258 Professor Sedgwick and Mr. Murchison on the 



sional seams of impure limestone, the whole rising from beneath the Devonian 

 system (PI. XXIII. fig. 4). The same series is seen in passing from Schwelm to 

 Meinershagen, concretionary bands of dark-coloured impure limestone occurring 

 irregularly throughout the zone. 



It appears, after a general view of the sections, that the vast group of fossiliferous grauwacke lying 

 beneath the schistose and calcareous beds last adverted to, is characterized not only by some of the fos- 

 sils we have been enumerating and by several species of small Orthis closely allied to those of the Silurian 

 rocks*, but also by certain peculiar forms which are as yet unknown to us in England; e.g. by the 

 Hysterolites of Schlotheim, and two species of Delthyris, the D. macroptera and D. microptera of 

 Goldfussf. 



In some of the lowest strata of this grauwacke, M'hich appear in the environs of Siegen and also at 

 Unkel and other places on the Rhine, we meet with several forms of Orthis (one of which has been 

 recognized as Orthis Pecten, another as Orthis Jlabellulum, a third as Orthis rugosa) associated with a 

 species of Terebratula, undistinguishable from Terebratula Stricklandii. This group of fossils seems 

 rather to be characteristic of the lower Silurian rocks. 



If, however, we attempt to separate the grauwacke of the Rhenish provinces 

 into formations, we meet with insurmountable difficulties ; first, because w^e want 

 distinct calcareous ledges or separating lines, each characterized by peculiar fossils 

 (as in the typical country of the Silurian rocks of England) ; and secondly, because 

 several of the same species of fossils, such as the Delthyris macroptera and D. mi- 

 croptera, and several of the small species of Orthis, are found throughout an enor-" 

 mous vertical thickness of strata. 



While speaking of the thickness of strata, we have every possible desire to 

 guard against the exaggerated interpretation which might be put upon our words 

 or derived from the sections we describe. The great transverse fissure in these 

 older rocks which offers a passage to the Rhine, in exposing the strata on its ab- 

 rupt sides, convinced us of the vast thickness to which the strata must have been 

 successively accumulated ; although at the same time it taught us that they have 

 been subjected to many convolutions, breaks and repetitions. Making, however, 

 every allowance for such phsenomena, as well as for the repeated interference 

 of volcanic rocks (with evidences of which the region is replete), we were still 

 enabled to detect certain dominant points of flexure, and to establish certain centres 

 of elevation, by which the thickness of the fossiliferous grauwacke of the Rhine, 

 which we place on the parallel of the Silurian system, is proved to be enormous J. 



* For example, in this part of the series we meet with two species of small Orthis, one very closely 

 allied to Orthis orbicularis, and the other to Leptana ( Orthis) lata of the Silurian system. 



t Delthyris macroptera and D. microptera are considered by MM. de Verneuil and d'Archiac as one 

 species, and as the Spirifera macroptera of Goldfuss. 



X According to barometrical admeasurements of Berg Meister Schmidt, the river Sieg below Siegen is 

 714, and the Dill at Dillenburg 679 feet above the sea. The height of the ground between the two 

 towns near the Kalten Eich is 1773 feet. The altitudes of the hilly region may be said to vary from 



