354 Prof. J. Joly on the Amount of 



observation being required (the electroscope being observed 

 when the solution is alternately in ebullition and at rest) to 

 detect with certainty a change amounting to a small fraction 

 of a scale-division per hour. It may, I believe, be accepted 

 as certain that what error may arise from confining the 

 examination to the acid solution is very small, not more than 

 a few per cent. 



Table I. contains only arenaceous and conglomeratic rocks; 

 mainly sandstones of various characters and ages. The 

 greensandstone is, of course, a rock differing from the 

 others in mode of origin. The sandstones are for the greater 

 part constituted of residual quartz or of quartz and felspar, 

 derived from older rocks. As the quartz is, itself, probably of 

 very low thorium content, it is not surprising that these rocks 

 are generally poor in thorium when contrasted with many 

 igneous rocks. They are, however, much more radioactive 

 than the calcareous rocks, in which, in most cases, the 

 thorium emanation cannot be detected even when consi- 

 derably larger quantities of rock are used in the experiments 

 (Phil. Mag. July 1910). 



Table I. Thorh™. 



grm.XlO— 5 per gram, 



1. Greensandstone. Werl, Westphalia. Cretaceous. (15) 0'20 



2. Sandstone. Obernkirchen, Teutoburger Wald. Wealden. (15) 030 



3. „ Vlotho, Westphalia. Keuper. (10) 114 



(17) 102 



4. „ (red). Heidelberg, Baden. Bunter. (20) 012 



5. ,, Bemireinont, V"osges. „ (20) 091 



G. „ Westhofen, W T estphalia. Carboniferous. (15) 074 



7. „ Freienohl, „ „ (15) 061 



8. Quartz Conglomerate. Donebate, Co. Dublin. Old Bed Sandstone. (14). 0*33 



9. Grauwacke-Quartzite. Allrode, Harz. Lower Devonian. (16) 0'74 



10. Quartzite (Taunus). Schlangenbad, Nassau. Lower Devonian. (15) ... 034 



11. Sandstone (red). Locb Torridon, Scotland. Torridonian. (19) 0*27 



12. Quartzite-schist. Western Spessart. Archaean (?). (17) 032 



Mean Q-54 



In the above table the weight in grams of material dealt 

 with is given in brackets. 



The finer-grained detrital rocks — slates and shales — are, in 

 contradistinction to the sandstones, derived from the more 

 soluble and friable constituents of the primary rocks : such 

 constituents as are reduced by denudative actions most 

 readily to small dimensions. They are on this account pre- 

 cipitated furthest from the land, and represent materials 



