546 GEOLOGY 



that uniformity extended through great ranges of latitude, for the 

 fossils of warm-temperate regions are in part the same as those in 

 Arctic regions. Some regions appear to have been temporarily 

 very arid. The Silurian period was perhaps comparable in duration 

 to the Ordovician. 



Foreign Silurian 



In Europe the Silurian strata have a distribution similar to that 

 of the Ordovician, though they are wanting in some regions where 

 the latter are present. The fact that the Silurian strata do not 

 appear at the surface over wide areas does not indicate their general 

 absence, so much as their wide-spread concealment. In most of 

 the northern part of Europe, outside of Britain, the system has been 

 little deformed. In the southern part of the continent, the Silurian 

 formations appear in small areas only amidst formations of lesser 

 age. In contrast with the Silurian rocks of the northern province, 

 those of the southern are much deformed. 



The Silurian formations of Europe, especially of the northern 

 province, are more largely composed of limestone than those of the 

 Ordovician, suggesting clearer seas. 



Geographic changes took place in Europe at the close of the 

 period as shown by the unconformity between the Silurian and 

 Devonian systems in some places (Great Britain and Ireland), 

 though conformity is the rule. 



The Ordovician and Silurian of other continents have not been 

 generally distinguished. The equivalents of the two systems as 

 distinguished in Europe and North America probably occur in all 

 the less well-known continents. 



THE SILURIAN LIFE 



The extensive withdrawal of the sea from the surface of North 

 America at the close of the Ordovician ^period reduced the area of 

 shallow-sea water available for the life which needed it. The severe 

 repressive evolution which followed was the great biological feature 

 of the transition from the Ordovician to the Silurian. With the 

 re-invasion of the interior by the mid-Silurian sea, there followed 

 an expansional evolution of the shallow-water fauna, which con- 

 stitutes the great biological feature of the middle of the period. 

 Toward the close of the period there was another restriction of the 



