90 HISTORICAL GEOLOGY 



Climate 



Red sandstones, salt, and gypsum in the Upper Ordovician of 

 northern Siberia clearly imply an arid climate in northern Asia 

 during the late Ordovician. So far as can be determined from the 

 character of the rocks, geographic conditions, and distribution of 

 the fossils, the climate of North America and Europe must have 

 been mild and much more uniform than now. Ordovician fossils, 

 even from Arctic lands, are very similar to those of low latitudes. 



Economic Products 



Many great marble quarries are located in metamorphosed 

 Ordovician limestone in New England and the Piedmont Plateau. 

 Also much non-metamorphosed limestone is quarried for building 

 purposes or burnt for lime in various parts of the United States. 

 In the Lehigh district of Pennsylvania much Trenton (argilla- 

 ceous) limestone is used in the manufacture of Portland cement. 



Among the greatest lead and zinc ore deposits in the world are 

 those of the Mississippi Valley, especially in Missouri, Wisconsin, 

 Iowa, and Illinois. These ores, which were originally disseminated 

 through the limestones, were dissolved and redeposited in more 

 concentrated form in openings in the rocks. 



Manganese ores of Arkansas and phosphate deposits of Ten- 

 nessee occur in limestones of this age. 



The great oil and gas field of Ohio and Indiana derives its 

 principal supply from the Ordovician rocks, especially the Tren- 

 ton limestone. The oil and gas were formed by the decomposition 

 of the rich organic accumulations in the limestones. 



Life of the Ordovician 



Abundance of Marine Life. — The Ordovician epicontinental 

 seas literally swarmed with marine organisms, few systems contain- 

 ing a fuller record of marine forms than the Ordovician because of 

 very favorable conditions of fossilization. As regards both diver- 

 sity and abundance of known organisms, this period is far superior 

 to the Cambrian. Schuchert states that over 1600 species of 

 animals are known from the middle Ordovician alone. It is to 

 be noted, however, that, with very slight exception, Vertebrate 

 animals are not known to have existed in the Ordovician. Also 



