SILURIAN AGE. 



165 



The area 5 (Niagara period) is divided into a, b, c, corresponding to 5 a, 5 b, 5 c, in 

 the preceding table. Other areas are similarly divided. The areas of Nos.° 7 and 8 in 



the series (the Lower Helderberg and the Oriskany) are not distinguished on the map 

 from No. 9. 



