( 320 ) 



absent in our diluvium. With regard to Groningen tliis was already 

 told us bj Van Calker ^). Afterwards I ])oin1ed out the sauie thing 

 for Kloosterholt ^), and it will a[)pear, too, that boulder-clay of 

 Hemelum contains as many Under-siliirian erralic-t)locks as the loam 

 of the places mentioned. 



That the al)0\e-named writers are of ditfereut opinions may be 

 easil}' explained by the way in which stones used to be gathered. 

 Formerly the hannner was hardly ever used and there is no doubt 

 that only those stones were gathered whose outward ap[)earance 

 drew the attention. Now, this very rarely hai)pens with Cnder- 

 silurian erratics. Tlie polyparia of syringophyllum organum L., 

 which are [jrobably without any exception Under-silurian, are con- 

 spicuous for their form, and wc really see that old collections contain 

 these fossils in large numbers. The Upjier-silurian erratics, however, 

 on the outside of which it is sometimes already visible that the}^ 

 are ricii iii fossils (which is i. a. I he case with limestones with 

 chonetes striatella Dalm), much sooner draw the attention. This is 

 especially the case with petrified corals, which mostly ha\e an 

 Upper-silurian age. They foi'm, indeed, the greater part of the old 

 collections. 



Even if one uses a hammer while gathering stones, one is sure to 

 find, in [)roportion, more rp])er-.Silurian erratics with determinable 

 fossils than Under-Silurian ones, because as a rule the former are 

 much richer in fossils than the latter. 



Moreover, in Upper-Silurian erratics Leperditia-valves are frequently 

 found. As, in consequence of their comparatixely small size and their 

 smooth surface, these valves are easily exposed to view and the 

 diiTerent kinds of Leperditia are easily distinguished and are charac- 

 teristic of different strata, one may, by means of these remains, deter- 

 mine the age of many Upper-Silurian erratics. 



A great part of our Under-Silurian erratics, however, consist of 

 pieces of tough, greyish lime-stone, which does not possess many 

 petrifactions, so that these pieces seldom gi\e a determinable fossil. 

 A'ery often an Asaphus, an Illaeinis or an Endoceras, found in them, 

 proves their Under-Silurian age, whereas these remains are too 

 incomplete to allow of their being ranged under a ciefinite division 

 of Under-Silurian erratics. 



This is even more the case with that kind of limestone of frequent 



1) Van Galker, Ueber das Vorkommen cambrischer und iinlersiluriscber Gescbiebe 

 be! Groningen. Zeitschr. d. deutscli. geol. Gesellschaft. Bd. XLIII. pag. 792. 



-) BoNSEMA, De sedimentaire zwerf blokken van Kloostekholt. Versl. v. d. Koiiinkl. 

 Akad. V. Weienschappen. 1898. pag. 448. 



