SILUHIAN SYS'I KM 181 



* 



IV 11 into the surrounding sea and were spread out on it* 

 floor, while between each epoch of eruption marine sediments of 

 tin- ii-iml kind were greatly accumulated. The part of the sea in 

 which these deposits were formed was probably not very far from 

 tin- >hores of a continent occupying a large area in what ifl now the 

 Atlantic Ocean. 



D. CONTINENTAL EQUIVALENTS 



Rocks of Silurian age occur in Scandinavia, Russia, Poland, 

 Thuringia, Bohemia, Carinthia, the Alps, Belgium, France, and 

 Spain. We can only refer to the more important areas. 



1. Scandinavia 



Undisturbed and unaltered Silurian strata not only occur in 

 the same parts of Norway and Sweden, and succeed those of the 

 Ordovician System (see p. 146), but they also form the island of 

 Gottland in the Baltic, which thus becomes a link between the 

 Swedish and Russian areas (see map, p. 66). 



The Silurian Series exhibits several different facies in different 

 parts of the Scandinavian region, and shows great variation in 

 thickness. It is most fully developed in Scania, where sedimenta- 

 tion in the form of graptolitic shales continued throughout the 

 whole of Valentian and Wenlock times, a bed of limestone only 

 occurring at the top of the system, the full thickness of which is 

 there about 4000 feet. This great series is divisible into zones 

 which can l^e correlated with those of Great Britain. 



In the Christiania district the deposits are very different, con- 

 sisting of an alternating series of limestones and shales, and the 

 total thickness is not much over 1200 feet, but is not complete 

 at top. 



In Oland and Gottland, again, to the east the facies is essentially 

 calcareous, shales prevailing only in the lowest part, and the whole 

 system is condensed into a thickness of something less than 200 feet 

 in Gottland. The base, however, is not seen, though the sequence 

 extends down as low as the equivalent of our May Hill sandstone 

 (Upper Llandovery), neither do the highest beds i-onu- into these 

 islands. * 



The Gottland limestones are shall< -\\-\\, it<-r and not deep-water 

 deposits. 23 They consist largely of crinoidal limestones, but these 

 are replaced locally by beds which are so largely composed either 

 of Stromatoporoids or Corals (or of both) that they have clearly 

 been formed by the growth of these organisms in situ. There 



