THE SILURIAN PERIOD 451 



existence of land plants in the preceding period. Nevertheless, the ab- 

 sence of the sediments of fresh-water lakes in America, where land 

 fossils are likely to be preserved, leaves us without evidence, although 

 it does not prove that there were no land plants or animals. 



Migration. — The presence of thirty or more identical species in the 

 Silurian strata of Iowa and northwestern Europe indicates that migra- 

 tion between the two continents took place. This presumption is 

 strengthened by the discovery of a peculiar genus of coral (Fig. 413 E, 

 p. 444) whose quadrangular opening was protected by a calca- 

 reous covering. Since the interior seas of North America had no free 

 communication on the east, it is thought that the migration took 

 place along a belt of shallow water which extended through Canada, 

 Alaska, and perhaps the Arctic region. 



Climate and Duration. — The climate of this period seems to have 

 been uniform over the entire world, as during the preceding periods, 

 there being no positive evidence of the existence of climatic zones. 

 The presence of salt and gypsum beds, locally 40 to 80 feet in thick- 

 ness, in the Silurian strata (Salina) of New York and Ohio is evidence 

 that desert conditions prevailed during a portion of the period, prob- 

 ably over a considerable area. 



The Silurian Period was probably not more than one half as long 

 as the Ordovician. 



Close of the Silurian. — The change from the Silurian to the 

 Devonian in eastern North America is even less clearly marked than 

 that between the Ordovician and the Silurian, the formations of one 

 often passing into the other without an unconformity. In portions of 

 Great Britain an unconformity separates the two systems, but in 

 other parts of Europe there is no break in the sedimentation. The 

 separation in such cases is based upon the fauna. 



REFERENCES FOR THE SILURIAN PERIOD 



Blackwelder and Barrows, — Elements of Geology, pp. 349-357. 



Chamberlin and Salisbury, — Geology, Vol. 2, pp. 368-417. 



Grabau and Shimer, — North American Index Fossils, Vols. I and 2. 



Schuchert, Chas., — P ale o geography of North America: Bull. Geol. Soc. America, 



Vol. 20, 1910, pp. 489-491. 

 Scott, W. B., — An Introduction to Geology, pp. 578-589. 

 Ulrich, E. O., — Revision of the Paleozoic Systems: Bull. Geol. Soc. America^ Vol. 22, 



1911. 



