THE SILURIAN PERIOD 



113 



and Saginaw, Michigan. The usual method of obtaining the salt 

 is by pumping brine from deep wells, and then evaporating. 



Much gypsum is mined along the lines of outcrop of Cayugan 

 strata in western New York. 



Oil and gas are obtained from the Clinton sandstone of Ohio, 

 and some gas from the Medina sandstone of New York. 



Life of the Silurian 



Plants. — Sea-weeds, though not abundant as fossils, are well 

 known, especially in the Medina-Oneida sandstones and con- 

 glomerate, and Clinton formation, all of which were deposited in 

 very shallow water. Knowledge of the land plants of the period 

 is still very meagre, though some rather doubtful specimens are 

 known. Perhaps the most authoritative example is a fossil Fern 





a b 



Fig. 62 

 Silurian and Devonian Corals: a, Cup-coral, Zaphrentis roemeri (M. Edwards 

 and Haime) (Devonian form); b, Honeycomb-coral, Heliolites pyriformis 

 (Guettard); Chain-coral, Haly sites catenulatus (Linn.). 



found in France, and this shows that the Pteridophytes at least 

 were in existence. Considering the profuse land vegetation of the 

 next (Devonian) period, it seems certain that their progenitors 

 must have been well represented in the Silurian, and that either 

 their remains may yet be discovered, or the conditions for their 

 preservation were unfavorable. 



Protozoans have not been found as fossils, but they must have 

 existed, because they are known from both the preceding and suc- 

 ceeding periods. 



Porifers. — Sponges were common, and in the Silurian strata 

 of western Tennessee they are exceedingly abundant. A genus of 



