TIME AND FORMATION SCALE 



1 103 



Subdivisions of Time and Formation Scales. 



The primary divisions of the geologic time scale are, as we have 

 seen, based on the changes in life, with the result that fossils alone 

 determine whether a formation belongs to one or the other of 

 these great divisions. The primary divisions now generally recog- 

 nized are as follows : 



Corresponding to each time division we have a formational divi- 

 sion, which represents the rock material accumulated during the 

 continuance of that time. As will be seen from the above table, 

 the formation scale now generally in use is made up partly of the 

 old names in vogue during Lyell's time, partly of the newer names, 

 and in part of distinct names applied to the rocks of these divisions 

 by American geologists and adopted by workers in other countries 

 as well. 



A number of terms have been proposed by which the subdivi- 

 sions of the time and formation scale are to be known, but at 

 present there is no unanimity in the usage of these terms. The 

 following are the most important of the proposals made, the num- 

 bering being in the order of magnitude of the categories : 



* In many text-books the Quaternary is included with the Tertiary under 

 Cenozoic, which is not the historic sense of the term. Post-Tertiary time is 

 essentially characterized by the presence of man and may be separated as 

 Psychozoic. 



t Walcott, 1889. From a tribe of North American Indians. 



t Proposed by Dana. 



