iio6 



PRINCIPLES OF STRATIGRAPHY 



Lower Lias or Sinemurian' 



1 1 zone of Liparoceras henleyi, etc. 



lo zone of Phylloceres ibex 

 9 zone of ^goceras jamesoni 

 8 zone of Deroceras armatum 

 7 zone of Caloceras raricostatutn 

 6 zone of Oxynoticeras oxynotum 

 5 zone of Arietites obtusus, etc. 

 4 zone of Arietites liirneri, etc. 

 3 zone of Arietites bucklandi 

 2 zone of Schlotheimia angulata 

 I zone of Psiloceras planorbe 



In the Trias, too, a number of distinct zones marked by species 

 of ammonites or other fossils are recognized. 



II. Dana's System. In the last edition of his Manual (3:^0(5), 

 Professor James D. Dana gives the following classification : 



Chronologic. 



I. Aeon (Ex.: Palaeozoic).* 

 2. Era (Ex.: Siluric). 



3. Period (Ex.: Palaeo-Siluric). 

 4. Epoch (Ex.: Clinton). 



Stratigraphic. 



I. Series (Ex: Palaeozoic). 

 2. System (Ex.: Siluric). 



3. Group (Ex.: Niagaran). 

 4. Stage (Ex.: Clinton). 



III. United States Geological Survey. The United States Geo- 

 logical Survey in its ruling of 1903 makes the period the unit of the 

 time scale and correlates with it the system of the formation scale, 

 thus following the usage of the International Congress. The sys- 

 tems recognized are : "Quaternary, Tertiary, Cretaceous, Jurassic, 

 Triassic, Carboniferous, Devonian. Silurian, Ordovician, Cambrian, 

 Algonkian, and Archaean." No complete scheme is formulated, only 

 the following terms being used : 



Chronologic. 



I I. 



2. Period. 



3 



4 



Stratigraphic. 



2. System. 

 3. Series. 

 4. Group. 



As far as this scheme was developed it is thus seen to correspond 

 to the one promulgated by the International Congress, with the 

 exception that group is used for the division of the fourth order 



* Examples added by the author. 



