256 Professor Sedgwick and Mr. Murchison on the 



as we follow them on their line of bearing ; and with a change in their mineral 

 character they are often seen to exhibit a peculiar change in the forms of organic 

 life. Of this we have already adduced proof by tracing the lower zone of the car- 

 boniferous system through Westphalia (ante p. 231 et seq.) : and we now apply the , 

 same reasoning in explanation of our view respecting the slates of Wissenbach. 



It has been shown, that in geological position these slates (see PI. XXIII. figs. 6 and 

 7) lie between limestones containing the same fossils as those of Elberfeldt and Iser- 

 lohn, and the fossiliferous grauwacke which constitutes the chief mountain masses 

 of the right bank of the Rhine. It has also been stated, that on the Westphalian 

 frontier, where the whole succession is clearly exhibited, these slates have no ex- 

 istence, and are there merely represented by a few courses of shale, in which we 

 found no characteristic fossils. But these underlying shales expand vastly in their 

 range towards the east ; and at Wissenbach, strata, occupying exactly the same 

 position, assume a much more decided development, and contain that marked group 

 of shells above mentioned which led us to our present conclusions. This view 

 is, we think, in perfect harmony with all that we have written or observed respect- 

 ing the development of the Palaeozoic rocks. 



In the sequel we shall endeavour to show, that the calcareous shale which un- 

 derlies the great limestone of the Eifel (and we may here remark that the fossils 

 of the shale and the limestone are always mixed together, and confounded in one 

 group by the local collectors), is the equivalent of the black slates of Wissenbach. 

 In the Eifel, however, the lithological development of this group is small, being 

 not much more expanded than in Northern Westphalia. The observer, therefore, 

 who drew his conclusions only from the structure of the Eifel, might (as we shall 

 show hereafter) place the group here described in the Devonian system ; while 

 he who judged from the sections of Wissenbach and the right bank of the Rhine, 

 and decided its age from the dominant fossil types, would consider it, as we do, 

 to be Silurian. 



Silurian Grauwacke, K^ of coloured Sections, PI. XXIII. figs. 1, 3, 4, 6, 7, 8, &c. 



The grits, psammites, and flagstones, &c., rising beneath the slaty groups last de- 

 scribed, contain some fossils {e. g. Homalonoti, Calymene, &c.) approaching closely 

 to upper Silurian types ; whilst other fossils of the same rocks resemble those 

 which characterize the lower portions of the system. 



Among the few fossils of this part of the series which are absolutely identical 

 with our English forms, is the Pentamerus Knightii, found at Greifenstein, between 

 Dillenburg and Wetzlar : but at this spot the section is unfortunately obscure, and 

 the undulations and breaks are so considerable, as to destroy all sure indication of 

 the exact geological place of the quartzose grits in which the shell occurs. They 



