Lead concentrated in ore deposits is called common lead or ore-lead. Its standard atomic weight (1936) is 207.21. The lead dispersed through rocks in minute quantities is called rock-lead. Rock-lead consists partly of common lead, which was originally present in the rock-material; and partly of radiogenic lead, which has been generated in the same rock-material as a result of the radioactive disintegration of uranium and thorium during geological time, the duration of which is now taken to be about 2,000 million years. From the available determinations of lead, uranium, and thorium in various rocks it is shown: (a) that the average atomic weight of granitic rock-lead should have progressively decreased during geological time from 207.21 at the beginning to 207.14 at the present day; (b) that the average atomic weight of basaltic rock-lead should have similarly decreased from 207.21 to 207.10. The atomic weight of rock-lead concentrated in the sublimate mineral, cotunnite, from the 1906 magma of Vesuvius, is 207.05, a result which confirms the inference that rock-lead has an atomic weight significantly lower than that of common lead. The atomic weight of ore-lead, however, is found to be 207.21 + or - .01, and--as far back as 1,300 million years--to be independent of the geological age of the ore. It follows from the evidence presented that ore-lead has not been derived from granitic or basaltic rocks, or from the sediments formed from such rocks, and that it has no genetic connection with acid or basic magmas. Ore-lead must therefore have ascended from depths below the sialic and basaltic layers of the lithosphere. Gregory's hypothesis that "the source of the ores appears to lie in a zone deeper than that of the ordinary igneous rocks" is thus largely confirmed, so far as lead ores are concerned. The data for peridotites, however, are too few as yet to justify the extension of this important generalisation to ultrabasic rocks and magmas.