874 J. BARRELL MEASUREMENTS OF GlJOLOGlC TIME 



of this evidence the assumption would appear to be justified that lead is the 

 final product of uranium." ^^^ 



As has been noted, Becker argued that the sequence of geological age 

 was in certain examples directly opposed to Boltwood's conclusions, but 

 in a preceding section it has been shown that Becker^s conclusions were 

 based on a series of errors. 



With an increasing recognition of the importance of the radioactive 

 methods, it is to be hoped that the problem will be directly attacked, and 

 from tiie standpoint of geologic science, rather than left almost wholly as 

 incidental investigations of tbe radio-chemist. Minerals high in uranium 

 are, however, rare. Those with only accessory uranium are seen to be 

 relatively unreliable because of the small amount of uranium, the still 

 smaller amount of uranium lead, and the large changes in the ratio which 

 may result from analytical errors, original lead, temporary thorium lead, 

 or other causes. 



It would seem, however, that the ratio of helium to uranium and 

 thorium might be made of high supplemental value in a manner which, 

 so far as the writer is aware, has not been used.. Holmes has pointed out 

 that the helium content of uranium and thorium minerals is always less 

 than the amount of helium which has been generated, because of the 

 leakage of the gas. Nevertheless, in fresh rocks which contain such min- 

 erals, no matter what their age may be, there is reason to believe that 

 the helium is mostly present, though in part outside of the parent mineral. 

 It is known that fresh unbroken ■ rock is practically impermeable, as is 

 shown by the storage of carbon dioxide under high pressure in micro- 

 scopic vesicles. The very freshness of the rock proves, furthermore, the 

 lack of permeation of surface waters. The helium generated in the small 

 minerals, such as zircon, will be in part ejected by the initial velocity of 

 the atom ; in part it will slowly diffuse into the adjacent minerals, owing 

 to the accumulating excess of vapor pressure within the mineral. In an 

 uncleaved rock it will, however, practically all remain within a very 

 limited distance. In those parts of a rock-mass where uranium and 

 thorium are concentrated in appreciable quantity, the amount of new 

 helium would be large in comparison with original helium. In a rock 

 containing zircon, biotite, minerals of the rare earths, or other minerals 

 holding uranium or thorium, if a cube several centimeters in diameter 

 were cut from the fresh rock and the whole subjected to analysis, the 

 amounts of uranium, tborinm, helium, and lead might be ascertained by 



13*^ B. B. Boltwood : On the ultimate disintegration products of the radioactive elements. 

 Part II. — The disintegration products of uranium. Am. Jour. Sci., vol. xxiii, 1907, pp. 

 83, 84. 



