^Y WA RD THR OUGH THE A G ES. 



123 



Fig. 55. What the North Amer cin 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- 



