older Deposits of the North of Germany and Belgium. 253 



of Devonian system. It remains for us to notice the lower groups of the descending 

 sections, and to show the analogies they present, both in their position and their 

 fossils, with the Silurian system. 



That such lower groups exist is proved by a series of parallel transverse sections 

 from the coal-field of Westphalia, across the Devonian strata above described, into 

 the more elevated and rocky regions which constitute the right bank of the Rhine. 

 Taking any one of the great roads which run to the south from Elberfeldt, Schwelm, 

 Hagen, or Lethraate, these schistose and grauwacke rocks, with occasional calca- 

 reous courses, are seen to rise up conformably, into massive ledges, from beneath 

 the Devonian system. 



The succession consists in general of thin-bedded, schistose grauwacke, passing into quartzose sand- 

 stone, having more or less the character of slightly micaceous flagstones of brownish and greyish colours, 

 very commonly having a ferruginous and sometimes a reddish tint. Occasionally these beds {e,g. near 

 Iserlohn) exhibit a passage into dark grey, slightly micaceous flagstones with rippled surfaces, separated 

 by argillaceous " way-boards," and they frequently contain a sufficient quantity of calcareous matter to 

 constitute seams of impure limestone. 



In these rocks we find a class of fossils entirely distinct from those forms which approach the carbo- 

 niferous types, and of which we were gradually losing the traces as we passed through the intermediate 

 or Devonian strata. The fossils here predominating are, several species of the genus Pterincea of Gold- 

 fuss (the Avicula of the upper Silurian rocks), one of which can scarcely be distinguished from the 

 Avicula reticulata. The genus Homalonotus also prevails among the same rocks, along with a great pro- 

 fusion of a small species of Orthis, a genus eminently characteristic of the Silurian system, together with 

 new forms of Spirifer unknown in the overlying system (see PI. XXXVIII., Silurian grauwacke 

 fossils). 



Black Slates of Wissenbach, 8fc., PI. XXIII. figs. 6 and 7. 



Besides the above-mentioned fossiliferous flagstones, certain strata are interpo- 

 lated between the true Devonian rocks and the " older grauwacke," which in some 

 districts on the left bank of the Rhine consist of flagstone, indurated shale and schist, 

 with thin calcareous courses. Judging from their organic remains, these shales 

 and slates must be considered as connected more with the Silurian than with the 

 Devonian system. Of this peculiar group the transverse Westphalian sections, 

 PI. XXIII. figs. 1 and 3, off"er only the imperfect rudiments in certain beds of shale 

 and schist immediately under the great limestone. We shall therefore describe 

 the group as it appears in those tracts where it is most developed. 



In following the strata upon their strike to Meschede and Brilon these 

 schists become thicker and more important (assuming a distinct slaty cleavage 

 transverse to the bedding), and are largely worked as roofing-slates. In that di- 

 strict we were not, however, fortunate enough to discover in them any organic 

 remains : but in the country of Dillenburg, the black slates of Wissenbach, which 

 we have shown to occupy the same geological position, are loaded with fossils. 



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