older Deposits of the North of Germany and Belgium. 263 



fossils, near theLahn mouth, are associated with two or three species of Orthis (one 

 of which is A^ery near to Leptana or Orthis lata) , and also with the very prevalent 

 fossils Delthyris macroptera and D. microptera ; they also occur in the arenaceous 

 and schistose beds, but chiefly in the form of casts. (See PI. XXXVIII.) 



Although we do not find among these fossil casts more than two or three forms 

 which clearly belong to the published Silurian types of Great Britain, we see at 

 once, that the group, as far as regards its fossil shells, is not merely distinct from 

 the carboniferous system, but also from the Devonian. A glance at the plates and 

 lists of the fossils establishes this point. 



Beds, we consider of a still older date, which rise out on the Moselle and at 

 Unkel on the Rhine, contain a vast profusion of a few species of the genus Orthis, 

 and assume very much the characters of the lower Silurian rocks, with which 

 they have one or two species in common. But, as we have before said, the absence 

 of clear subdividing lines, and the great dislocations and convulsions of these 

 tracts, forbid us to dwell further upon the descending order, and to restrict our- 

 selves to broad distinctions. All that we can say with certainty is, that, of the 

 fossil beds in question, those which contain the Homalonoti, Orthocerata, and Pteri- 

 neeB, indicate the upper Silurian division : while those which are marked by the pre- 

 valence of the genus Orthis indicate the lower. The peculiar shells Delthyris ma- 

 croptera and D. microptera occur through such a deep vertical range of the series, 

 that they cannot be considered as the types of any one portion of it. We conclude 

 therefore, that the slates of Wissenbach, and the shales and schists of similar age 

 which underlie the Devonian limestones on the left bank of the Rhine, may repre- 

 sent a portion of the upper Silurian group of England ; and that the fossiliferous 

 grauwacke of the Rhine may as a whole stand both for all the middle and lower 

 parts of the Silurian system. 



The presence of certain Trilobites and Orthoceratites, both in the Wissenbach 

 slates and in the underlying grauwacke, teach us that some of the races of animals 

 entombed in these rocks lived through very long periods of time. Again, we 

 repeat that the lithological development of these deposits does not permit us to 

 subdivide them into any well-marked physical groups ; and hence they are, both 

 lithologically and zoologically, much less distinct than the Silurian system of 

 England. 



Eruptive Rocks, Mines, Dislocations, 8fc. 



The short account we have given of the remarkable association of sedimentary 

 and trappean rocks in the neighbourhood of Dillenburg, may help to explain the 

 relations of similar rocks in other mineralized districts on the right bank of the 



2 M 2 



