MEASUKEMENTS BASED ON RADIOACTIVITY 847 



the pegmatite dikes of that island. During all that time its store of 

 helimn has been leaking awa}^^ and present measurements give only a 

 minimum estimate of its age. 



In the course of his work Strutt investigated phosphatic nodules and 

 iron ores from sedimentary rocks, and zircons and sphenes from igneous 

 rocks, the two latter being among the most radioactive of the commoner 

 rock-forming minerals. The results for iron ores and zircons are the 

 most valuable and instructive, for they represent widely different periods, 

 and they show that, in spite of the unavoidable leakage, the older minerals 

 contain far more helium than the younger ones. 



In Table A some of the highest of the ages based on Strutt's results 

 are given for each of the geological periods represented. It will be seen 

 that with few exceptions they stand in close relation to the geological ages 

 of the minerals. For comparison, the corresponding ages based on the 

 accumulation of lead are given in parentheses in the cases where these 

 have been measured. The figures clearly bring out the limitations of the 

 helium method. x\ll that it can tell us is that the age of a mineral is 

 greater than a certain minimum value. 



PLEOCHROIC HALOS"*^ 



The presence of uranium and thoriimi in rocks is sometimes revealed in 

 a most beautiful way. When mica and tourmaline and a few other min- 

 erals are examined in thin sections under the microscope, small circular 

 spots, known as pleochroic halos, are sometimes seen. At the center a 

 minute crystal of zircon, or of some other radioactive mineral, can usually 

 be detected. In 1907 Joly demonstrated beyond doubt that these intensely 

 pleochroic spots were due to the radioactivity of the tiny inclusions at 

 their centers. -^ 



The a-rays, or helium atoms, discharged from the different radio- 

 elements have not all the same velocity. Those from uranium can pene- 

 trate about an inch of air, and they give up most of their electrical charge 

 just before coming to rest. Since they must be discharged equally in all 

 directions, they come to occupy a spherical surface around the central 

 uranium. The helium atoms from radium travel more rapidly and thus 

 penetrate farther, forming another spherical surface. Those from radium 

 C have the maximum range and form a sphere which incloses all the 

 others. The ranges of the helium atoms from the thorium family are of 

 the same order, but slightly greater than those from the uranium family, 

 and consequently the resultant spheres are of somewhat greater diameter. 



The distance to which a helium atom can penetrate depends on the 



»3By Arthur Holmes. 



LXIII — Bull. Geol. Soc. Am., Vol. 28, 1916 



