Borehole gravimetric measurements were made at major Phanerozoic formation contacts and within the Precambrian sedimentary rocks to a depth of 4 km in the McClure-Sparks No. 1-8 drill hole. The inplace densities of these units were calculated from the corrected observations. The densities of the Phanerozoic rocks vary considerably owing to the presence of evaporites contrasting with the elastic and carbonate rocks, and the mean density of the Phanerozoic rock column is 2.61 g/cm3. The borehole gravimeter density of Precambrian elastic rocks is 2.77 g/cm3, 0.05 g/cm3 greater than the density obtained from sample cuttings. The source of this discrepancy is unknown, but one possible explanation is the gravimetric effect of mafic igneous rocks, which may be more common within the Precambrian sedimentary rocks than is indicated by intersection of these rocks in the drill hole.