URANIUM AND GEOLOGY JOLY. 357 



with its distinguished author was permitted to me. This address will 

 be concerned with the application of my results to questions of geo- 

 logical dynamics. 



Did time permit I would, indeed, like to dwell for a little on the 

 practical aspect of measurements as yet so little used or understood ; 

 for the difficulties to be overcome are considerable and the precau- 

 tions to be taken many. The quantities dealt with are astoundingly 

 minute, and to extract with completeness a total of a few million 

 millionths of a cubic millimeter of the radio-active gas — the emana- 

 tion — from perhaps half a liter or more of a solution rich in dis- 

 solved substances can not be regarded as an operation exempt from 

 possibility of error ; and errors of deficiency are accordingly frequently 

 met with. 



Special difficulties, too, arise when dealing with certain classes of 

 rocks. For in some rocks the radium is not uniformly diffused, but 

 is concentrated in radio-active substances. We are in these cases 

 assailed with all the troubles which beset the assayer of gold who is 

 at a loss to determine the average yield of a rock wherein the ore is 

 sporadically distributed. In the case of radium determinations this 

 difficulty may be so much the more intensified as the isolated quanti- 

 ties involved are the more minute and yet the more potent to affect 

 the result of any one experiment. There is here a source of discrej)- 

 ancy in successive experiments upon those rocks in which, from 

 metamorphic or other actions, a segregation of the uranium has taken 

 place. With such rocks the divergences between successive results 

 are often considerable, and only by multiplying the number of experi- 

 ments can we hope to obtain fair indications of the average radio- 

 activitj^ It is noteworthy that these variations do not, so far as my 

 observations extend, present themselves when we deal with a recent 

 marine sediment or with certain unaltered deposits wherein there has 

 been no readjustment of the original fine state of subdivision, and 

 even distribution, which attended the precipitation of the uranium in 

 the process of sedimentation. 



But the difficulties attending the estimation of radium in rocks and 

 other materials leave still a large balance of certainty — so far as the 

 word is allowable when applied to the ever-widening views of sci- 

 ence — upon which to base our deductions. The emanation of radium 

 is most characteristic in behavior; knowledge of its peculiarities en- 

 ables us to distinguish its presence in the electroscope, not only from 

 the emanation of other radio-active elements but from any accidental 

 leakage or inductive disturbance of the instrument. The method of 

 measurement is purely comparative. The cardinal facts upon the 

 strength of which we associate radium with geological dynamics, its 

 development of heat, and its association with uranium are founded in 

 the first case directly on observation and in the second on evidence so 



