780 THE WONDERS OF GEOLOGY. Lect. VIII. 



products were erupted prior to the granite of those regions. 

 Even in the Devonian strata near Tavistock, and in South 

 Devon, volcanic ash is intermingled with the argillaceous 

 slates and limestones.* 



17. Silurian strata of Europe and America. — From 

 the short period that has elapsed since the establishment of 

 the Silurian System, it is scarcely possible accurately to deter- 

 mine the position of all the foreign sedimentary deposits, for- 

 merly known to geologists by the general term of Transition 

 rocks ; but so far as recent observations have ascertained 

 the characters and relations of the most ancient fossilifero us 

 strata on the Continent, they are all referable to the same 

 geological period as the Silurian formations of England. 



In France the oldest palaeozoic rocks are unquestionably 

 Silurian. In Bohemia, and around Prague, similar strata 

 are largely developed, and abound in trilobites, and other 

 characteristic fossils. f 



Throughout Scandinavia crystalline rocks occupy the 

 surface of the country to a vast extent, and are covered in 

 many places by sedimentary strata containing Silurian 

 fossils. In Christiania, Lower Silurian deposits occupy a 

 long trough of primary rocks ; and the little islands in the 

 Bay contain Upper Silurian strata. J 



Throughout a large part of the province of Skaraborg, 

 in the south of Sweden, the Silurian strata are perfectly 

 horizontal; the different subordinate formations of sand- 

 stone, shale, and limestone, occurring at corresponding 

 heights, in hills many leagues distant from each other, with 

 the same mineral characters and organic remains. It is 

 clear that they have never been disturbed since the time 



* Memoirs of the Geological Survey of Great Britain. 



t Barrande, on the Silurian System of Bohemia. See Geological 

 Journal, vol. iii. part ii. 



t See Sir R. I. Murchison's Memoir on the Geology of Sweden, in 

 Geological Journal, vol. iii. part i. 



