SILURIAN SYSTEM: AGE OF INVERTEBRATES. 



297 



Also as narrow bands on the flanks of nearly all the Eockv Mountain 

 ranges ; 5. Also considerable areas in Basin region the outlines of which 

 are little known. 



Physical Geography. — At the beginning of the Silurian (Primor- 

 dial), as already said, the land was approximately the Laurentian area 

 (Fig. 26 6). The Silurian, which embraces the great Y-shaped Lau- 

 rentia area on the southeast, south, and southwest, was then the sea- 

 bottom border of the coast of the Primordial continent. The Silurian 

 bordering the Appalachian Laurentian was also then a sea-bottom 

 bordering the Primordial continent in that region. It is probable, 

 also, that the Silurian of the Rocky Mountain region also borders 

 Laurentian areas, and these areas represent Primordial continents, and 

 the Silurian border the marginal sea-bottom of that time. The other 

 patches mentioned in the interior were probably bottoms of open seas. 



Now, the Silurian area represents so much of Silurian sea-bottoms 

 as were raised into land-surfaces during or at the end of Silurian times, 

 and not subsequently covered by sea.* Therefore, at the beginning of 

 Silurian times the land was the Laurentian area ; while at the end of 



sc 



Fig. 268.— Geological Map of New York: a, Archaean: PS. Primordial: LS. Lower Silurian; US, 

 Upper Silurian; d, Devonian; SC, Subcarboniferous; C, Coal-measures. 



the Silurian times the land was increased by the addition of the Silurian 

 area. This addition was not all made at once, but very gradually. 



* This is true as a broad, general fact ; but patches of Silurian may also be exposed 

 by removal of later deposits by erosion. 



