SILURIAN SYSTEM 187 



The thickness of this series of beds has not been estimated 

 because of the frequent plications. There is also some doubt about 

 the position of the Erbray limestone which contains some Devonian 

 species, such as Spirifer Decheni, Cryphaus pectinatus, and at the 

 top Harpes venulosus. 



In Normandy (Cotentin) the Silurian similarly consists of sand- 

 stone at the base succeeded by black carbonaceous shales (ampelites), 

 the zonal details being at present unknown. 



In Southern France (Languedoc) the Silurian presents a series 

 of black shales and limestones about 650 feet thick. The lower 

 part yields Mon. priodon, M. Flemingi, and higher up M. colonus 

 and M. bohemicus. Black limestones at the top contain Cardiola 

 interrupta, Slava bohemica, Maminka comata, and other Bohemian 

 species (of E 1 and 2) with numerous fragments of Orthoceras, the 

 fauna being of essentially southern type. 



7. Pyrenees, Spain, and Portugal 



In these countries the Silurian has for the most part an 

 argillaceous facies like that of Brittany. In the Pyrenees it con- 

 sists of black carbonaceous shales containing graptolites of Valentian 

 and Wenlock species, but there are some calcareous shales, and in 

 the Haute Garonne a black limestone contains several species of 

 Orthoceras with Monograptus priodon. 



In Catalonia there is a more complete succession, as below : 



Black calcareous shales, Mon. colonus, M. Nillssoni. 



Soft black shales with M. priodon. 



Black nodular shales with M. turriculatus. 



Pale shales with M. proteus (Valentian). 



A similar succession is found in Portugal (Alemtejo and 

 Barrancos), where the top bed is a thin band of limestone com- 

 parable to that of Rozan in Brittany. 



E. CONDITIONS OF DEPOSITION 



The general geographical conditions over Northern and Western 

 Europe during the earlier part of Silurian time seem to have been 

 similar to those prevailing in the Ordovician period. There was 

 the same mass of continental land in the Atlantic area, along the 

 borders of which thick deposits of sand and mud were accumulated, 

 but there seems to have been more land over Central England and 

 in the area of the North Sea, for it was probably from such land 

 that the great mass of arenaceous material forming the Wenlock 



