THE ORDOVICIAN PERIOD 



519 



on the other hand, the igneous rocks are interstratified with sedi- 

 mentaries, and are therefore thought to have been ejected beneath 

 water. 1 This is one of the most extensive, as well as one of the most 

 ancient, volcanic tracts of Europe. From north England and 

 Wales the system thins in all directions. In Scandinavia and 

 Russia it has but a fraction of the thickness which it possesses in 



Fig. 384. Diagram showing the relations of land and water in western 

 Europe in the Ordovician period. The shaded parts represent areas of 

 marine sedimentation. (After DeLapparent.) 



Britain. In southern Europe the system does not attain great 

 thickness, and limestone is more abundant than in the north. The 

 strata are exposed about various mountains where local disturbances 

 have upturned them, and where erosion has cut off the beds which 

 once overlay them. 



In Bohemia, though the system does not appear at the surface 



1 Geikie, op. cit., pp. 946 and 949. 



