62 THE STORY OF THE EAETH AND MAN. 



oceanic areas similar to those of the Lower Silurian ; 

 In these we find vast and thick coral and shell lime- 

 stones, the Wenlock of England and Niagara of Ameri- 

 ca, as rich in life as the limestones of the Lower Silu- 

 rian, and with the generic and family forms similar, 

 but the species for the most part different. In America 

 these limestones were followed by a singularly shallow 

 condition of the plateau, in which the surface was so 

 raised as at times to be converted into separate salt 

 lakes in which beds of salt were deposited. On both 

 plateaus there were alternations of oceanic and shal- 

 low conditions, under which the Lower Helderberg 

 and Ludlow beds, the closing members of the Silu- 

 rian, were laid down. Of the Atlantic beds of this 

 period we know little, except that the great lime- 

 stones appear to be wanting, and to be replaced by 

 sandy and muddy deposits, in some parts at least ot 

 the margins of the area. In some portions also ot 

 the plateaus and their margins, extensive volcanic 

 outbursts seem to have occurred ; so that the Ameri- 

 can plateau presented, at least in parts, the aspect of 

 a coral sea with archipelagoes of volcanic islands, the 

 ejections from which became mixed with the aqueous 

 deposits forming around them. 



Having thus traced the interesting series of geo- 

 graphical conditions indicated by the Silurian series, 

 we may next take our station on one of the submerged 

 plateaus, and inquire as to the new forms of life now 

 introduced to our notice ; and in doing so shall include 

 the life of both the Lower and Upper Silurian. 



