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opinion on this head. I do not in the least question Dr. Jonker's 

 assertion, that in some places he "can decidedly conclude from the 

 relief whether we have to do with boulder-sand or with clay", 

 because there is such a large diiFerence in the resistance which 

 boulder-clay and sand offer to erosion ; but in most parts of Drenthe 

 it is impossible to judge from the appearance of the surface, whether 

 the ground underneath is sand or clay : this I learnt from consulting 

 the mentioned register of borings and also from numerous small borings 

 on my own account. The parts of the bottom-moraine from which, in 

 the opinion of Dr. Jonker, the boulder-clay has "disappeared", and 

 the "intermediate stages between original boulder -clay, and altogether 

 washed-out boulder-clay", therefore undoubtedly, have been varieties 

 existing from the beginning. 



With regard to the occurrence of flint, which among our erraties 

 has been rather disregarded, there really exists an important differ- 

 ence between boulder-clay and boulder-sand. This J learned especially 

 too through the "comparative mechanical analysis", recommended by 

 Dr. Jonker. Further I recollect that among the stones found in 

 the sand on the Hondsrug, 1 did not come across a single flint; 

 on the other hand I met with flint in all the clay-pits in the 

 neighbourhood, and, taking also into account the small fragments, 

 1 found it even largely represented. The single exception which 

 Dr. Jonker observed in a pit of loamy sand near Groningen, is no 

 proof against this general experience. Besides, the bed w^as only loamy 

 sand, not boulder-clay. He too found flint in several clay pits on 

 the Hondsrug in Drenthe. In the fact, that on the whole (for it is 

 necessary to compare places lying outside the Drenthe Hondsrug as 

 well, because our Northern diluvium is generally considered as 

 belonging to one and the same glacial epoch) there is, with regard 

 to the presence of flint, an evident difference between boulder-clay and 

 boulder-sand, I find another proof in favour of my opinion that, 

 generally speaking, the one has not proceeded from the other by 

 a wash-out. Neither the occasional absence of flints from boulder- 

 clay nor the occasional presence of these stones in boulder-sand, are 

 proofs against the general tendency of my argument. 



in the preceding I have endeavoured to give a succinct refutation 

 of the objections raised by Dr. Jonker against one of the solutions I 

 proposed to account for the vaulting of the Hondsrug, — a question 

 which is only of secondary importance. 



But 1 gladly avail myself of the opportunity to discuss a point 

 of far greater importance, on which Dr. Jonker has expressed an 



