356 Amount of Thorium in Sedimentary Rocks. 



reproduced below. These rocks are regarded as for the 

 greater part of sedimentary origin, although highly altered, 

 and of Mesozoic age. Some are calcareous, some quartzose. 

 It is, therefore, difficult to classify them with the materials 

 grouped in the foregoing tables. The general means are, for 

 the same reason, somewhat misleading. After subtracting 

 from these of the Usernmulde two rocks of calcareous 

 character, the mean for the Usernmulde (9 rocks) is 

 1'10 x 10~ 5 . The mean for the Tessinmulde exclusive of 

 the dolomite (7 rocks) 0*53 x 10~ 5 . The first is in close 

 agreement with that of Table II. : the second is too low ; but 

 the origin of some of the more basic rocks in the Tessinmulde 

 is sufficiently doubtful to deprive the result of much of its 



weight. 



Table III. 



Usernmulde. Thorium. 



grain x 10— 5 per grain. 



1. Usern gneiss. (1144) 1*4 



2. Quartz-schist. (9-6) M 



3. Black lustrous slate. (8'0) 02 



4. Grey cipolin. (9*47) 04 



5. Quartzitic cipolin. (lTof)) 04 



6. Black lustrous slate. (10-66) 10 



7. Usern gneiss. (94) 1*3 



8. Sericite-sehist. (8'0) 17 



9. Black lustrous schist (7'84) 2*4 



10. Quartz-mica. (846) <0o 



11. Usern mica-gneiss. (S'38) - 5 



Tessinmulde. 



12. Hornblende-schist. (8 -64) <:0'3 



13. Calcareous mica-schist. (9 02) 0*5 



14. Hornblende „ „ (7'99) 0(> 



15. Amphibole garnet mica-schist. (8"38) 1*0 



16. Quartz-schist (95) <0'3 



17. Amphibole mica-schist. (947) 05 



18. Quartz „ „ (8-8) 0o 



19. Dolomite. (8*66) 0-4 



If we assume that the results on sedimentary rocks, recorded 

 in this and the previous paper on the subject, may be accepted 

 as approximately representative, we find that whereas the 

 calcareous rocks show a small, almost negligible, quantity 

 of thorium, the detrital sediments contain easily measured 

 amounts of thorium in almost every case ; the argillaceous 

 group having almost double the amount contained in the 



