872 J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



representative of all time, concluded that it was impossible to accept the 

 results based on the hypothesis of the uniformit}^ of radioactive disin- 

 tegration. He pointed out that this would imply that the present rate 

 of supply of sodium to the sea was from ten to fourteen times the mean 

 rate throughout past time. This seemed to him so impossible that to his 

 mind it was easier to think of some unknown factor modifying the rate 

 of atomic disintegration and resulting in a marked slowing of atomic 

 decay with increasing age of the uranium mineral. It has been sliown, 

 however, on geologic grouoids that there is good evidence for holding that 

 the present rate of denudation is ten or fifteen times the mean rate whicli 

 has existed since the opening of the Paleozoic. There is thus brouglit 

 about a general accordance between independent lines of evidence. 



There is seen to be even less justification for Becker's assumption that 

 the contribution of sodium from igneous rocks to the sea has been regu- 

 larly decreasing with the passage of time. So far as the stratigraphic 

 evidence goes, it may or may not have had a higher mean rate in the Pre- 

 cambrian. Areas of Archean rock were then, it is true, more widely 

 exposed, but the repeated baseleveling implies long periods when the 

 lands had been worn low and erosion was very sluggish. Through much 

 of the Paleozoic and Mesozoic, however, the evidence is clear that the 

 continents were widely covered with seas or their deposits, and that the 

 relatively small areas which supplied the waste were generally of low 

 relief. This paleogeographic interpretation is distinctly against the postu- 

 late of a regular decrease of new sodium through geologic time, especially 

 a decrease following a logarithmic curve. 



The question of the uniformity of rate of atomic disintegration, which 

 is the basis of all calculations on the age of radioactive minerals, has been 

 discussed and, although in the nature of things^ a uniformity of disin- 

 tegration of uranium through geological time can not be directly demon- 

 strated, the evidence from the measured rates of decay of its products 

 points strongly to a similar imiform rate for the parent element. Nothing 

 is known to support a hypothesis that there is an increase in stability of 

 unstable atoms by a process of aging. The temperature and pressure 

 limits, through which the uranium minerals have existed since their for- 

 mation, are not greater than those within which its more unstable prod- 

 ucts have been studied in the laboratory. The effects of the profound 

 temperatures and pressures of the earth's interior are, as yet, wholly 

 unknown, but the conditions which exist within the outer crust have been 

 duplicated by experiment. 



But may not these moderate ranges affect the rate of disintegration of 

 the more stable parent? Tliat can be tested by geological evidence. If 



