THE ORDOVICIAN SYSTKM 149 



bad-areas, and the position of this land seems to be fairly well 

 indicated by the fact that the fauna found in Bohemia differ* 

 greatly not only from that of Northern Europe but also from those 

 of the French and British Ordovician. Thus the number of 

 species common to the Ordovician of Britain and Sweden is much 

 larger than the number common to Sweden and Bohemia. It 

 seems clear, therefore, that the Bohemian area belonged to a 

 different life province, and that the land from which the Bohemian 

 sediments were derived lay to the south and south-east of that 

 country, over the region now occupied by Hungary, Roumania, 

 Turkey, the Black Sea, and Asia Minor. 



It is probable that nearly the whole of Central and Western 

 Europe was covered by the Ordovician Sea, though there may 

 have been some islands of a fair size. We have indeed evidence 

 of one such island in the centre of the English area, for the 

 absence of Arenig, Llanvirn, and Llandilo rocks to the west of the 

 Longmynd proves that part of Shropshire was land during the 

 greater portion of the period, while the presence of the Caradoc 

 sandstone shows that it was submerged (in part at least) toward 

 the close of Ordovician time. 



Of the size and extent of this island we have little to guide us 

 in forming an opinion, but it cannot have reached very far north 

 since Ordovician strata occur at Settle in Yorkshire, and its chief 

 extension was probably eastward. Where the Cambrian is exposed 

 in the Midland counties, as in the Lickey Hills (Worcestershire) 

 and near Nuneaton (Warwickshire), we find it succeeded either 

 by Silurian or Carboniferous rocks, so that probably the whole of 

 the Midland area from Shropshire to Northamptonshire was land 

 during the Ordovician period. 



Another island may have existed in the area of the Ardennes 

 where Devonian rocks rest directly on Cambrian. 



With regard to the Atlantic region the existence of continental 

 land to the west and north-west of Ireland at this time is proved 

 not only by the enormous thickness of Ordovician deposits in 

 Mayo, but also by the presence of American forms of trilobites in 

 the Arenig limestones of that area (see p. 141). It will be re- 

 membered that the Cambrian limestones of Scotland contain a 

 fauna of American affinities, and this fauna is supposed to have 

 migrated along the shores of a North Atlantic land. In Arenig 

 time this land seems to have been greatly enlarged so as to touch 

 the north-west of Ireland, and thus to afford similar facilities for 

 the migration of American species to the Irish area. 



How far this Atlantic continent extended over Scotland we do 

 not know at present, but though the old supposition that parts of 



