CHAPTER VII 



THE ORDOVICIAN (LOWER SILURIAN) PERIOD 



Origin of Name, Subdivisions, etc. 



In the preceding chapter we learned how the basal portion of 

 Murchison's great Silurian system came to be called the Cambrian. 

 In 1879 Lap worth proposed to divide the remaining Silurian system 

 into two parts, the lower portion to be called Ordovician, and the 

 upper to retain the name Silurian. The term Ordovician was taken 

 from an old tribe (Ordovici) which once inhabited Wales. When 

 it is realized that one of the most profound stratigraphic breaks 

 (unconformities) in the whole Paleozoic group lies within Murchi- 

 son's old Silurian system, and between what are now called the 

 Ordovician and Silurian systems, the justification of Lapworth's 

 proposal is evident. In America and England the Ordovician 

 system is now generally recognized, though on the continent of 

 Europe the term Lower Silurian is still largely employed instead. 

 The following tabular arrangement will serve to make clear the 

 history of these terms : 



(Murchison, 1835) 



(Sedgwick) 



(Lapworth, 1879) 



{ Upper Silurian 



Silurian system { T aM . 



Lower feilurian 



Upper Silurian 

 f Lower Silurian 

 \ Cambrian 



Silurian 



Ordovician 



Cambrian 



Since the North American Ordovician was first carefully studied 

 in New York state, the section there has become, to a very consid- 

 erable degree, the standard to which the subdivisions in other 

 parts of the continent are referred. During recent years several 

 unconformities, though rather minor ones, have been discovered 

 in the New York Ordovician, so that this section is not as perfect 

 or continuous (stratigraphically) as was formerly supposed, certain 

 records being entirely missing. Following are the principal sub- 

 divisions of the New York Ordovician system according to the 

 most recent classification by the State Geological Survey: 



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