General Classification of Geologic Time and Stkata. 135 



From this it results, that, as we go southward from Lake On- 

 tario, we ascend in vertical altitude and also in the geologic 

 column. Our youngest Palaeozoic rocks, which are Lower Car- 

 boniferous, are near the Pennsylvania boundary. 



These physiographic features are well shown on the accom- 

 panying relief map of New York. 



For a detailed discussion of the geography of New York see 

 Examination bulletin 11, University of the State of New York, 

 by Wm. Morris Davis. 



GENERAL CLASSIFICATION OF GEOLOGIC TIME 



AND STRATA 



It must be realized by the student at the outset that all classi- 

 fication is to some extent arbitrary. There was throughout the 

 earth as a whole a continuous process of erosion and sedimenta- 

 tion and a continuous chain of life. Locally, through changes of 

 level, sedimentation was varied and life interrupted from time to 

 time' For convenience in discussion, a scheme of arrangement 

 has -been adopted which is based on the more conspicuous of 

 these breaks in life and sedimentation. 



According to the classification most generally accepted, the 

 principal divisions of the geologic time scale are called aeons or 

 times and designated by the following names which are based on 

 the principal features of life development: 



Cenozoic, latest time, characterized by forms closely related 

 to those of the present day. 



Mesozoic, middle time of life development. 



Palaeozoic, early time; ancient forms of life well developed. 



Proterozoic or Agnotozoic, life not well known as yet. 



Archaean time of the most ancient rocks with only suggestive 

 traces of life. 



