O N WA R D THR OUGH THE A G ES. 



123 





N I 



Fig. 55. What the North Arner'cm Continent had become at the end of the 

 Silurian Age. (The modem continent is indicated by dotted lines; the 

 rivers by broken lines.) 



during the Lower Silurian Age will answer for a represen- 

 tation of the nature of the events which followed during 

 the Upper Silurian and Devonian ages. Successive extinc- 

 tions, wrought by the lapse of time, or by violent geological 

 revolutions, followed by successive creations of higher and 

 higher forms, and the annexation of successive belts to the 

 pre-existing land — these constituted the great secular feat- 

 ures of the world's history down to the dawn of the period 

 when air-breathing animals were to have birth (Fig. 56). 



The first period of the Upper Silurian was that during 

 which the Niagara limestone was accumulated — a forma- 



