of the Radio-active Elements. 83 



the assumption that the lead and uranium are in a constant 

 ratio to one another in the thorianite from the Galle district. 





Table II. 





Sample No. 



uo 2 +uo 3 



PbO 



I 



32-7 



2-56 



II 



10-3 18-9 



2-29 



III 



28-2 



2-29 



IV 



28-7 



2-50 



V 



27 



2-99 



VI 



28-0 



2-90 



In the paper by Dunstan and Jones an analysis of a speci- 

 men of thorianite from the Balangoda district, showing U0 2 4- 

 U0 3 = 13'4 per cent and PbO = 2*54, suggests a close agree- 

 ment of the ratio of lead to uranium in this mineral with the 

 same ratio in the material from the Sabaragamuwa Province. 



The minerals given in the preceding table are all primary 

 minerals, in the general sense in which this term is used. In 

 the following table (Table III), the ratio of lead to uranium 

 has been calculated for some secondary minerals from the same 

 localities. 



Table III. 



Per Per Eatio 



No. Mineral Locality cent cent Pb Analysis by 



U Pb tj 



1. Uranophane, 



Mitchell Co., N. C, 55 0'56 0-01 Genth, Am. Ch. J., i, 



2. Uranophane, 88, 1879. 



Arendal, Nor., 40 1*6 0-04 Nordenskiold, G. For. 



3. Thorogummite, Forh., vii, 121, 1884. 

 Llano Co., Tex., 19 2-0 0-10 Hidden and Mackintosh, 



this Journal, xxxviii, 

 480, 1889. 



These analyses all agree in giving a lower ratio for the 

 secondary minerals than for the primary minerals from the 

 same localities. The most common alteration product of 

 uraninite known as gummite can be left out of present con- 

 sideration since lead is apparently one of its natural, chemical 

 constituents. 



The actual value of the ratio varies considerably for the 

 primary minerals from different localities, the maximum value 

 being about six times the minimum. It is beyond the writer's 

 province to discuss the data bearing on the geological ages of 

 the different deposits, but he is indebted to Professor Joseph 

 Barrell of Yale University for the statement that, so far as 

 the knowledge of the latter extends, the relative values of the 



