684 PRTNCTPLES OF STRATIGRAPHY 



Erie it is only 17 feet. Still further west in Canada the thickness in- 

 crea.ses again to over a hundred feet. The 17 feet of shales in the 

 Lake Erie section apparently represent the 250 feet of the eastern 

 section, but owing to lack of accumulation, probably because the wa- 

 ter remained shallow and currents were active, the amount of de- 

 posit was slight. That this shallow area seems to have formed a 

 barrier between the eastern and western faunas is indicated by their 

 diversity. 



In contradistinction to this marked variation is the persistence 

 in thickness of the underlying Encrinal or Morse Creek ''" liriiestone. 

 This bed is only about 2 feet thick, but it has been traced almost con- 

 tinuously from western Ontario (Thedford) to the Genesee Valley, 

 a distance of over 200 miles, and throughout the extent it retains its 

 lithic character, thickness and uniformity of organic content. (Gra- 

 bau-39.) 



Comprehensive Formations. 



Under this title may be grouped deposits which apparently 

 without break continued over long periods of time. That 

 such exist has frequently been observed, though many appar- 

 ent lithic units have been found to consist of several members 

 separated by disconformities. The Hunton limestone of Okla- 

 homa was formerly believed to be a unit, extending without break 

 from the Niagaran to the Oriskanian. It is now known to consist 

 of at least four distinct and disconformable members separated by 

 great time gaps (Reed-80). The Durness limestone of northern 

 Scotland, believed to range from Lower Cambric to Ordovicic, is 

 now known to consist of a basal member of Lower Cambric age, 

 separated by a hiatus and disconformity from the Lower Ordovicic, 

 the Middle and Upper Cambric being wanting. (Grabau-40.) That 

 such comprehensive formations occur, nevertheless, is shown by the 

 deposits on the f^oor of the modern deep sea, where teeth of Ter- 

 tiary sharks (Carcharodon) occur side by side with remains of or- 

 ganisms now living. Here sedimentation is so slow that the time 

 interval since the Tertiary, when these sharks died, has not been 

 sufficient to completely bury their teeth. 



So far, however, no such comprehensive sediments are known 

 from the older geological formations, though a commingling of or- 

 ganisms of various geological horizons is not uncommon. This 



* The "Encrinal Limestone" of western New York is not the same as the 

 Tichenor of central New York. 



