52 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 6, 



transition-zone from the Lower to the Upper Silurian is the deposit 

 laden with Pentameri, to which the name of Llandovery Rocks has 

 been assigned, because it was long ago described by me at and around 

 the town of Llandovery in South Wales. Whilst the Wenlock fauna 

 is quite recognizable in Esthonia, as in Norway, the fossils of the 

 highest zone of the former country, including many large Crusta- 

 ceans (Eurypteridce), with Lingula cornea and Trochus helicites, are 

 strikingly characteristic of the Ludlow rocks. 



Such agreement between those remote foreign rocks and our strata 

 of the same age is the more remarkable, when we consider the 

 difference in thickness and the simplicity of lithological characters 

 of the deposits in these northern countries, as compared with the 

 vastly extended British types. The thousands of feet of diver- 

 sified British Silurian rocks, whether slates, schists, shales, conglo- 

 merates, sandstones, quartz-rocks, limestones, mudstones, with vast 

 sheets of interstratified igneous rocks, are represented in the Baltic 

 provinces of Russia by little more than one lithological character 

 only. There, they are all closely united calcareous masses, any 

 one of which has so great a resemblance to another, that, without 

 the aid of Palseontology, they could never be separated or distin- 

 guished. The united Lower and Upper Silurian constitute, in fact, 

 to use the language of Count Keyserling, "but a single volume of 

 limestone of small capacity and thickness, capable of division into 

 leaves by that person only who is acquainted with their included 

 fossils ;" the whole series not exceeding 2000 feet, and all the strata 

 graduating into each other by conformity of deposition. 



The interest, however, which attaches to the full illustration of 

 these Northern European types of the oldest known fossiliferous 

 rocks, does not terminate with the precise comparison between them 

 and the British contemporary formations*. 



The same species, even, are found to prevail far to the westward in 

 northern latitudes. Small as are the deposits, in vertical dimensions, 

 both of Scandinavia and Russia, the very same succession of life 

 which they offer is found to be persistent from those countries 

 across the British Isles to the heart of North America. The vastly 

 extended Lower Silurian rocks of the United States and Canada, 

 and the enormously spread Upper Silurian of the Arctic Regions, 

 are, like those of our own country, the absolute historical equivalents 

 in time of the thin and simple series described on this occasion, and 

 exhibit a vast number of forms specifically identical with those I 

 have enumerated. 



Other species, however, of Silurian typical genera prevail through- 

 out the southern region of Europef. Already we know that the 

 Silurian rocks of France and Spain, as illustrated and compared by 



* It is true, that in Russia there are fewer British species than in Sweden and 

 Norway, but this is just what we might expect from the relative distances of 

 those lands from our own country. 



f With additional researches, may not this Silurian type of Southern Europe 

 and Siberia be found to have its equivalents in the central and southern parts 

 of America ? 



