733 



where no admixture with the blue-grej mud had as jet taken 

 place; it consists chiefly of remains from plants and animals with a 

 few grains of quartz of at most 150 — 300 n length. 



A higher number was found for the following samples: 



Boring 12 with as many as 2.17 X 10 "^^ grammes. This is the 

 highest amount that has been found. "Blue-black" clay ; "consists 

 of very fine mineral dust, mixed with very feiv grains of sand, at 

 most 80 — 120|w long; feiv organic remains. (Treatise p. 38). Neither 

 is there any sample in this series in which the very fine mineral 

 dust is mixed with so little sand and organic material. It was also 

 pointed out that there was a resemblance between the Diatoms to 

 those from the slime of the JVIeuse near den Briel. (Treatise p. 51). 

 M with 1.09 X 10-12 grammes. TFwith 1.49 X lO-i^ grammes. 



I think that from this will appear that the quantity of radio- 

 active substance is smallest in samples of soil with much sand or 

 with much organic substance and that this quantity will be the 

 higher as the settled sliiïie of the river lacks these elements. This 

 is clearest in the case of boring 12 and the only samples where 

 the quantity also exceeds 1.00 X 10— i- grammes, are these vevy 

 slimes from Meuse and Waal. If we have to choose between an 

 origin from the dunes or slime of the river then I think I can 

 safely conclude that the radio-active substance in the alluvium of 

 Rockanje comes especially from the side of the rivers and in a far 

 less degree from the dunes. 



Apparently there is also radio-active substance in the sand of the 

 dunes (cf. sample C), though in a far less degree. Whether this is 

 the same substance as in the stronger active clay will perhaps be 

 settled later on by others. Moreover it remains possible that also 

 that slight quantity in the dunes yet exists of minerals originally 

 carried along, togethei- with slime of the river and which have settled 

 on the bottom of the sea on the shore. I must still add that, of course 

 independent of the quantity of sand and organic remains, the slime 

 settlements may possess different radio-active intensity, even if they 

 come from the same river. The motion of the water, either by the 

 current or the whirl of an eddy, may cause by fractionated settlement 

 on various spots a varying quantity of active minerals, altering with 

 the strength of that motion. 



We will now consider the numbers of the samples of boring 

 (2, 4, 6, 8, 9, 10, 11, 12, 13) a little more closely. They belong to 

 that part where in a deep basin hollowed in the sand, slime of the 

 river was thrown down in salt and later on in brackish water, 

 while in later times the slime of the uppermost layers was mixed 



