Analyses (1) and (2) of early Tertiary pitchblende from Gilpin County, Colorado; (3) of early Precambrian monazite from the Huron Claim near Winnipeg, Manitoba; (4) of a monazite from near Las Vegas, New Mexico; (5) of a euxenite from Trout Creek Pass, Colorado, are reported with lead age ratios as follows: (1) 0.0146; (2) 0.0167; (3) 0.257; (4) 0.106; (5) 0.059.

The first four were executed with the help of a grant from the Elizabeth Thompson Science Fund, and the proportion of isotopes in the lead have been determined by A. O. Nier. The fifth was helped by the Penrose Bequest of The Geological Society of America. Some of the figures given in this paper have already been incorporated in the Report of the Committee on the Measurement of Geologic Time (1940).

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