S6 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ June S, 



If among these we recognised several of our characteristic 

 British species, we were indeed much struck with the profusion of 

 new forms. This fact is the more remarkable considering the very 

 small areas occupied by the chalk in Scania in reference to its enor- 

 mous development in Britain, Russia, and other countries. 



To the overlying erratic and modern deposits of Scania T have 

 adverted in a former communication. 



Conclusion. 



In terminating this memoir, I have to remark, that the examina- 

 tion of certain districts of Scandinavia, made during last summer 

 in company with M. de Verneuil, has substantiated the conclusions 

 at which I had previously arrived by visits to other tracts of that 

 region, and by an inspection of many collections, that the Silurian 

 system is there most clearly separated into Upper and Lower groups, 

 and cannot usually be further subdivided. 



In Norway the Lower Silurian rocks are chiefly schists and black 

 limestones ; in large tracts of Sweden they are more expanded, and 

 with a sandstone base containing no fossils except fucoids, and with 

 the same alum-bearing schists as in Norway, the overlying calca- 

 reous matter expands into masses of considerable thickness, laden 

 with Orthoceratites and Trilobites, and surmounted by black Grap- 

 tolite schists. 



In Russia the lower member of these strata consists of soft shales 

 with a few fucoids only (a mere unconsolidated mudstone), which is 

 overlaid by a sandstone and grit, sometimes a conglomerate, con- 

 taining the peculiar shells Ungulites or Oboli with a few Orbiculae. 

 These beds, the lowest containing fucoids only, as in Sweden, and 

 the next having small horny bivalves, are surmounted by thick beds 

 of earthy limestone not harder than our most slightly coherent 

 secondary strata, in which a great mass of fossils are distributed, 

 many of them, particularly the Orthidee with simple plaits, being 

 typical of Lower Silurian strata in different parts of the world. 



Now, with this variation in mineral characters, or in other words, 

 with this change of the original condition of the deposit, we are pre- 

 sented in each region with local peculiarities of zoological develop- 

 ment. Certain genera, and even the same species of Trilobites, Or- 

 thidae and Orthoceratites, are indeed common to the lower group of 

 each of these northern kingdoms ; but a species which is common in 

 the one is often very rare in the other, and each district has a 

 greater or less number of fossils peculiar to it. 



If even then within the limits of the tracts around the Baltic, 

 where no contemporaneous eruptive operations have interfered with 

 the deposits of this age, we find that the contents of rocks (pre- 

 cisely on the same parallel) vary considerably in their zoological 

 contents, we might expect that the Lower Silurian type of Great 

 Britain and Ireland should vary according to the conditions of de- 

 posit in districts remote from each other even in our own islands, 

 concerning which it must not be forgotten, that the thick marine 



