MEASUREMENTS BASED ON RADIOACTIVITY 851 



some quantities the amount is likely to vary from mineral to mineral, and 

 the lead ratios will lack that constancy which is the criterion of their 

 value. Moreover, the lead from such minerals is of two kinds — "ordi- 

 nary" lead and "uranium" lead — distinguishable by their atomic weights, 

 though not by chemical methods of analysis. If the lead is wholly original 

 its atomic weight should be about 207.1; if generated from uranium the 

 atomic weight should be about 306.2. Values between these figures imply 

 a mixture of the two types. The criteria pointing to the absenice of orig- 

 inal lead in perceptible quantities are (a) constancy of the lead ratios in 

 a series of fresh, primary minerals of the same geological age, and (/;) an 

 atomic weight value of the order 206.2. 



In cases where the lead ratios are not constant, or vvliere the atomic 

 weight is too high, the presence of original lead is to be suspected and 

 the ratios become worthless as an index of age. The thorium minerals 

 from Ceylon afford an instructive instance. ^^ 



We may now proceed to consider a number of analyses of minerals 

 which may fairly be used for the measurem^ent of geological time. In 

 Table B many of the analyses compiled by Boltwood in 1907 are quoted, 

 together with several new ones which have since become available. 



I. The uraninite of Glastonbury, Connecticut, United States of Amer- 

 ica, occurs in pegmatite dikes which are associated with a granite prob- 

 ably of late Carboniferous age. The granite intrudes Lower Carboniferous 

 strata and is certainly pre-Triassic. 



III. Unfortunately, uraninite does not occur in the Devonian igneous 

 rc^cks of the Christiania district of Norway. As has been already shown, 

 analyses of thorite are far from reliable as tests of age, and the writer 

 has felt reluctantly obliged to abandon them from that point of view. 

 The remaining analyses give ratios varying from 0.04 to 0.062, tTie dis- 

 crepancies being here due to the difficulty of accurately determining small 

 quantities of lead. The average age given in the table may be too bigh, 

 and must be regarded as indicating no more than an approximation to 

 the correct figure. It is significant that the somewhat older granites of 

 County Carlow give a maximum age of 470,000,000 years as determined 

 "by the pleochroic halo method. 



»3 Since the amount of uranium present is slowly decreasing as the helium and lead 

 accumulate, it is clear that the amount of uranium Uo originally in the mineral must 

 have been greater than the amount of U now present. For periods of time less than 

 2,000 million years, the average amount of uranium present in a mineral throughout its 

 history — the time average — is almost exactly (Uo + U)/2. Now Uo is given by U + Pb 

 I He or by U + 1.15 Pb. Consequently (Uo + U)/2 = U + 0.575 Pb, and a more ac- 



' Pb 



curate expression of the age of a mineral than that given above is ^ ^ q 575 p^ ^ 8.000 



miiiinn vpn rq ( ^ — ^r" X 7,500 million years. — J. B. ) 



million years. \^ ^ + 0.575 Pb / 



