164 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 31, 



In Britain, however, the Pentamerus oblongus is unquestionably a 

 Lower Silurian type, being found in the Llandeilo rocks as well as in 

 the Upper Caradoc, and never in the Wenloek formation*. 



But at whatever horizon the division be draw^n ( — a horizon 

 which every far-travelled geologist knows must vary in different 

 countries — ), it is undeniable, that in Scandinavia, Russia, and 

 Bohemia there is not the slightest trace of that local dislocation 

 which has partially affected the British strata between the Lower and 

 Upper Caradoc. 



In Norway and other parts of Scandinavia, the overlying strata of 

 shale, limestone, and sandstone which are laden with Upper Silurian 

 forms are everywhere perfectly conformable to the Lower Silurian ; 

 and, even in the Bay of Christiania, the Wenloek limestone and shale 

 charged with a profusion of Corals and Shells are seen to undulate 

 upon and with underlying masses of the black Lower Silurian slates, 

 in the horizontal space of a few hundred yards. 



Feeble in vertical dimensions as the Upper Silurian is said to be 

 by M. Kjerulf, his statement is quite in accordance with my own 

 observations, which would not assign more than 80 or 90 feet to the 

 limestone subdivision, and 70 or 80 to the flag-like sandy strata 

 which, containing Chonetes {Leptcena) lata and other fossils, repre- 

 sent the Ludlow rocks, and pass up into the bottom-beds of the Old 

 Red Sandstone, as seen on either side of the great plateau of Ringe- 

 rigge exposed in my original section. Even in Gothland, that large 

 island which is exclusively composed of Upper Silurian rocks, M. de 

 Verneuil and myself could never recognize a greater united thickness 

 than 200 or 300 feet, including a sandy representative of the Ludlow 

 rocks, with some subordinate courses of an oolitic limestone. Yet, 

 there also the fossils are very decisive of the age of the rocks, parti- 

 cularly those of the Wenloek limestone ; whilst the meagre represen- 

 tative of the Ludlow rocks, whether seen at Mount Hoburg in Goth- 

 land or near Ofved-Kloster and the Lake Ring in Scania, where it is 

 a purplish tilestone, is w^ell marked by its OrthonotcBy Cypricardice, 

 Leptcena lata, Avicula retroflexa, and Cytherina (Lepej'ditia) 

 Balfhicaf. 



Having previously dwelt upon the great geological value of such 

 data and comparisons in showing that the age of ancient deposits is 

 never to be indicated by their thickness merely, I have naturally 

 great satisfaction in seeing my views confirmed, as respects Norway, 

 by an accurate local observer. 



In addition to the indications on his map, M. Kjerulf has commu- 

 nicated to Mr. D. Forbes a few notes, both on the sedimentary 

 rocks and on the eruptive and metamorphic rocks. 



Thus, in speaking of the Devonian or Old Red Sandstone, which, 



* M. Kjerulf classes the Pentamerus-limestone with the Upper Silurian, and is, 

 I have no doubt, correct ; for in Norway, as in Courland, the Pentamerus oblongus 

 is almost always associated with a profusion of Wenloek species. — [R. I. M., 

 May 23, 1855.] 



t Quart. Journ. Geol. Soc. vol. iii. pp. 25, 34, &c. 



