1 146 



PRINCIPLES OF STRATIGRAPHY 



Maps of this kind are more easily read when colors instead of 

 symbols are used. Another type of complex map is that which at- 

 tempts to show changes in outline of the lands and seas during the 

 period represented. Such are the maps of Freeh (13), in which 

 transgressions and regressions of the sea are represented by differ- 

 ences in color. Haug, too (21), indicates the areas of trans- 

 gression upon his maps, and, furthermore, outlines the limits 

 of the geosynclines. And, in addition, his maps are facies 

 maps. 



Special maps aim to show the outlines of lands and seas at a 

 definite time period, as at the end of the Lower, Middle or Upper 

 Cambric (Fig. 264 a-c), or at the beginning of a time period. 

 Such maps may be either simple or complex. Examples of the 



Fig. 264. Maps showing the probahle distribution of land and sea around 

 the Atlantic basin in Cambric time. a. At end of Lower Cam- 

 bric, b. At end of Middle Cambric, c. At end of Dictyonema 

 ilabelliforme time. (A. W. Grabau.) 



former are the maps published by Grabau (19) for the Cambric 

 (Fig. 264), and for the various stages of the Ordovicic (18). The 

 maps published by Schuchert, Chamberlin and Salisbury, Willis, 

 Haug, De Lapparent, and, indeed, by most authors are general 

 maps to cover a whole time period, though in some cases this period 

 may be very small. 



Construction of Pal.eogeogRx\phic Maps. In the construction 

 of palaeogeographic maps it is first of all necessary to bear in mind 

 that modern geographic maps can at least serve only as a distorted 

 base for such depiction. Thus the Appalachian region of North 

 America, and the region of the Alps in southern Europe are areas 

 where the earth's crust has been greatly foreshortened, and where, 

 hence, localities far apart at an earlier time were brought close 

 together. It is, of course, impossible to allow for such foreshorten- 

 ing, if the localities where certain formations crop out to-day are 



