30 HISTORICAL GEOLOGY 



reasonable basis upon which to work. Within the last few years, 

 however, several sets of paleogeographic maps, notably those by 

 Bailey Willis 1 and Charles Schuchert, 2 have been prepared. The 

 Willis maps are used in this text, but the student will do well to 

 compare the Schuchert maps with these as the different periods are 

 taken up. 



Willis gives a general statement of the lines of evidence used 

 in the construction of his maps as follows : " A certain period having 

 been selected as that which should be mapped, the epicontinental 

 strata pertaining to that time interval have been delineated. The 

 phenomena of sedimentation and erosion have then been correlated, 

 with a view to determining the sources of sediment and topographic 

 conditions of land areas, and from these data the probable positions 

 of lands have been more or less definitely inferred. Thus, certain 

 areas within the continental margin are distinguished as land or sea, 

 and these areas may be defined as separate bodies or connected 

 according to inferences based upon isolated occurrences or upon 

 later effects of erosion. It is assumed that the great oceanic basins 

 and such deeps as the Gulf of Mexico and Caribbean have been 

 permanent features of the earth's surface at least since some time 

 in the pre-Cambrian . . . 



"From the geographic conditions thus developed, inferences 

 regarding the climate and the life habitats of the time may be 

 drawn. If now we turn to the records of paleontology, and com- 

 pare the distribution of faunas and floras 3 with the conditions 

 of distribution which should result from the inferred physical 

 phenomena, we may check the whole line of reasoning and 

 by a readjustment draw a step nearer to the truth. This is the 

 method which has been pursued in making the maps of North 

 America." 4 



It should be borne in mind that such paleogeographic maps 

 are generalized and rather tentative as regards many details — 

 generalized because each map represents a considerable time period 

 so that certain more local geographic changes during the period 



1 B. Willis: Jour. Geol, Vol. 17, 1909. 



2 C. Schuchert: Bull. Geol. Soc. America, Vol. 20, 1910. 



3 The term "fauna" refers to an assemblage of animals populating a 

 given area during a certain epoch. In a similar sense the term "flora" is 

 applied to an assemblage of plants. 



4 B. Willis: Jour. Geol, Vol. 17, 1909, pp. 201-202. 



