764 Distribution of Thorium in the Earth's Surface. 



some cases, where no positive result was obtained, is arrived 

 at by dividing the smallest detectable amount of thorium, 

 under the conditions of the experiment, by the number of 

 grams of the substance ; in the case of the sea-waters by the 

 number of cubic centimetres. 



Thorium in 

 grams per gram. 



Lava, Vesuvius, 1906. 10 grams 3'6xl0~ 5 



1895-99, 10 grams 3-1 



1832, 10 grams 3'4 „ 



1794, 10 grams : 09 „ 



1631, 10 grams 2S „ 



„ St. Helena, 10 grams 09 „ 



Gneiss, St. Gothard Tunnel, 8-38 grains 19 „ 



Mica-schist (sedimentary), St. Gothard Tunnel, 9'1 grams 1'H ,, 



Gneissose granite, St. Gothard Tunnel, 10 grams 2-1 ,, 



Gneiss, Simplon Tunnel, 10 grams 1*6 ,, 



Diabase, Fit'esbire, 10 grams less than 0*3 ,, 



Shale, Moffat Dale, 45 grams 05 „ 



Granite, Co. Wicklovv, 20 grams 0-9 ,, 



Carboniferous Limestone, Armagh, 22*5 grams ... less than 0*2 „ 



Marsupites Chalk, 32 grams less than 09 ,, 



Eed Clay, N. Pacific, 2'4 grams less than 1*3 „ 



,, Central Pacific, 10 grams 0*5 „ 



Radiolarian Ooze, Central Pacific, 6*3 grams less than 0*5 „ 



Manganese Nodule, S. Pacific, 12 grams less than 0*2 „ 



Sea- Water, Coast of Dublin, 1400 ccs less than 2-1x10-8 



,, S. Atlantic, 1790 ccs less than 1"6 ,, 



1500 ccs less than 20 ,, 



Indian Ocean, 2800 ccs 1:1 „ 



The number of observations is as yet too small to merit 

 any full discussion or justify definite conclusions. But the 

 fact that every rock examined, with three exceptions, was 

 found to contain thorium, lends considerable support to the 

 view that it is a generally prevalent constituent. The quan- 

 tities observed in some cases are such as might afford an 

 explanation of the local occurrence of thorium emanation in 

 the atmosphere, although the general average of the few rocks 

 examined is hardly such as to account for a general prevalence 

 of the emanation to the extent to which it has been observed 

 in particular localities, unless thorium is breaking up faster 

 than is generally assumed. It may turn out, however, that 

 the surface soils are richer in thorium than the rocks from 

 which they are derived. The results on the Yesuvian lavas 

 seem to accord well with Blanc's observations on the abund- 

 ance of thorium-emanation in the atmosphere at Rome, the 

 leucitic lavas entering extensively into the surface-geology of 

 the district. The quantities of thorium observed in some of 

 these materials exceeds the average uranium content of rocks 

 about three times. It is true that the lavas in question 

 possess a very unusual radium richness, so that the local 



