2IO REPORTS ON INVESTIGATIONS AND PROJECTS. 



The broader results of this research are found in the comparative 

 studies which trace for one age the geographic condition of all lands 

 and pursue through all ages the geographic history of the earth. To 

 illustrate : In Cambrian time, North America and Eurasia became 

 to a great extent submerged. Was this true of other continental 

 areas? Throughout the northern hemisphere there ensued a long 

 period of rest from mountain growth. Was this a general condition 

 of the whole earth ? During a succeeding period, the Silurian, 

 geographic changes were frequent and complex ; land areas were 

 larger ; in some districts mountains grew and were eroded. To 

 what extent were these manifestations of earth-warping general ? 

 Following the course of geographic history through succeeding ages, 

 there is, for lands where we know the geology, an apparent perio- 

 dicity ; extensive seas and low lands alternated with limited seas and 

 higher lands. These are evidences of intermittence in the earth's 

 internal activity, and if they should be shown to be general we 

 would arrive at an important line of inference regarding the mani- 

 festation of the energy. 



This suggestion of periodic warping has an important bearing on 

 organic evolution. In the wide, shallow seas of a long period of 

 quiescence, life was favored and kinds multiplied ; in the shrinking, 

 deepening seas of an epoch of warping, the stress of limited habitat 

 was great, and the destruction of many forms resulted. Migrations 

 with expanding habitats, retreat, and extinction with lessening seas 

 or lands have influenced life histories, and we shall find in better 

 knowledge of such causal conditions a better understanding of the 

 obscure problems of paleontology. 



Climate also is probably variable as geographic conditions vary, 

 and one object of interpretation of continental histories is to deter- 

 mine the importance of this relation. 



Correlation. — Within the narrow limits of a geographic district, 

 for whatever period of time there exists a continuous sedimentary 

 record, we may read the events of that local geographic history 

 without necessarily referring them to divisions of the world's geo- 

 logic calendar ; but to compare parallel provincial histories we must 

 correlate contemporaneous events, not only for each continent, but 

 also for all continents. 



The means of correlation are facts of stratigraphy, paleontology, 

 climatology, and epeirogeny. This enumeration is not only in the 

 order of their recognition during the growth of our science, but also, 

 in a measure, according to their nearness to a fundamental cause. 

 The physical characters of strata, which are remote effects of a 



