THE SILURIAN PERIOD 



351 



mingham, Alabama, Clinton iron ore, coal, and limestone are found 

 together. This fortunate combination has made that region one 

 of the great centers of the iron and steel industry, and a place of 

 much importance in the industrial upbuilding of the southern states. 



The interior sea again enlarged. As the period progressed, 

 the sea seems to have encroached slowly upon the land, much 

 as it did during the Cambrian. One broad arm extended 

 northward across Canada and perhaps into the polar regions. 

 As the lands were worn 

 lower and the shores ad- 

 vanced eastward in the 

 United States, the zones 

 of deposition migrated 

 accordingly, so that not 

 only did the Medina 

 sandstone come to over- 

 lap the Oneida conglom- 

 erate, but the limestone 

 of the West, with its 

 peculiar iron-bearing 

 shoreward phase, over- 

 spread the Medina as 

 far as central New York. 

 From the fact that its 



massive layers form the FlG - 357. Diagram of a limestone cliff in 

 ,.,. , . , Montana, showing levels (x) at which fos- 



Ciltt Over Which the sils of different ages were found. 



How may the absence of Silurian fossils 

 be explained ? 



Niagara River plunges 

 in its famous cataract, 

 the limestone is known as the Niagara formation. It is, of 

 course, much thicker in the Mississippi Valley, where it seems 

 to have accumulated through most of the period, than in the 

 New York region, where it began to be deposited considerably 

 later. The Silurian furnishes an illustration of the well-known 

 fact that a single rock formation in one part of the country 

 may be equivalent in time to several distinct and unlike 

 formations in another place. 



