,000,000 and up Sanidine cooled to about 500°C Rubidium-strontium Mica When rock last 5,000,000 and up cooled to about 300°C Potash When rock last 50,000,000 and up feldspar cooled to about 500°C Whole rock Time of 100,000,000 and up separation of the rock as a closed unit Uranium-lead Zircon When crystals 200,000,000 and up formed Uranium-238 Many When rock last 100-1,000,000,000 fission cooled (Depending on material) [Illustration: _Time scale of the Earth, drawn to scale. See also the Holmes time scale on page 46._]