1 144 PRINCIPLES OF STRATIGRAPHY 



Trenton) differed, likewise, in a marked degree from the preced- 

 ing Beekmantown faunas. The Sikiric fauna of North America, 

 also an expansional fauna, differed radically from that of the preced- 

 ing Upper Ordovicic (Weller-52), which was largely exterminated 

 by the late Ordovicic or early Siluric retreat of the sea. The com- 

 parative uniformity of expansional faunas, over wide areas, such 

 as that of the Mid-Ordovicic, the early Siluric, the Mid-Devonic, 

 and the later Jurassic and Cretacic, shows that such periods are 

 eminently fitted to furnish a basis for practically worldwide cor- 

 relation. 



Finally, it may here be noted that, if the theory of polar pendu- 

 lations, as outlined in Chapter XXIII, should prove to have a 

 sound basis in fact, we must modify our conception of movements 

 of the water body, to the extent of recognizing the coincident ris- 

 ing of the sea-level in the region approaching the Equator, and the 

 fall of the sea-level in the regions approaching the poles. Thus 

 movements of the water body would not be uniform over the earth, 

 but compensatory, rising in the equatorial and falling in the polar 

 regions. For an attempt at correlation on such a basis the student 

 is referred to Simroth's book "Die Pendulations-theorie." (44.) 



Pal^ogeography and Pal^ogeographic Maps. 



"Palseogeography," says Dacque, "may be compared to a fire 

 which has smoldered long under cover, but which has at last broken 

 forth with all-consuming energy" (12). Attempts to restore the 

 outlines of continents and seas during former geological periods 

 were essayed by geologists and palaeontologists before the middle of 

 the nineteenth century. Since then the subject has smoldered 

 under cover of the detailed investigations carried on in other fields 

 by the students of the earth sciences, until, in recent years, it has 

 burst forth with almost volcanic violence, and palaeogeographic maps 

 and palaeogeographic discussions have become the order of the day. 



The term Palceogeography is credited to Robert Etheridge, and 

 its birth is given as in the year 1881 (Schuchert-39),* but palaeo- 

 geographic maps were made much earlier. Schuchert regards those 

 given by Dana in the first edition of his manual (1863) as the earli- 

 est ; but earlier maps, though less definite in character, had been 

 published, as, for example, those by Goodwin Austen for England 



*Dacque mentions A. Bou^, who in 1875 used the terms paldogeologische Geo- 

 graphic and geologische Paldo-Geographie. (Sitzungsberichte K. K. Akad. Wiss. 

 Wien. Math. Nat. Kl. Bd. 71, i*« Abt., s. 305-405.) 



