252 Professor Sedgwick and Mr. Murchison on the 



becomes much more crystalline, and is in parts a complete marble, of greyish, whitish, mottled red and 

 blue colours, and is much charged with corals identical with those of Devonshire. 



The same rock, offering a still greater number of mineral varieties, is seen near Dietz, where the lime- 

 stone is in contact with porphyry and trap. When polished the marbles of Dietz and Villmar are indeed 

 perfectly identical with our rocks of Babbicombe and Torquay, both in colour, composition, and in the 

 species of corals with which they abound. The Strigocephali and other shells characteristic of the 

 group in the Rhenish provinces are not merely found in these marbles, but occasionally in the associated 

 beds of schaalstein and schist. 



To the north-west of the town of Dietz the Devonian limestones are underlaid by 

 a great thickness of slaty rocks, some parts of which doubtless represent the fossi- 

 liferous slates of Wissenbach, like which they repose upon, and graduate down- 

 wards into, arenaceous flagstone and grauwacke. Near Dietz, however, these schists 

 are much more expanded than in any other portion of the tracts we are here de- 

 scribing. The transverse section (PL XXIII. fig. 9) will help to explain the 

 relations of the rocks of igneous origin to those of sedimentary deposit. In many 

 places where trap rocks abound, the limestone loses nearly all traces of bedding (as 

 for example near Hadamar), and occasionally becomes dolomitized. We consider, 

 however, all the calcareous masses near the banks of the Lahn and within the 

 limits of the elliptical area we have been last considering, to belong to one geo- 

 logical epoch; though, in a country so dislocated and obscured by plutonic erup- 

 tions, it is no easy task to readjust the different broken and altered fragments. 



The previous conclusion is drawn from the structure of the great group of rocks 

 we have been last describing, from their relation to older groups, and, above all, 

 from their characteristic fossils. The enormous apparent thickness of the group, 

 under any interpretation of the transverse sections, might at first sight seem to put 

 it out of all relation to the great Devonian limestone of Westphalia and (as we 

 shall afterwards prove) of Belgium. But we have already shown that we may rid 

 ourselves of a part of this difficulty by the supposition of an inclined basin or trough, 

 for the reality of which we think there is good evidence. Again, the great thick- 

 ness of the group is a natural, and we might say an inevitable consequence, of the 

 great development of the contemporaneous erupted and regenerated trappean 

 rocks, which (both in the country of Dillenburg and on the Lahn) are so asso- 

 ciated with the Devonian series as to form an integral portion of it. 



Silurian Rocks (Map, colour 8. Sections, PI. XXIII. fig. 1, letters h and i.) 



We have endeavoured to show that certain groups of strata, above described, 

 occupy, in the descending sections, the place of the old red sandstone of England, 

 and that they present a group of fossils intermediate between those of the carbo- 

 niferous and Silurian systems. We have therefore designated them by the name 



