Age determinations were made of three monazite samples from carbonate rock and four zircon samples from the associated shonkinite of the Mountain Pass district, San Bernardino County, California. The determinations made by the alpha-counting and total-lead method gave an average age of 950 million years for monazite and 840 million years for zircon. One of the monazite samples contains 2.65 per cent thorium, 0.002 per cent uranium, and 0.1130 per cent lead, giving a maximum age of 955 million years uncorrected for original lead. The Pb208/Th232 age of this monazite is 925 million years, based on an isotopic analysis of the monazite lead corrected for original lead by an isotopic analysis of galena lead from the same deposit. The monazite lead is 98.854 per cent radiogenic of which 98.342 per cent is Pb208 and 0.512 per cent is Pb206. The isotopic composition of the galena lead is similar to that of Precambrian galena from other localities.
Optical properties of the monazite are: α = 1.779, β = 1.781, γ = 1.833, 2V (calc.) = 26° (+), r < υ, Z Λ c = 7°, and Y = b. The density measured on the Berman microbalance is 4.98; the density calculated from the rule of Gladstone and Dale, n – 1/d = K, is 5.03. Other data include spectrographic and chemical analyses of the monazite.