394 SILURIAN PERIOD 



diversity of temperature in the Silurian period, and much to suggest 

 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. This probably was one of the shorter Paleozoic periods. 



Foreign 



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 widespread concealment. In most of 

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

 little deformed. In contrast with the Silurian rocks of the northern 

 province, those of the southern are much deformed. 



There were important geographic changes in some parts of 

 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). In some parts of western Europe there were 

 overthrust faults of great extent. Locally (Scotland) the thrust was 

 as much as ten miles, 1 and had for a result, the thrusting of Cam- 

 brian and even Archean formations, over the Silurian. 



The Ordovician and Silurian of other continents have not been 

 generally distinguished. Equivalents of the two systems probably 

 occur in all the less well-known continents. 



LIFE 



The extensive withdrawal of the sea from North America at 

 the close of the Ordovician period reduced the area of shallow water 

 available for the life which needed it. The severe repressive evolu- 

 tion 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 constitutes the great 

 biological feature of the middle of the period. Toward its close, 

 there was restriction of the epicontinental sea, complicated with 

 intense salinity in the eastern interior, and there followed a second 

 repressive evolution through which the Silurian fauna passed into 

 the Devonian. 



Theoretically, the history of the land life should have been the 



!Quart. Jour. Geol. Soc., 1884 and 1888. 



