of the Northern and Central Regmis of Bassh. 131 



the equivalents of the Silurian system of the British Isles. Tlie detailed 

 order of these beds was long ago given by Strangwaya ; but at the early 

 day when he wrote, the study of organic remains was not sufficiently ad- 

 vanced to enable him to determine the exact place of these beds in the 

 geological series, nor to point out their true relations to the adjacent 

 masses. Many of the fossils have since been described by the native 

 authors, Pander and Eichwald, and recently some very characteristic 

 forms by M. de Bucli. 



The Silurian deposits consist in ascending order, of blue clay, interme^ 

 diate grity and overlying limestone) &c. In the first of these no organic remains 

 have yet been found ; and the intermediate sandstone or grit is alone dis- 

 tinguished by a remarkable form unknown in western Europe {the Ungu- 

 lite), which the authors consider to be nearly allied to Orthis. In the 

 limestones, and certain overlying. flagstones first described on this occa- 

 sion, organic remains abound ; and they agree well in the leading cha- 

 racters on which the Silurian system was established, viz. that the forms 

 of Trilohite, Orthoceratite, and Orthis, are distinct from the types of the 

 overlying members of the palaeozoic series. 



The most prevalent fossils are the Orthoceratites vaginatus, Asaphus 

 expansus, lU^nus crassicaiida, the peculiar Crinoidean Spheronites (allied 

 to the Ischadites of the Upper Silurian rocks), and a vast profusion of 

 many species of Orthis. Although, upon the whole, the Silurian fossils 

 of Russia difter more than those of Sweden from British species of the 

 same age (as might indeed be expected from their more remote distance), 

 certain shells are identical with those published from England ; among 

 which are enumerated, Leptcena depressa {L. rugosa, Dalm.), Leptcena 

 sericea, Lingula Lewisii, Orthis canalis {0. clegantula, Dalm.), &c. ; and 

 according to M. Eichwald, two or three species of Trilobites.* 



With the exception of some very trivial dislocations in the low hills 

 south of St Petersburgh, the Silurian rocks are so uniformly horizontal, 

 that in the fine quarries on the banks of the WolkofF, the authors were 

 able to prove a difference of 2° or 8° to the S.S.E. only by pouring water 

 on the surface of the rocks. 



These Silurian deposits occupy the islands of Oland, Gothland, &c. in 

 the Baltic, and trend along the shores of Courland in a broad band from 

 W.S.W. to E.N.E., till they are lost under vast heaps of granitic detritus 

 between the lakes Ladoga and Onega. Near the latter, these deposits 

 are deflected to the north, and there meet with great ridges of trappean 

 rocks, which run from N.N. W. to S.S.E. In that region all the deposits 

 are iu a metamorphic condition ; the limestones present no distinct traces 

 of fossils ; and the authors having satisfied themselves that there was no 

 chance of observing any further evidence of a descending order between 

 such rocks and the great primarized granitic chain of Scandinavia and 



* See Professor Eichwald's work, published since the authors' visit to 

 Russia, entitled *' Silurische-Scliicbten-system in Estliland." 



