2 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



Paleogeographic studies have been made repeatedly from one side or 

 the other and maps illustrating the results presented to the scientific world. 

 The results have almost invariably contained some assumptions or con- 

 clusions immediately recognized as highly improbable or utterly impossible 

 by men conversant with other phases of the problem. The biologist builds 

 bridges of impossible position and extent to accommodate the observed dis- 

 tribution of plants or animals and the physical geologist describes lands or 

 seas where the evidence of life denies the possibility of their existence or of 

 their assumed character. 



For this reason the following attempt at a concrete outline of a paleo- 

 geographic problem is presented to direct attention to the multiplicity of 

 factors that must be considered. One half of this outline will appear 

 obvious and unnecessary to the stratigrapher and the other half will appear 

 equally superfluous to the biologist. It is hoped that in directing attention 

 to both sides the errors that have formerly crept into such studies may be 

 in some degree avoided, and especially that students now preparing them- 

 selves for such work may realize the necessity for a broader training. 



The present chapter is intended simply as an outline and few points 

 have been discussed in detail. Numerous references are given to important 

 articles which will direct the student to the literature, to extended dis- 

 cussions, and to fuller information. 



B. OUTLINE OF POINTS TO BE CONSIDERED IN ANY 

 PALEOGEOGRAPHIC PROBLEM. 



I. STRATIGRAPHIC LIMITS OF THE UNIT TO BE CONSIDERED. 



The paleogeography of any geological unit becomes increasingly difficult 

 of solution as the size of the unit is increased, whether in space or time. As 

 the size of the unit is increased generalities must necessarily take larger 

 and larger part in the observations and the conclusions will be proportion- 

 ately looser and less capable of direct proof. In order to obtain the most 

 definite results the limits, both geographic and stratigraphic, must be as 

 confined and as strictly and accurately determined as possible. Such isola- 

 tion of the unit demands not only a consideration of the stratigraphic and 

 geographic limits, but of the homogeneity of the deposit in all ways, mineral- 

 ogic, physical, biologic, etc., which reflect the uniformity of the conditions 

 under which the bed was formed. Vertical, horizontal, and accidental 

 changes are all too frequently neglected or undervalued and incongruous 

 elements included in what should be a carefully restricted unit. Among 

 the accidental inclusions may be listed, as a suggestion of the many possi- 

 bilities, creep or slide of material from above onto an exposed surface, 

 reworked material, residual matter included in cavities of an older formation 

 (as the Devonian matter included in rifts of Silurian limestones near 



