100 A CENTURY OF SCIENCE 



creating the least change in geologic nomenclature, and 

 of doing the greatest justice to our predecessors that the 

 present conditions of our knowledge will allow, the fol- 

 lowing scheme is offered: 



Silurian period. Llandovery to top of Ludlow in Europe. 

 Alexandrian-Cataract-Medina to top of Manlius in America. 



Champlain (1842) or Ordovician (1879) period, Arenig to top 

 of Caradoc in Europe. Beekmantown to top of Richmondian 

 in America. 



Cambrian period. In the Atlantic realm, begins with the 

 Paradoxides, and in the Pacific, with the Bathyuriscus and 

 Ogygopsis faunas. The close is involved in Ulrich's provi- 

 sionally defined Ozarkian system. "When the latter is estab- 

 lished, the Ozarkian period will hold the time between the 

 Ordovician and the Cambrian. 



Taconic period. For the world-wide Olenellus or Mesonacidae 

 faunas. 



Paleogeography. 



When geologists began to perceive the vast significance 

 of Button's doctrine that "the ruins of an earlier world 

 lie beneath the secondary strata, " and that great masses 

 of bedded rocks are separated from one another by 

 periods of mountain making and by erosion intervals, it 

 was natural for them to look for the lands that had fur- 

 nished the debris of the accumulated sediments. In this 

 way paleogeography had its origin, but it was at first of 

 a descriptive and not of a cartographic nature. 



The word paleogeography was proposed by T. Sterry 

 Hunt in 1872 in a paper entitled "The Paleogeography 

 of the North American Continent, ' ' and published in the 

 Journal of the American Geographical Society for that 

 year. It has to do, he says, with the "geographical his- 

 tory of these ancient geological periods." It was again 

 prominently used by Robert Etheridge in his presidential 

 address before the Geological Society of London in 1881. 

 Since Canu's use of the term in 1896, it has been fre- 

 quently seen in print, and now is generally adopted to 

 signify the geography of geologic time. 



The French were the first to make paleogeographic 

 maps, and Jules Marcou relates in 1866 that Elie de 

 Beaumont, as early as March, 1831, in his course in the 

 College of France and at the Paris School of Mines, used 



