732 



and [ will only point out some conclusions to be drawn from the 

 values found by Miss Folmer. Tliis is all the more important, 

 because we are concerned here for the first time with a great 

 number of determinations of radio-activity of a geologically amply 

 described country. 



As has been fully described in I he treatise (pp 29 — 31) we are 

 concerned here with clay, organic material, and sand. In the 

 microscopic investigation into the organisms of the samples I have 

 always mentioned the greater or smaller presence of mineral dust, 

 organic remains and grains of sand, together with their size. Of 

 course, it may not be lost sight of that mineral dust need not 

 exclusively be slime from the rivers, but that also part of it may 

 come from the dunes. And the more we meet with grains of quartz 

 in the layer, the greater the chance will be that also part of the 

 finer minei-al dust originates in the dunes or the bottom of the sea. 

 Reversely, very small grains of cpiartz are thrown down in the 

 slime, for in the freshly settled slime of the Meuse and the Waal 

 they are to be found in a small quantity: I shall leave undecided 

 whether they have been blown into it, carried along or rooted up. 

 Yet it is very well possible to conclude from all the figures Miss 

 Folmer has given, to which of the three mentioned elements : clay, 

 organic remains, or sea-and dunesand the radio-active parts belong. 



The quantity of radio-active substance varies in the 24 samples: 

 boring 2, 4 etc. A. B. C. D. M. W. R. I. II. Ill in 14 of the 24 cases 

 between rather narrow limits: 0.49 and 0.78 > lO—''-^ gr. of radium 

 pro gramme dried substance. 



The following samples possess a lower number; 



Bonng 2 with 0.43 X 10— i- grammes. Besides fine mineral dust 

 and a few grains of sand of 200 — 300 /t length at most, especially 

 much organic material. 



Boring 17 with only 0.1 )< lO-^^ grammes. Consists of blue-grey 

 sand (sand with little slime). 



Boring 23 with 0.36 and 0.37 .^^ 10 ~^'^ grammes of sand; the 

 grains of sand in this sample are at most 450 n long. Mixed with 

 very flue mineral dust. 



A with 0.39X10^12 grammes. According to notes in 1915: 

 excessively rich in Diatoms; a great many organic remains, little 

 mineral dust". 



C with only 0.23 X 10~i'^ grammes. According to notes in 1915: 

 "very regular sand without any organic mixtures. 



R with only 0.24 X 10~'"-^ grammes. This sample is the same as 

 that described in the treatise p. 17 under No. 2: "brown mud" 



