Terrestrial Surface Materials. 699 



It will be seen that these depart considerably from the 

 means obtained on composite rock-powders, using the method 

 by fusion : the intermediate and basic groups, more espe- 

 cially, are about 100 per cent, of their value too low if the 

 method by fusion gives correct results. 



I have in the above analyses of results by solution omitted 

 some results of my own * which afforded generally higher 

 values than those obtained by other investigators using the 

 same method. No clue to this difference is as yet forth- 

 coming, and until the results by fusion were obtained it 

 seemed best to assume that some difference in the prepara- 

 tion of the solutions was responsible, in which case the 

 higher results would be the more reliable. However, many 

 of my determinations by solution stand some fifty per cent, 

 (or even more) of their value above the fusion results, and 

 as it seems improbable that emanation can be retained when 

 the latter method is employed, it seems safer to set aside for 

 the present such determinations of mine as have not been con- 

 firmed by the use of the electric furnace. A remarkable fact 

 is that just some of the highest of my results have been so 

 confirmed. Thus, in the results obtained by composite fusions 

 given above, the mixed powders of seven Vesuvian lavas 

 give 12' 6. The results obtained by solution on these same lavas 

 would afford a mean value of 10*8 t« Again, eleven of the 

 St. Gothard granites gave 7'2 by solution, and six other 

 examples of this granite gave by fusion 6*0 $. I have also 

 omitted some high values obtained by Gockel using the 

 solution method §. These results give a mean of 74*6 for 

 six rocks. His low results are obtained by a radiation 

 method, and are not easily compared with the measurements 

 by solution. 



fn his first paper {loc. cit.) on the subject of terrestrial 

 radioactivity, Strutt pointed out that the acid rocks appeared 

 to contain a larger amount of radium than the basic rocks. 

 The fusion results support this conclusion. There are extremes 

 of radioactivity found in all the chemical groups. These may 

 be regarded as exceptional, or set off one against the other. 

 We see such extremes in the basic Vesuvian lavas and in the 

 granites of the St. Gothard or of Karangan ||. The fact 



* Address Section C, British Assoc. 1903, and '"Radioactivity and 

 Geology,' Constable & Co. 

 t Joly, Phil. Mag. Oct. 1909. 

 | Joly, Phil. Mag. Feb. 1912. 



§ Gockel, Jahrbuch der Had. u. Elekt. B. vii. p. 487. 

 || Buchner, loc. cit. April 1912. 



