112 HISTORICAL GEOLOGY 



of Europe are not as thick as those of the two immediately preced- 

 ing systems, being from 3000 to 5000 feet in the British Isles, and 

 generally less elsewhere. Igneous rocks of Silurian age are almost 

 unknown. 



In other continents Silurian rocks have seldom been well studied 

 and separated from the Ordovician, though they are definitely 

 known in China, Africa, Australia, and South America. 



Climate 



The general distribution and character of the rocks and their 

 fossil content point to more uniform climatic conditions than those 

 of today. Fossils in the Arctic Silurian rocks are not essentially 

 different from those of low latitudes. 



From central New York across to Michigan at least, there was 

 an arid climate during the Salina epoch, as already mentioned, but 

 this was probably only local. 



Economic Products 



Silurian sandstones and limestones are extensively quarried for 

 building purposes, or the limestones burned to make quick-lime. 

 The waterlimes of late Silurian age were until quite recently con- 

 siderably used for the manufacture of hydraulic cement, especially 

 in the Hudson Valley of New York state. 



We mentioned the widespread and almost universal occurrence 

 of hematite iron ore in the Clinton formation. This ore is mined to 

 some extent in central and western New York, but in the Birming- 

 ham, Alabama, district, which is the second greatest iron mining 

 region of America, the Clinton formation is the source of the ore. 



Another important economic product of Silurian age is the salt 

 of the Salina formation. In New York alone salt beds underlie 

 most of the western part of the state or an area of about 10,000 

 square miles. Sometimes there is one bed and sometimes several 

 interstratified with other rocks. Single beds locally attain a thick- 

 ness of from 50 to 80 feet. In the southern part of the state the 

 salt is most deeply buried under later rocks, a well at Ithaca having 

 passed through 248 feet of salt in seven beds below 2244 feet from 

 the surface. Toward the north the beds gradually come near the 

 surface. Important salt beds also occur near Cleveland, Ohio, 



