( 111 ) 



In f lie communications on the geology of the Netherlands, col- 

 lected by the geological committee, number 35, Dr. Lorik shows 

 convincingly that the surface of the "Geldersche Vallei" represents 

 a terrace of a faded branch of the Rhine. 



"The peat is followed by a thick layer of sand which has not been 

 washed away from the hills on either side, but has been supplied 

 by a branch of the Rhine which thus built a terrace", (p. 95). 



The fact, that the fluviatile sand reposes upon the marine fauna 

 of the "Eemsteisel" with temperate character and is still separated 

 from it by peat and clay, proves in my opinion that the terraces 

 have been formed by the rivers in an old-alluvial period with our 

 present climate. I hope to revert soon to this subject. 



The third examiner of the Dutch sand districts was Dr. .1. 1,. ('. 

 Schroeder van der Koek, and it may be superfluous to draw out 

 his great merits another time, also in regard to this geological 

 problem. To his accurate sand inquiry we owe this division'). 



I. Quartz-am flbol-sands : 



a. quantity less than 0,4. Southern Diluvium. 

 It. quantity more than 0,4. Northern Diluvium. 



II. Quarte-garnet-sands : 



a. small quantity. Increase. Alluvium. 



b. great quantity. Decrease. Alluvium. 



He applied his rules to this study of the environs of Deventer 

 with much acuteness. 



Schroeder van der Kolk could not continue his successful re- 

 searches because of his busy life in Delft and his feeble health, and 

 his death has put an end to all further expectations. 



The difficulties are tar from being conquered now. In general the 

 rules of Schroeder van der Kolk are quite right. Yet it is not 

 sufficient for the classification of each sand to determine the quantity 

 of heavy minerals. The activity of the forces of nature being too 

 much complicated, the effect cannot appear in a so simple form in 

 all cases. In each case all circumstances must he taken into con- 

 sideration or we shall often come to an erroneous conclusion. The 

 following example shows this necessity. 



The lower parts of the Rhine-diluvium in Limburg are formed by 

 grey sands. In these sands grains of basalt are tolerably numerous. 

 Even when the grains larger than '2 mM. and those smaller than 

 0,2 mm. are removed, the quantity of heavy minerals is still 0,5 to 

 0,6, the specific weight of basalt being 2,9 to 3,1. The grains of 



l ) In this form Schroeder van der Kolk gave his division at the 6 th Physical 

 and Medical Congress in 1897 (Transactions page 409). 



