30 GEOLOGICAL BIOLOGY. 



United States Geological Survey Definitions of Formation and 

 Period. — This principle is clearly enunciated in the rules 

 adopted by the United States Geological Survey for the 

 direction of the Survey.* "Among the clastic rocks there 

 shall be recognized two classes or divisions, viz. : structural 

 divisions and time-divisions." " The structural divisions 

 shall be the units of cartography, and shall be designated 

 formations. Their discriminations shall be based upon the 

 local sequence of rocks, lines of separation being drawn at 

 points in the stratigraphic column where lithologic characters 

 change. . . . The time-divisions shall be defined primarily 

 by palaeontology and secondarily by structure, and they shall 

 be Z2\\^6. periods'' (p. 65). We have thus reached the stage 

 in the making of the geological time-scale at which the ideas 

 of the geological forjnation and the geological period have 

 become thoroughly differentiated. The geological period as 

 a time-unit is primarily defined by the characters of the fossil 

 remains in the rock, so that the elaborating further and mak- 

 ing more precise the geological time-scale must come from a 

 direct study of the life-history of organisms as recorded in 

 the stratigraphical formations. 



The classification of time-divisions made on this principle 

 by the United States Geological Survey is expressed in the 

 Tenth Annual Report as follows : 



Period. Letter Symbol. Color used in Mapping. 



Neocene N Orange 



Eocene E Yellow 



Cretaceous K Yellow, Green 



Jura-Trias J Blue, Green 



Carboniferous C Blue 



Devonian D Violet 



Silurian S Purple 



Cambrian C Pink 



Algonkian A Red 



English Usage. — The English geologists maintain the dom- 

 inance of the systems as the basis of classification, and deal 

 with the geological formations as prime factors, considering 

 the periods as secondary and as dependent upon the forma- 



* Report of the Director in the Tenth Annual Report, 1890, pp. 63-65. 



