254 B. B. Boltwood — Ultimate Disintegration 



of their production, obtaining in this manner an approximate 

 knowledge of their relative ages. In dealing with the question 

 of radio-active change, where the element of time is such an 

 important factor in the solution of nearly every problem, the 

 advantages to be derived from the careful study of the radio- 

 active minerals can therefore scarcely be overestimated. 



From a knowledge of the chemical properties and the crys- 

 tallographic, optical and other physical properties of a given 

 mineral specimen, together with an understanding of its occur- 

 rence and of the other mineral substances with which it is found 

 associated, it is generally possible to definitely determine whether 

 the mineral was formed simultaneously with the mass of mate- 

 rial or geological formation in which it now occurs, or whether 

 it is of more recent production, having originated through the 

 action of percolating waters or of subterranean vapors or gases 

 on some original constituent. In the former case, when all 

 available data indicate that the formation of the mineral was 

 coincident with that of the mass of rock in which it occurs, the 

 mineral can be classed as primary ; in the latter case, when it 

 has apparently originated through the alteration of primary 

 compounds, it can be considered as secondary. The term 

 secondary can also be applied in a restricted sense to such 

 minerals as occur in veins, where the general character of the 

 vein indicates that it has originated through the formation of 

 fissures in existing strata and that the contents of the vein is 

 of an age inferior to that of the mass of rock by which it is 

 bounded. 



In applying these considerations to the greater number of 

 minerals which have up to this time been observed to contain 

 radio-active constituents, it may be considered as fortunate that 

 these minerals occur under conditions which would seem to 

 render the task of assigning the individual species to one or the 

 other of the above classes a relatively simple one. 



The most prominent radio-active mineral, uraninite, more com- 

 monly known as pitchblende, occurs both as a primary consti- 

 tuent of granitic rocks and also as a constituent of metalliferous 

 veins cutting geological formations of a relatively recent geologi- 

 cal period. When occurring in a granitic rock the uraninite is 

 frequently quite perfectly crystalline in form and the rock itself 

 is of the type called pegmatite ; the most noted localities fur- 

 nishing specimens of this primary uraninite being southern 

 Norway, particularly in the neighborhood of Moss, North 

 Carolina, Llano Co., Texas, and Connecticut. Prominent local- 

 ities where uraninite occurs as a constituent of metalliferous 

 veins are Johanngeorgenstadt, Marienberg and Schneeberg in 

 Saxony, Joachimsthal and Pribram in Bohemia, Cornwall in 

 England, and Colorado and South Dakota in the United States. 



