Proceedings of the A. A. A. S. 513 



believe that a false conception is abroad, it is proper that 

 in seeking the true one we begin with the elements. 



The surface of the land is constantly degraded by erosion, 

 and the material removed is spread on the floor of the ocean 

 forming a deposit. This process has gone on from the 

 dawn of geologic history, but the positions and boundaries 

 of land and ocean have not remained the same. Crust 

 movements have caused the submergence of land and the 

 emergence of ocean bottom, and these movements have 

 been local and irregular, districts here and there going up 

 while other districts have gone down. The emergence of 

 ocean bottoms exposes the deposits previously made and 

 subjects them to erosions. In transportation from its region 

 of erosion to its place of deposition, detritus is assorted,. and 

 so it results that deposits that are simultaneous are not 

 everywhere the same. Many of these variations in deposits 

 are correlate with depth of water and distance from shore, 

 and it results that elevation and subsidence in regions of 

 continuous deposition, produce changes in the nature of the 

 local deposit. If now we direct attention to some, limited 

 area, and study its geology, we find that under the opera- 

 tion of these general processes it has acquired a strati- 

 graphical constitution of a complex nature. Its successive 

 terranes are varied in texture. Breaks in the continuity of 

 deposition are marked by unconformities. The fossils at 

 different horizons are different, and when they are examined 

 in order from the lowest to the highest, the rate of change 

 is found to vary, being in places nearly imperceptible and 

 elsewhere abrupt. 



It is by means of such features as these — that is, by litho- 

 logic changes, by unconformities and by life changes — that 

 the stratigraphic column is classified into groups, systems, 

 series and stages. A system is a great terrane separated 

 from terranes above and below by great unconformities or 

 great life-breaks, or both. Smaller unconformities, smaller 

 life changes and lithologic changes are used for the demar- 

 cation of series and stages ; and, on the other hand, excep- 

 tionally great unconformities and life-breaks are used to 

 mark groups. As the same criteria determine groups, sys- 



