248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



3,000 and 5,000 feet.^ The variation in geothermal gradient as a 

 possible index of oil or gas has been studied by Van Orstrand and work- 

 ers with K. C. Heald for the American Petroleum Institute. 



The difficulties in obtaining age results from temperature gradients 

 are to some extent the same as those inherent in all progressive proc- 

 esses — uncertainty as to initial conditions and as to how far results 

 have been affected by other factors. The change of rate of activity 

 in time is also uncertain, but usually we can know in what direction 

 it is. We also know Uttle, accurately, as to the diffusivity of rocks as 

 they lie in the ground, saturated with water fresh or salt, or dry and 

 containing only oil or gas. 



Minor methods of estimating geologic time, the accumulation of 

 meteorite material, of gases in the atmosphere, organic changes, 

 variation in the position of the pole, and others of the 40-odd methods 

 which geologists have tried, which arc occasionally useful, may bo 

 passed over to come to those which seem the most important and 

 widely useful — "popcorn" methods of measuring geologic time by 

 atomic decay and the accumulation of the products thereof. 



GEOLOGIC TIME RECKONED FROM ATOMIC DISINTEGRATIONS 



If one watched a popcorn machine and noted the number of 

 kernels popped in a second, and noted how much was popped, one 

 could figure how long it had been nmning. If, after a few minutes in 

 the dark, one looks at a luminous dial or hands of a watch with a 

 pocket lens, one can generally see that the luminosity is not quiet, but 

 is due to a shower of sparks, each of which represents an exploding 

 atom. While in detail these explode irregularly, yet ordinarily in any 

 radioactive element there are many flashes per second per milligram, 

 and the number per hour is practically constant and characteristic 

 of the element, regardless of any variation of pressure, temperature, 

 or electric charge that is likely to occur naturally in the earth's crust. 



The study of these explosions has been the subject of many physical 

 investigations. Three rays are said to be emitted by the explosion, 

 none of them visible but causing luminosity in certain compounds. 

 The alpha ray is really a nucleus of the gas helium, and the residual 

 atom has its atomic weight lowered by 4. The Geiger counter is an 

 electric device for counting them, a method much more accurate than 

 visually counting them in a scintilloscope. A recent counting study 

 is that of Robley D. Evans and G. D. Finney.^^ The counting has been 

 continued by C. Goodman. The physical difficulties and the uncer- 



» See Recent geothermal measurements in the Michigan copper district, by James Fisher, L. R. Ingersoll' 

 and Harry Vivian, with discussion by A. C. Lane, P. Weaver, and L. Gilchrist, Trans. Amer. Inst. Min. 

 Met. Eng., vol. 110, Geophysical prospecting, pp. 628-635, 1934, and the papers referred to in Lane's 

 discussion. 



«> Finney, O. D., and Evans, R. D., The radioactivity of solids determined by alpha-ray counting. Phys. 

 Rev., vol. 48, pp. 603-611, Sept. 16, 1935. 



