GEORGIC PERIOD 519 



From southern Newfoundland, among others, Walcott reports Holmia 

 oroggeri, Microdiscus, Hyolithellus, Fordilla troyensis. Paterina labra- 

 dorica, and Helenia oella. The Labrador biota is closely allied to that of 

 Vermont and New York. 



These lists bring out anew the well known fact that the Georgic 

 faunas are cosmopolitan, and that Olenellus and Holmia, the two leading 

 trilobites, are common to the Pacific and Atlantic. In the Pacific realm 

 Olenellus may have given rise to the eurypterids, while in Atlantic waters 

 Holmia appears to have developed into Paradoxides. In the former 

 ocean, also, originate the large-tailed trilobites, which swarm in the Mid- 

 dle Cambrian and dominate these deposits as far east as the Appalachian 

 region. 



The writer has not yet been able to make more than one paleogeo- 

 graphic map of the Georgic period, but it will probably not be long before 

 additional ones can be developed. Perhaps the transgression will then be 

 shown to have been from the south, in all probability earliest in the Cor- 

 dilleran sea of the Great Basin area. With Holmia present at York, 

 Pennsylvania, and on Newfoundland, it may be shown that at these places 

 the Atlantic began its earliest invasion of the continent, and finally that 

 there was some withdrawal of the continental seas along the Atlantic 

 region owing to the secular changes that here closed the Georgic period. 

 In the main, it is this evidence that distinguishes the Lower Cambrian as 

 a period or system. At first there was a widely emergent land, but slightly 

 transgressed by the oceans, and finally even the small Atlantic seaway 

 vanished, being brought about by an uplift that here greatly altered the 

 former topography of the land. The name Georgic has been chosen for 

 this period to conform with Georgian series and Olenellus epoch as used 

 by Walcott. 



If the sediments of the Cambrian are compared with those of the 

 Orclovician, it will be seen that the former consist chiefly of coarse clas- 

 tic deposits, this being especially true of the Georgic. For many 

 years Walcott has persistently collected the faunas of the latter period, 

 yet today he probably has less than 250 American species from the thou- 

 sands of feet of strata. This scarcity of faunal development, which is 

 composed mainly of trilobites with chitinous tests and of brachiopods 

 having phosphatic shells, suggests cool waters, but of equable temperature, 

 as shown in the cosmopolitan character of the Olenellus fauna. The 

 widespread Lower Cambrian tillites (Australia, China. Norway, ? South 

 Africa) of course furnish positive evidence of at least local glacial condi- 

 tions. On the other side, however, is the equally marked fact of the 

 occurrence of thick reefs of Archo?ocyathus in Inyo county. California. 

 XLVI — Bull. Geol. Soc. Am., Vol. 20, 1908 



