Thorium in Sedimentary Rocks. 355 



from which the more resistant grains have been sorted by- 

 gravity. As the latter are generally quartz or felspar, and 

 hence substances which in most cases are poor in radioactive 

 constituents, it is to be expected that the argillaceous group 

 of detrital rocks would reveal a more considerable richness 

 in thorium than the arenaceous. Table II. shows that this 

 is, indeed, the case. Comparison with Table I. shows that 

 only two of the sandstones, Nos. 3 and 5, have quantities of 

 thorium equal to those generally prevailing in the argillites. 

 A few surface materials of recent date are included in the 

 table. These possess the same degree of richness in thorium. 



TABLE II. Thorium. 



grm. X 10— 5 per gram. 



1. Brick-Earth. Eosslare, Co. Wexford. Eecent. (18) 113 



2. Clay. Friesdorf, Bonn. Eecent. (15).., 0*91 



3. Loess. Heidelberg. Pleistocene. (18) 1'04 



4. Bundnerschiefer (folded). Piz Ot, Ober Eugadin. Jurassic. (15) 091 



5. Schiste Lustre. Simplon Tunnel. Jurassic. (15) 104 



6. Bed Marl. Ballymurphy, Co. Antrim. Keuper. (12) 014 



7. Boofing Slate. Weissenbach, Nassau. Upper Devonian. (15) 112 



8. „ „ Caub, on the Ehine. „ „ (16) P40 



9. Slate. Valentia, Co. Kerry. Devonian. (15) P30 



10. „ (dark Killas). Cornwall. „ (15) 1-16 



11. Grauwacke. Wipperfurth. Ehen-Prussia. Middle Devonian. (15)... 2'40 



12. Clay-Slate. Ma<idesprung, Harz. Lower Devonian. (15) 087 



13. ,, ,. Weilburg, Nassau. Devonian. (14) P17 



14. Slate (green). Kingscourt, Co. Cavan. Silurian. (15) P30 



15. Phyllite (green). Lossnitz, Saxony. Lower Silurian (?). (15) 1'94 



16. Shale (black). Moffat, Scotland. Silurian & Ordovician. (10) P00 



17. Eoofing Slate. Penrhyn, N. Wales. Cambrian. (16) 0*96 



18. Slate (Oldhamia). Bray Head, Co. Wicklow. Cambrian (?). (15) 0'82 



Mean 114 



The highest result obtained, the Grauwacke No. 11, was 

 checked by a second examination of the preparation, when 

 the first result was almost exactly confirmed. The lowest, 

 the Keuper Marl No. 6, refers to a material deposited under 

 continental conditions, probably in inland waters, and is 

 therefore of somewhat different character to the others. It 

 contains very little calcium carbonate. Excluding this 

 material, the general mean for the argillaceous group rises 

 to 1'20 x 10~ 5 gram thorium per gram. It will be noticed 

 that there is a remarkable sameness in the foregoing results : 

 fifteen rocks vary between the limits 0*82 and 1*40. 



The results which I have already published (Phil. Mag. 

 July 1909), when dealing with the St. Gothard rocks, are 



