CLOSE OF ORDOVICIAN 377 



Foreign. — In Europe the Ordovician rocks appear to have been 

 laid down in two distinct seas separated by a ridge of land. The 

 northern area extends from Ireland far into Russia, while the 

 southern is represented by numerous scattered masses. These 

 rocks cover a much wider surface than do the Cambrian. In 

 Great Britain, especially in Wales, they form very thick masses of 

 shales and slates, with but little limestone, intercalated with much 

 volcanic lava and tuff. In Scandinavia these rocks are nearly 

 horizontal limestones and shales, and in Russia they cover very 

 large areas and are so perfectly undisturbed that many are still 

 incoherent sediments. In the southern sea were laid down the 

 Ordovician strata of Bohemia, Germany, northwestern and central 

 France, Spain, Portugal, Sardinia, and Morocco. 



The very marked difference between the fossils of the two great 

 European areas, and the fact that the Ordovician fossils of other 

 continents agree with those of northern Europe, while those of 

 the southern district are peculiar, indicate that the latter region 

 was a partially closed sea, which occupied the Mediterranean 

 basin, though extending far beyond its present limits. 



Asia appears to have been principally dry land during the Ordo- 

 vician, but with a broad Indo-Chinese sea covering its eastern 

 shore. Africa has Ordovician rocks only in the Mediterranean 

 region; in the southern part of the continent nothing older than 

 Devonian is known. The Australian continent of Cambrian times 

 was partially submerged in the Ordovician, and strata of this date 

 were laid down in New Zealand, Tasmania, and the southern part 

 of Australia. But little of South America was submerged and that 

 continent may have been considerably larger than at present. 

 Rocks of this system are known only from Bolivia and Argentina. 



Close of Ordovician. — The period was a time of tranquil depo- 

 sition of sediments, with some oscillations of level and changes in 

 the depth of water and position of the shore line, as indicated by 

 the alternations of the strata. At the close of the period came 

 a time of wide-spread disturbance, upheaval, and mountain mak- 

 ing, the traces of which may still be plainly observed in North 

 America and Europe, especially along the Atlantic slope of each 



