7 72 Radio ictivity of Igneous Rocks from Antarctic Regions. 



chemically or petrographically. Nos. 1 and 5 in the table, 

 probably taking their origin in the same lava-flow, similarly 

 show a striking equality in their radium and thorium contents. 



Some of the lowest results obtained were from erratic 

 granites. These contained but a small quantity of biotite, 

 and consisted almost entirely of orthoclase and quartz. With 

 the single exception of No. 11, the granites showed a com- 

 plete absence of precipitation, and are probably therefore 

 approximately correct. 



Speaking generally, the results do not bear out the con- 

 clusion of Farr and Florance in their research on the radium 

 content of the Sub- Antarctic islands of New Zealand (Phil. 

 Mag. Nov. 1909), that the radioactivity depends roughly 

 upon the acid or basic character of the rock. The mean 

 value, 2*15, obtained for the basic rocks of Mt. Erebus is 

 in good agn ement with the minimum value 2'38 found by 

 them (he. cit.) on a specimen of lava from the same locality. 



The radioactivity of No. 6 in the table is of especial 

 interest. This rock is stained red probably under the action of 

 infiltering geyser waters, and it seems probable that in this case 

 the kenyt owes its relatively high radioactivity to this cause. 



In addition to the constancy shown in the quantities of 

 radium and thorium present in the different groups of rocks 

 in the table, these quantities seem to show a decidedly definite 

 relationship between themselves, from the highest to the 

 lowest reliable values obtained ; a rise in the radium present 

 being almost invariably accompanied by a nearly corre- 

 sponding rise in thorium present. This feature which, in 

 the case of some weathered andesites examined, might find 

 an explanation in the effects of the weathering action itself, 

 cannot be similarly explained in the case of granite and the 

 tin weathered Antarctic rocks, where a similar relationship 

 appears to hold. 



In the case, however, of the kenyt No. 6 in the table, the 

 same cause which probably gave rise to its relatively high 

 radium content appears at the same time to have propor- 

 tionately increased the thorium content, for the ratio of 

 radium to thorium present fairly approximates to the general 

 mean ratio. Hence in this case the radioactivity, if due 

 chiefly to the external effects o£ infiltrating waters rather 

 than to any peculiarity inherent in its original magma, 

 involves a similar relationship between the radium and thorium 

 in those waters. 



In conclusion I desire to express my thanks to Professor 

 Joly for his interest in the work. 



Geological Laboratory, 



Trinity College, Dublin, 



March 17th, 1911. 



