This section contains data on the abundances of uranium, thorium, and potassium in geological materials. These elements are singled out for special attention because their radioactivity gives them unique importance to the thermal budget of the earth and the other terrestrial planets. The concentrations given here can be converted to rates of heat production by use of the data in Table 23-7. Uranium and thorium are trace elements; their concentrations rarely exceed a few tens of parts per million. Potassium, on the other hand, is a major element in many rocks, and its concentration is given in any complete chemical analysis. This difference in abundance, by a factor of the order of 104, has meant that commonly potassium has been determined by different methods and for different purposes than have uranium and thorium. Determinations of all three elements on the same sample are still regrettably rare. Because of this, it proves desirable to tabulate potassium separately in many cases. Analyses of the concentrations of these elements in rocks have been made for many years, but it is now clear that most of the older determinations led to results that were too large, sometimes by orders of magnitude. The errors were most serious in materials where the concentrations are low, such as ultramafic rocks and meteorites. The presence of systematic errors in old uranium and thorium analyses has been known for some time, but it is only recently that errors in potassium values have been demonstrated. The latter errors