14 THE SCIENTIFIC MONTHLY
computation of the time rates of deposition and maximum thickness of strata from the base of the Cambrian upwards; but recent advances in our knowledge of the radioactive elements preclude the possibility of any trustworthy determination of the age of the elements through the methods suggested by Joly and Rutherford.
We thus return to the estimates based upon the time required for the deposition of stratified rocks as by far the most reliable, especially for our quest of the beginning of the life period, because erosion and sedimentation imply conditions of the earth, of the water, and of the atmosphere more or less comparable to those under which life is known to exist. These geologic estimates, which begin with that of John Phillips in 1860^ may be tabulated as follows:
Time required roR the Processes or Past Deposition and or Sedimentation AT Bates Similar to Those observed at the Present Dat^*
1860. John PhiUips 38- 96 miUion yean.
1890. De Lapparent 67- 90 miUion yean.
1893. Walcott 65-70 mmion years.
(27,640,000 years since the base of the Cambrian Paleozoic; 17,5000,000 years or upwards for the pre-Paleozoic.)
1899. Geikie 100-400 mimon yean.
(Minimum 100 million years; maximum — slowest known rates of deposition— 400 milUon years.)
1909. Sollas 34- 80 million yean.
(The larger estimate of 80 million years on the theory that pre-Paleozoic sediments took as much time as those from the base of the Cambrian upwards, allowing for gaps in th^ stratigraphic column.)
These estimates give a maximum of 64 miles as the total amount of sedimentation^ which is equivalent to a layer 2^300 feet thick over the entire face of the earth.*^ From these purely geologic data the time ratio of the entire life period is now calculated in terms of mil- lions of years^ assuming the approximate reliability of the geologic time scale. The actual amount of rock weathered and deposited was prob- ably far greater than that which has been preserved.
In general these estimates are broadly concordant with those reached by an entirely different method, namely, the amount of sodium chloride (common salt) contained in the ocean, to understand which we must take another glance at the primordial earth.
The lifeless primordial earth can best be imagined by looking at the lifeless surface of the moon, featured by volcanic action with little erosion or sedimentation. The surface of the earth, then, was chiefly
s« Becker, George F., 1910, pp. 2, 3, 5. «T CJlarke, F. W., 1916, p. 30.
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