A result that arrives, by way of atomic physics, at the same conclusion, reached by the broken ladder of geologic descent, radiates its own implications. Various and many minerals have been cousined in this search. The lead ratios of Europe Asia, North America, and Gonwanaland have been examined as well as the accumulation of helium in minerals and meteorites. The end of this labor leads to a statement that carries us back one step further in our conception of the earth's age. "No more definite statement can therefore be made at present than that the age of the earth exceeds 1,46o million years, is probably not less than 1,6oo million years, and is probably much less than 3,ooo million years."
Part V, by Ernest W. Brown, brings to bear the astronomical data which can be mustered. Celestial mechanics, however, do not help a very great deal. A review of the earth's eccentricities and inclinations does not contribute much to the discussion. The retardation of rotation by the sun and the moon may have had considerable effect on the lengthening of the earth-day but how to convert this into a measure of time is still out of reach. "The age of the earth is probably of the order of magnitude stated, but that on this evidence (celestial mechanics) it may be one third to three times this age." HOWARD W. ELKINTON.