CHAPTER XVII 

 THE SILURIAN 1 PERIOD 



This system has been divided into a number of subdivisions which 

 in New York are as follows : 



Rondout water lime 



Cobbleskill limestone 



Salina shales, salt, water lime 



Lockport and Guelph dolomites 



Rochester shale 



Clinton shale, sandstone, limestone, and iron ore 



Medina and Oneida sandstone and conglomerate 



In eastern North America the Silurian strata, for the most part, 

 rest unconformably upon the deformed and eroded Ordovician rocks. 

 In the Middle States the Lower Silurian is usually absent, and the 

 Middle Silurian strata rest unconformably upon the Ordovician or 

 upon older rocks, showing that during early Silurian times the central 

 portion of the continent was land. In Montana and Utah the strata 

 of the Ordovician, Silurian, and Devonian are apparently conformable, 

 and their separation is more or less arbitrary because of the scarcity 

 of fossils. 



Geography of the Silurian. — - The period can, for convenience, be 

 divided into three epochs, (i) During the first (early lower) the 

 epicontinental seas were apparently restricted to three principal bays 

 (Fig. 410) : one stretching up the Mississippi Valley to northern Illinois ; 

 a second extending across Newfoundland and northern New Brunswick; 

 and a third occupying the Appalachian trough, and stretching east 

 and west over central New York and Ontario. Later in the Lower 

 Silurian the seas were, for a time, withdrawn from the Appalachian 

 trough and New York. (2) The middle (later lower) of the period 

 (Fig. 411) saw an expansion of the seas over a large portion of Canada 

 to the Arctic Ocean and over the United States east of the Mississippi 

 River, and an extension of two seas on the west, one from California 



1 The name Silurian has been taken from the Silures, an ancient tribe which dwelt in Wales 



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