PLATE 4 

 Silurian Tectonic Map 



The Atlantic Ocean and Gulf of Mexico are left uncolored because accumu- 

 lating evidence suggests that North America was once attached to and part 

 of a single great continent which cracked and drifted apart. The spreading 

 apart is presumed to have brought these ocean basins into existence, starting 

 in late Paleozoic time. 



