GEOLOGY — WILLIS. 205 



if considered with reference to any one continent, will give the 

 sequence of its geographic conditions, or which, if considered with 

 reference to any one epoch, will constitute a map of the world at 

 that time. 



The bearing. — This historical atlas of the world, summarizing 

 all the geologic record, will constitute a work of reference upon 

 which to base discussion and research in all departments of geology. 

 Summing up available evidence, stating alternative interpretations, 

 giving conclusions where facts are conclusive, but justly weighing 

 doubt, the publication should promote advance in the science in the 

 highest degree and should itself advance with the science. Coirre- 

 lation of strata and faunas, the crux of geologic historical investiga- 

 tion, is involved in the proposed research ; but the facts of geographic 

 variations, once placed in sequence, will throw much light on its 

 problems. Organic evolution is studied without reference to the 

 important factor of environment so long as we have no definite 

 knowledge of the shifting lands and seas, which have been the scene 

 of its conflicts ; this contribution of paleogeography will place it 

 upon a foundation. The problems of those climatic fluctuations 

 which have ranged from tropics in the Arctic to glaciation near the 

 Equator remain indeterminate so long as the geographic variable is 

 unknown. That variable should be evaluated by this research. The 

 initial cause of continental variations, intermittence of the earth's 

 internal activity, will be followed throughout geologic time, and the 

 contribution to epeirogeny and orogeny will thus constitute also a 

 contribution to the data of geophysics. The proposed research, if 

 broadly, liberally, and judiciously executed, must serve students in 

 all branches of geology and in many of its allied sciences. 



DISCUSSION. 



Data available for interpretation. — Geologic literature contains a 

 large amount of data relating to the proposed material. Stratigraphic 

 sequences, structural relations, and paleontologic correlations yield 

 the facts with which the paleogeographer begins, and he should base 

 his work, so far as possible, on the results of geologic surveys already 

 accomplished. For the United States and Europe the existing ma- 

 terial is adequate to furnish a tolerably complete conception of the 

 continental histories, and the same is true, though in a less degree, 

 of Canada and Mexico. The literature relating to South America, 

 Asia, Africa, and Australia is sufficient to afford a suggestive outline 

 of continental conditions at different times, through which we may 

 arrive at an understanding of the critical localities for further research. 



