396 GEOLOGY. 



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

 Ordovician, suggesting clearer seas. In Great Britain, the three prin- 

 cipal groups of the system, commencing below, are (1) the Llandovery, 

 (2) the Wenlock, and (3) the Ludlow. 



In general the Silurian rests conformably on the Ordovician, but 

 this is not true in the British Isles, where the younger system rests 

 on the upturned and eroded strata of the older. As in America, the 

 system contains little igneous rock. 



Its thickness is less than that of either of the two preceding systems, 

 being but 3000 to 5000 feet in Great Britain. It is perhaps need- 

 less to add that this lesser thickness does not necessarily denote a 

 shorter period of time. It may equally well stand for a less rapid 

 rate of accumulation. 



The Ordovician and Silurian of other continents have not been 

 generally distinguished. The equivalents of the two systems as dis- 

 tinguished in Europe and North America probably occur in all the 

 less well-known continents. Strangely enough, the faunal resemblance 

 of the Silurian of other regions, even of other continents, to that of 

 the northern province of Europe, is stronger than that of this province 

 to the southern province of the same continent. 



Climate. 1 



There is nothing to indicate great diversity of temperature in the 

 Silurian period, and much to suggest that its uniformity extended 

 through great ranges of latitude, for the fossils of warm-temperate 

 latitudes are in part the same as those in Arctic regions. Certain 

 regions (New York and vicinity) appear to have been temporarily 

 (Salina epoch) arid. 



THE SILURIAN LIFE. 



The extensive withdrawal of the sea from the surface of the North 

 American Continent at the close of the Ordovician period very greatly 

 reduced the area of shallow water available for the forms of life that 

 had so richly peopled it during that period. A repressive evolution 

 of much severity necessarily followed. This constitutes the great 

 biological feature of the transition from the Ordovician to the Silu- 

 rian. With the extensive re-invasion of the interior by the Mid-Silurian 



1 Schwarz, Jour. Geol., 1906. 



