ART. 21 



ORDOVICIAN TRILOBITES ULEICH 



73 



and their origin and evolution and of the physical changes that 

 caused their local advent and extinction or permitted intermigration 

 of diversely originating types that is needed to dispel prevailing 

 obscurities. 



DISCUSSION OF CORRELATION TABLE 



Basis of the generalised time scale. — ^The column giving the gen- 

 eralized time scale in the accompanying correlation table is built up 

 of stratigraphic units of thoroughly determined age relations in 

 different parts of eastern North America. The Silurian part of the 



