274 Professor Sedgwick and Mr. Murchison on the 



elusions to which they lead us ; but they naturally and necessarily differ when we 

 come to questions relating to minute subdivisions. 



The actual sequence of the deposits, and their relative order in an ascending or 

 descending series, can only be made out, in the first instance, by the examination 

 of true mineral groups and natural sections. And such groups are of inestimable 

 value in making out the relations of physical regions, either actually continuous, 

 or not very widely separated from one another. But as all such groups are the 

 results (at least in a great measure) of mere local mechanical causes, we might 

 infer hypothetically (what indeed we know by experience) , that even among forma- 

 tions undoubtedly of the same age, an agreement in the minute subdivisions is not 

 to be looked for. We do not find such an agreement between the subdivisions of 

 the Silurian system in Herefordshire, Shropshire, and North Wales ; still less do 

 we find any exact mineralogical agreement of these subdivisions, as developed in 

 a part of Lancashire and Westmorland. 



In questions of doubt, the mind naturally seizes on the first analogies that pre- 

 sent themselves ; and we think it was perfectly natural for Monsieur D'Halloy and 

 Professor Dumont to classify the three lower members of their terrain anthraxifere 

 respectively with the Ludlow, Wenlock and Caradoc formations of the Silurian 

 system : and we were so far misled by the beautiful analogy which seemed to be 

 presented by the mineralogical development of the region, that we were disposed 

 to adopt this view after our first examination of the Belgian sections, though it 

 threw our previous classifications on the right bank of the Rhine into great diffi- 

 culties. Our present classification not merely reheves us from these difficulties, but 

 appears to be based on a sound induction from all the facts of the case ; and with 

 this remark we pass on to a brief consideration of the lower Belgian formations, 



5. Upper Division of the Terrain ardoisier of the Ardennes, <Src. — This formation is, in its upper mem- 

 bers, blended with the base of the formations already noticed, and its lower members graduate insensibly 

 into the central and more crystalline slates of the Ardennes (or systeme ardoisier inferieur). It has been 

 shown by Professor Dumont to occupy the position thus indicated ; to be wrapped round the central 

 slate rocks of the Ardennes ; and in a similar position to surround the central masses that appear in the 

 prolongation of that chain towards the Eifel. As our several traverses only tended to confirm the 

 general justness of his views, we accept them without hesitation. But there is another question, what 

 are the British equivalents of this formation ? In replying to it we can only appeal to our fossils, unfortu- 

 nately very few in number, and to the fossil evidence oiFered us by Professor Dumont and some other 

 geologists. But we shall afterwards confirm our views by an appeal to other evidence, derived from the 

 fossils in analogous beds under the Eifel limestone ; and by the general structure and relations of the 

 Rhenish provinces (out of the limits of Belgium), in which fossils are more abundant. 



In the first place, then, we found in several parts of this formation large winged Spirifers {Delthyris 

 macroptera of Goldfuss, &c.) of species unknown in England, but analogous to forms which, on the 

 right bank of the Rhine, we have shown to be associated with known Silurian fossils. For example, 

 specimens of this kind have been found at Houffalize, Wiltz, Weidinza, Longwily, &c., places on the 



