715 



they should belong to the small qiuiiilitv ol" dust from the dunes 

 carried into the lake by the wind. But as it must be very well 

 possible to make out by radio-active measurements of samples from 

 the soil, whether the radio-active substance present in that soil had 

 entered into it from the side of the dunes or rather from the side 

 of the slime-carrying rivers, I thought that it was better experi- 

 mentally to settle a question which was geologicall}' so important. 

 For that purpose I have collected a great number of samples as 

 will be mentioned below. After Dr. Büchnër informed me, in 

 answer to my letter, that he did not intend to continue these 

 researches himself, I was happy enough to find Miss H. J. Folmer 

 at Groningen willing to undertake them. Consequently this communi- 

 cation is chiefly the result of researches made by her at Groningen. 



First samples 2, 4, 6, 8, 9, 10, 11, 12, 13, 17, 20, 23 were 

 destined for inquiry, taken from the boring made at Rockanje. From 

 these it was to appear in what layer the activity was strongest, 

 after that the geological constitution of those layers and the conditions 

 under which they had been thrown down, had already been 

 discussed before in detail. Samples 20 and 23 are, however, from 

 layers which existed long before the origin of the Lake and of the 

 isle of Voorne '). These samples are called : boring 2, 4, etc. 



To these others were added : 



R. Taken 25 m. to the North East of the Grotto at 40 cm. 

 depth, consisting of brown, entirely organic mud mixed with some 

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



A. Taken from the bottom of the water, on the side of the 

 Noorddijk, behind the Windgat, where formerly the dike often burst, 

 on the North West side "). 



B. Taken from the bottom of a ditch close to the Molendijk in 

 the Strijpepolder, in order to examine whether the mud at a short 

 distance from the Lake is still strongly active. 



C. Sand taken from the dunes nearest the Lake (at =b 800 m. 

 distance) whereabout formerly the Sv\yn was situated. 



D. Light-grey clay, taken at 25 cm. under the meadow of the 

 Drenkeling behind the VIeerdam Dike of the Lake. 



31. Slime, freshly, settled, from the Meuse at Grave, which Mr. 

 J. DF,N Doop from Grave kindly provided me with. 



1) Of. treatise pp. 50 — 55. 

 3) Gf. treatise pp. 90—93. 



