( -^51 ) 



remains to prove tliat tlio orratics traceable to the Ardennes niav 

 claim a considerable share in the formation of the mixed dilnvinm '). 

 Cambrian sy.ttem. Tlie principal part of the Ardemies is built 

 np of layers belonging to the Cambrian system, which A. Dumont 

 originally snb-divided into thi'ee groups, namely Devillian, Revinian 

 and Salmian^). The Devillian and Revinian systems were afterwards 

 united by J. Gossklet, ') into one series, called the devillo-revinian, 

 which consists of pliyllites, alternating with bands of greyish black 

 and dark bluish grey quartzites. These layers may be seen exposed 

 principally near Revin and Deville, on the banks of the Mense, near 

 Rocroi and Stavelot, and also near Givonne, to the north of Sedan/) 

 These quartzites are often crossed in \-arious directions bv fine veins 

 of quartz and — a distinctive feature by which thev are easily 

 recognized — they often contain small cubes of pyrite, which in 

 some cases has been in a greater or lesser degree changed into 

 hydroxyde of iron. Now and then specimens are found in which 

 the orginal mineral has entirely disappeared, only the impression of the 

 cubes being left. J. de Windt*) has given microscopical descriptions 

 of these crystalline quartzites, but has omitted to mention one 

 special characteristic in which they show great conformity with the 

 phyllites. In reference to the latter, E. Geinitz was the fii-st to point 

 out that the enclosed crystals of magnetite and pyrite are sur- 

 rounded by a zone of quartz, thus forming elongated lenses. ") 

 From the manner in which these minerals have grown together, 

 as well as the chlorite, he was led to the conclusion that the\' 

 were coeval. This theory has been refuted by A. Ren.vrd. Althouo-li, 

 with Geinitz, he believes the magnetites and pyrites to have 

 been formed at the same time as the mass of the rocks he 



1) In all probability this share will be found to be mucli larger than is thought 

 at present, because a great many lotky fragments, among others quartzites and 

 sandstones, are now ascribed lo the diluvium of the Rhine although they are also 

 present in that of the Meuse. 



') Mémoire sur les terrains Ardcnnais et Rhénan-Mémoires de TAcad.-rov. de 

 Belgique XX. Bruxelles 1847, p. 8. 



■■') Esquisse géologique du nord de la l-'ranee. Lille, 1880, p. 10. 



■') It cannot be made out which of these localities have provided the boulders. 

 They are represented in the accompanying map ^as if they were coming from 

 Revin, the chief locality. 



°) Sur les relations lithologiques entre les roches considérées comme cambrien- 

 nes des massifs de Rocroi, du Brabant et de Stavelot. Mém. cour. de l'Acad. rov. 

 de Belgique LVl. Bruxelles 1898, p. 21, fi8. 



"j Der Phyllit vou Rimognes in den Ardennen. Tschermak's Mineralog. und 

 Petrogr. Mitthig. UI. \Vi«a. 1880, p. 533. 



