717 



These seven samples enable us to compare : 



1 the mud of the Lake with that of the environs. 



2 the Rockanje mud with neighbouring dunesands and with the 

 slime from the Meuse, the Waal, consequently from the Rhine and 

 Meuse basins. 



For purposes of comparison Miss Folmer still added mould from 

 the garden round the laboratory at Groningen and Fango of Batta- 

 glia and of an unknown find-spot. 



After this series had been investigated it appeared from reasons 

 to be mentioned below, that it was indeed necessary to extend the 

 number. First Mr. Trouw at Rockanje kindly sent us three samples 

 taken from the same spots where Mr. Büchner collected the samples 

 in 1913, which he found so extremely radio-active. 



These samples are called : 

 I (about 50 m. to the South- West of the Grotto at a depth of 150 cm.) 



II (from the South-East Corner of Betjenskelder 60 cm. deep) 



III (ibidem 150 cm. deep). 



In conclusion we were obliged to collect yet another series of 

 samples scattered over the whole area and at various depths. To 

 that end I have chosen the spots so as to include entirely the area 

 of the spots from which samples had been taken already, which 

 were said to have shown radio-activity by means of radiograms or 

 emanation (cf. fig. I). They were collected 1 from 10 spots at and 

 about those places where the radio-active samples were collected in 

 1913 (see above) and besides from 5 spots from the bottom of the 

 ditch that skirts the area of the Lake and Betjenskelder on the North. 



From these 15 spots 46 samples were taken at different depths 

 varying from 30, 60, 90, 120, 150, and 180 cm. All this is sufficiently 

 indicated in the figure together with the numbers of the samples. 

 We have taken these samples by means of a boring machine 2 m. 

 long, consisting of a tube of galvanized iron. Its stock can be pushed 

 through these layers of clay and peat. To the bottom is attached a 

 box in the shape of a hollow cylinder, on the flattened side of which 

 is a slide, which can be opened by means of a thin rod, running 

 upwards through the iron tube; this slide, therefore, can be opened 

 at the top and fastened by a screw, after the boring machine has 

 been placed at the required depth. With the crank at the top of 

 the tube one turns the whole thing round a couple of times, unscrews 

 the slide-rod, closes and fastens the slide again and draws the whole 

 upwards. In this way we made perfectly sure that we drew up only 

 mud from the depth as wanted. These samples Miss Folmer put 



