348 HISTORICAL GEOLOGY 



closed the Ordovician period is frequently spoken of as the 

 " Taconic revolution." 



Similar events in Europe. In Europe the deposition of 

 sediments in Ordovician time was in many ways like that in 

 the United States, and at the close it suffered a similar inter- 

 ruption. The rocks of Wales and Scotland were highly folded 

 into a series of mountains which were gradually worn down 

 during the Silurian period. The fact that the crust was 

 simultaneously wrinkled on both borders of the Atlantic Ocean 

 suggests that a slight subsidence of the great oceanic area may 

 have been directly responsible for the disturbance. Yet it 

 cannot be said that this is proved. 



QUESTIONS 



1. Sun cracks have been found on the bedding planes of the 

 Lower Ordovician limestone in the Mississippi Valley. From this, 

 what do you infer as to the depth of the water in which this lime- 

 stone was deposited ? How does this compare with limestones in 

 general ? 



2. Judging from what you know of the Archaean and Algonkian 

 systems, what was the character of the rocks from which the Ordo- 

 vician sediments were derived ? Why does the Ordovician system 

 consist of limestone, shale, and sandstone, rather than pieces of these 

 older rocks cemented together? 



3. What is the chief process of change at work on the surface 

 of a land which is too low to be eroded by streams ? 



4. Can you suggest why nothing is known about the sediments 

 which were deposited in the Ordovician period off the eastern shore 

 of Appalachia ? 



5. What phase of metamorphism would be most likely to obliter- 

 ate all traces of fossils in the Ordovician rocks of the Taconic Moun- 

 tains ? 



6. Can you see any reason for thinking that vertebrates were 

 in existence long before the fishes whose plates have been found in 

 the Ordovician rocks ? 



