SILURIAN SYSTEM 189 



Sea of Western and Central Europe seem to have remained 

 uniform for a considerable time and with little change except that 

 of a slow subsidence ; so that little but shale was deposited, while 

 here and there bods of limestone were accumulated, especially in the 

 north-east. During the later part of the period crust movement 

 took place, causing elevation and raising much of the northern 

 part of the region above the level of the sea. 



With regard to the composition and construction of the Silurian 

 deposits some interesting questions present themselves, especially 

 in connection with the limestones. These often consist of a varied 

 assemblage of organic remains, all the creatures then living in the 

 sea which possessed calcareous shells or structural parts contributing 

 to their formation, but occasionally some particular class of animals, 

 or even one particular species, flourished in such numbers at 

 certain places that their remains constitute almost the entire mass 

 of the resulting bed of limestone. 



Thus in some parts of the sea-floor there must have been fields 

 of Crinoids, growing in myriads as they do now in parts of the 

 Caribbean Sea. By the growth and death of successive generations 

 of these fixed " stone lilies," and by the subsequent disintegration 

 of the stems, cups, and arms, beds of crinoidal limestone were 

 slowly accumulated. 



In other places certain species of Brachiopoda flourished in 

 such numbers that they must have occupied the sea-floor almost to 

 the exclusion of other creatures, and their remains consequently 

 form a Brachiopod limestone. Occasionally, too, one species so pre- 

 dominates over the others as to be the main constituent of the 

 limestone, as in the case of the Pentamerua borealis limestone of 

 Russia (see p. 183). 



Other important components of Silurian limestones are Stroma- 

 toporoids and Corals which often occur together, but sometimes 

 separately form the chief constituent of a bed or mass of limestone. 

 Some coralliferous limestones are of such a nature that it has been 

 imagined they were coral-reefs of the same kind as those built up 

 by corals at the present day. This, however, is very doubtful for 

 the following reasons. 



In the first place, modern coral-reefs are either shore-formations 

 resting unconformably on other rocks (fringing reefs), or they are 

 steep-sided masses of limestone built up in the open sea with deep 

 water on one or both sides of them ; and in areas of subsidence 

 such reefs often reach to a great vertical height from the level from 

 which they started. Now the only ancient coral-reefs that are 

 likely to have been preserved are those which had been formed in 

 areas of subsidence, where their lower parts would be flanked and 



