250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



they were molten. Helium derived from the atmosphere can be 

 recognized by the neon which goes with it. The heUum might, of 

 course, diffuse. But when the hehum is present in such small quan- 

 tities as a few cubic centimeters per ton of rock, and especially as it is 

 right in the network of a crystalline structure, it does not seem to lose 

 enough to prevent determination of relative age, if work is done on 

 fine-grained basaltic specimens taken fresh and compact, and from a 

 considerable distance below the surface. Results on granitic rocks are 

 unaccountably low. 



On the other hand, in radioactive minerals containing a consider- 

 able amount of a radioactive element, the lead ratio is the more signifi- 

 cant, and, if there is considerably more helium in the mineral than in 

 the surroundings, the hehum diffuses. 



The hehum methods are then likely to give ages that are too low, 

 and the ratio of lead to the uranium and thorium, if the ordinary lead 

 has not been determined, ages that are too high. Insofar as they 

 check, we can be reasonably sure of not being far off, except for a pos- 

 sibility that the rate of change is markedly different now from what it 

 has been in the past. In the case of the Silver Leaf Mine-Huron 

 Claim region, the age by the Sr (87) : Rb (87) ratio also checks within 

 10 percent. 



For instance, a number of ages of Triassic rocks later than the Car- 

 boniferous have uniformly given ages by the helium method less than 

 200 million years. On the other hand, there are over a dozen analyses 

 of minerals in pegmatites intruded before the Appalachian uplift, and 

 certainly not later than the Carboniferous, which would indicate ages 

 between 200 and 300 million years. Thus, the probability is very 

 great that 200 to 250 million years ago some at least of the Carbon- 

 iferous coals were being formed. 



If a mineral or a rock contains a number of elements disintegrating at 

 known rates to different products, we have an excellent check, pro- 

 vided we can distinguish them and their products. Indeed, if we 

 know the proportions of the products and can be sure their production 

 began at the same time (as will be true, for instance, for the various 

 leads produced by the isotopes of which uranium is composed, of 

 which the most abundant have atomic weights 206 and 207) we can 

 infer the ages, since the lead from older minerals will have a larger 

 proportion of the products of the more rapidly disappearing isotopes. 



Prof. A. von Grosse ^* has especially emphasized this, and John L, 

 Rose ^^ has computed several ages from the relative strength of the 

 spectral lines of the different leads. Yet, as ordinary lead is a mixture 

 of the same isotopes, the isotope that comes from thorimn Pb (208) 

 and a small quantity of an isotope Pb (204), unless we can find how 



»• Von Orosse, A., Journ. Phys. Chem., vol. 38, pp. 87-494, 1934; al.so paper before the Amer. Chem. Soc., 

 Rochester, September 1937. 

 » Rose, John L., Phys. Rev., vol. 50, pp. 792-796, November 1938. 



