DIASTROPHISM AND THE FORMATIVE PROCESSES 397 
the beginning of the Paleozoic, from one hundred million to four 
hundred million years; (b) for that since the beginning of the 
Proterozoic, from three hundred million to twelve hundred million 
years; (c) for that since the earliest Archeozoic whose age has been 
estimated, from four hundred million to sixteen hundred million 
years.* 
12. The time occupied in the evolution of terrestrial life is 
regarded as one of the most dependable evidences of the earth’s 
age, though its testimony is of a rather general nature. Since 
the evolution from the early Paleozoic to the present is confessedly 
only a small part of the whole evolution, it was taken as 1/10 
in the computations.” 
13. Combining biologic, geologic, and radioactive estimates, 
the total period of life-evolution is taken roundly as lying between 
one billion and four billion years. 
14. The length of the period during which the rate of planetesi- 
mal infall was compatible with life, previous to the earliest deter- 
mined Archean, is thus made to range between six hundred million 
and twenty-four hundred million years.4 
15. Making allowance for the formative stage that precedéd 
life-evolution, the whole age of the earth is taken tentatively as 
falling somewhere between three billion and five billion years. 
All these estimates are of course only intended to serve working 
purposes in the light of the latest evidences; the whole matter is 
to be kept sub judice awaiting further light. 
CONSIDERATIONS ADVERSE TO HIGH ESTIMATES OF DIASTROPHISM 
16. Great thicknesses of shallow-water sediments do not 
necessarily imply great sinking of the crust. Measured in the 
usual way, thicknesses much greater than any observed may be 
laid down in the normal process of continental outgrowth with- 
out necessarily involving any crustal sinking at all. Very thick 
1 “The Bearings of the Size and Rate of Infall of Planetesimals on the Molten or 
Solid State of the Earth,” Article XIII, Jour. Geol., Vol. XXVIII (1920), pp. 675-77. 
2 Tbid., p. 675. 3 [bid., p. 676. 4 Ibid. s 
5 “Foreset Beds and Slope Deposits,” Article VI, Jour. Geo_= Vol. XXII (1914), 
pp. 271-72. 
