CHAPTER XV 



Transition from Ordovician to Silurian. In eastern United 

 States and western Europe the Ordovician period seems to be 

 distinctly set off from the Silurian by the so-called Taconic 

 revolution. Elsewhere, however, the transition from the 

 one to the other was quiet and not marked by notable disturb- 

 ances. Some portions of this continent emerged from the sea 

 and became low plains, from the surfaces of which little debris 

 could be eroded. In Oklahoma, on the other hand, and in 

 the western states generally, the surface appears to have 

 remained submerged beneath the sea. These things are 

 clearly shown by the succession of the sedimentary rocks. 

 Thus, as mentioned on a preceding page, the Silurian strata 

 lie in marked unconformity upon the folded Ordovician rocks 

 in the New England region. In Tennessee, Minnesota, and 

 some other states, the two systems are parallel in bedding, 

 but are separated by an irregular weathered surface which 

 is in reality an unconformity. In Utah and Montana the 

 Silurian system is only a part of a thick succession of lime- 

 stones which contain Ordovician fossils below and Devonian 

 fossils above. 



Clastic sediments along the eastern land. The oldest 

 sediments referred to the Silurian period are unlike in different 

 parts of the country. Along the western flank of the newly 

 made eastern highlands quantities of gravel and sand brought 

 down by swift rivers were spread out in thick banks which 

 thinned toward the west. The gravel, now consolidated into 

 hard conglomerate, is known as the Oneida formation. Where 

 it has since been tilted up on edge it forms mountain ridges, 

 because the softer rocks on each side of it have been more 

 B. & B. GEOL. 20 349 



