older Deposits of the North of Germany and Belgium. 269 



side, to rise (in their true natural order) from beneath them. (See PI. XXIII. fig. 12.) This mode of in- 

 vestigating the relation of contiguous groups of strata is obviously not confined to mineral basins, but 

 admits of very general application. And before we quit the subject we may remark, that the evidence 

 of superposition, obtained by this laborious method, cannot long be in conflict with that derived from 

 vertical sections. For violent dislocations, such as we are considering, are but of limited extent ; and as 

 soon as we have passed beyond them, the formations are seen in their natural order, and the superposi- 

 tion becomes at once defined by the phaenomena of a single vertical transverse section. 



§ 2. Successive Groups between the Coal-field of Liege and the Ardennes, S^c. 



Leaving the Belgian coal-field, let us next consider the successive groups of strata 

 on the side of the Ardennes, and along the sections from the neighbourhood of 

 Liege to the Eifel. This coal-field forms the first terrain of M. Dumont, and by 

 whatever line we make our traverse from it to the centre of the Ardennes, we find 

 it succeeded by a second group of strata, the terrain anthraxifere of that author, 

 which is composed as follows : — 



An upper limestone (the mountain limestone of England), (systeme calcareux superieur). 



A group of flagstones and earthy slates (systeme quartzo-schisteux superieur). 



A lower limestone (systeme calcareux inferieur). 



A series of grits, flagstones, and slates, of more complicated structure than No. 2, and sometimes 

 having subordinate masses of coarse conglomerate (systeme quartzo-schisteux inferieur^. 



This terrain anthraxifere ranges on the side of the Ardennes symmetrically with 

 the coal-field, but is not limited by it ; as its lower members extend far beyond the 

 bounds of the true carboniferous strata. 



The preceding terrain passes, by absolutely imperceptible gradations, into a third 

 group of strata (the systeme ardoisier superieur of M. Dumont), occupying a regular 

 zone on the horizontal section. 



Again, this third group is succeeded by a fourth (the systeme ardoisier infe'rieur of 

 Dumont), composed of fine roofing slates, alternating with great beds of quartzite, 

 of more ancient aspect than the preceding, and forming the central mass of the 

 Ardennes, as proved by following the respective systems on their lines of strike. 



In the very centre of the Ardennes, and surrounded by the ordinary rocks of 

 the 4th group, are some slates of unusually crystalline structure, and containing 

 in some places an incredible quantity of small crystals of octohedral magnetic iron 

 ore ; in other places numerous crystals of ottrelite, or numerous red stains, appa- 

 rently derived from some decomposing ferriferous mineral. These peculiar slates, 

 and the associated quartzites, are placed by M. Dumont in a 5th group. Led by 

 the physical structure of our own country, and the fact that beds of porphyry ap- 

 pear among them in the great gorges of the Meuse, we should rather consider 

 them as a lower and altered portion of roofing slates of the Ardennes. This is, 

 however, a question of little moment ; for these crystalline slates form a geo- 

 logical centre ; and in whatever direction we take our departure from them, the 



VOL. VI. — SECOND SERIES. 2n 



