16 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



most difficult parts of a problem to solve, especially if those limits lie in the 

 open-sea basin rather than in a gulf or bay. 



The character of sedimentation in shallow seas has recently been discussed 

 by Barrell. 1 



d. Lateral changes in the character of the deposits of a marine unit, 

 either those in sequence from the shore outward or those parallel to the 

 strand-line, introduce most puzzling elements into the problem. Uncon- 

 sciously the assumption is always made by the student, especially in his 

 earlier experience, that a unit must be homogeneous throughout its extent 

 alike in all its parts. The tracing of a formation by actual continuity may 

 easily be rendered a difficult if not impossible task if the possibility of lateral 

 changes is not kept in mind. Once the direction of the shore is determined, 

 it 'is commonly assumed that each zone parallel to this will be made up of 

 similar material and carry similar fossils in all parts. In the Paleozoic, 

 before zones of climate were established or sharply differentiated, and when 

 it is very possible that for a majority of the time the topography of the shores 

 was far less diversified than now, the chances for the existence of long zones 

 of similar deposits and fossils were far greater than in later periods, but even 

 in that time it is to be expected that more rugged coasts gave place to wide 

 estuaries, and that wide sandy beaches stretched along the coast at intervals ; 

 lowlands stretching back from the sea or offshore bars and reefs would have 

 their characteristic effects, all contributing to diversify the deposits of any 

 definite interval of time. Animal life is not conditioned by temperature 

 alone; the physiographic conditions suggested above would cause a diversity 

 of food -supply and habitat which would compel a diversity of life, and 

 currents controlled by the nature of the coast would influence the distribu- 

 tion. Then, as now, any zone of reasonable length would have varied 

 deposits and life in its different parts. 



e. Positive and negative areas. It is obvious from an inspection of paleo- 

 geographic maps that the submergences of the continents have repeatedly 

 occurred over well-defined areas. This is clearly seen in the continent of 

 North America. Schuchert's Paleogeographic maps 2 and Ulrich's table of 

 submergences 3 bring out the plan, and it is recognized in the mapping of the 

 positive and negative areas first suggested by Willis. 4 



From the Atlantic deep around the northern and southern ends of 

 Appalachia and, rarely, across it in the vicinity of New Jersey; from the 

 Arctic deep broadly across the Hudson Bay region, or more commonly from 

 the northwest down the course of the Rocky Mountain prism and then 



1 Barrell, Jos., Rhythms and Measurements of Geologic Time. Rhythms in Sedimentation, 



Bull. Geol. Soc. Amer., vol. 28, p. 776, 1918. 



2 Schuchert, Chas., Paleogeography of North America, Bull. Geol. Soc. Amer., vol. 20, 1910. 



3 Ulrich, E. O., Revision of the Paleozoic Systems, Bull. Geol. Soc. Amer., vol. 22, pp. 



346-347, 1911. 



4 Willis, Bailey, A Theory of Continental Structure Applied to North America, Bull. 



Geol. Soc. Amer., vol. 18, p. 389, 1907. 



