( 523 ) 



It stands to reason that erratics of this type must be more plentiful 

 still in the dislriet South of the Rhine ; in fact, similar quartzites 

 lia\e heen found in the dilnviuiu of the Meuse for a long time past. 

 In the Pi'ovince of Limburg they are looked upon as the most com- 

 mon kiiul of erratics. At,ph. Erens came across one 3 M. high, 2.6 M. 

 long and O.B M. broad '). According to this author, they are also 

 found in quantities in the Province of North Brabant, although they 

 are not so large as those of Liniliurg. .1. Lohik found rocks of this 

 conq)osiii()n on the heaths at Mook and at Scliaik, also in South Holland 

 on the beach of Springer in Goedereede and near Rockanje in the 

 island of Voorne. 



"Porphyroids." But the most conclusive |)roofs that immense quan- 

 tities of rocky fragments nuist have been transported from the Ardennes, 

 are furnished i)y the so-called Porphyroids. This rocky formation is 

 confined to the districts of Revin and Deville, where, more particu- 

 larly in the neiglibourliood of Laifour and Mairus, they form 

 dikes from 0.1 to 20 M. wide, corresponding to the kiyers of the 

 devillo-re\inian group. At present only 17 places are known where 

 this exceedingly characteristic formation ') may be encountered. 

 Dispersed in a bluish gray or greyish groundmass, may be seen 

 porphyritic crystals of bluish quartz and of feldspar. Owing to 

 their peculiar position and their schistose structure, many geologists 

 have classified these rocks among the series of crystalline 

 schists, — whilst others have ascribed to ihein an eruptive origin. 

 Ch. de t.a Yat.lee Polssin and A Renard, who ha\e given the 

 most detailed description of these rocks, favoured the former view ^); 

 Bakrois, Dalbree, Gosselet, von Lasaulx and others, on the con- 

 trary, justly considered them to be quartzporphyry, an opinion which 

 A. Renari) also finally accepted. 



Although tiiese porphyroids can ha\'c but a miuiniuni share iu liie 

 formation of the Ardennes, they are frequently met with iu dihnial 

 deposits. In lïelgium, (4. Dewaeqle only noticed them near Liege ^), 

 which proves that but little attention has been paid to them in that 



1) Recherches sur les foruiations ililuviemies du sud des Pays-Bas. Archives 

 Teyler (2) III. &'eme parlie. Haarlem 1891, p. 23. 



^) J. Gosselet. L'Ardenue. Paris 1888, p. 86. 



■') Ménioire sur les caractères mineralogiiiues et stratigraphiques des roches dites 

 plutüuieiiiu's de la Belgicjue. Mémoires cour. etc. de I'Acad. roy. de Belgique XL. 

 Bi-uxelles 1S7(J, p. 237 — 247 (also Zeilschr. d. D. geol. Ges. 1876, p. 750—769). 



•*) Prodroiiii- d'une description géologique do la Belgi(|ue. Bruxelles et Liége 

 IsdS, p. •_':!7. 



