Time-table for North America. 15 



(2) By widespread changes in the physical geography. That 

 is, there are at these times a highly diversified or young topog- 

 raphy, decided alterations in the continental outlines, the 

 making of new or the breaking down of old land connec- 

 tions (the land bridges which permit intercontinental organic 

 migrations), and marked changes in the oceanic currents, all of 

 which also lead to marked variations of temperature and often 

 to actual glacial periods. 



(3) By marked and widespread destruction of the previously 

 dominant, prosperous, and highly specialized organic types. 

 This is produced partly by the physical changes and partly by 

 the extensive migrations that are more conspicuous at these 

 times and that therefore invite the spread of death-dealing 

 parasitic diseases. 



(4) By the marked evolution of new dominant organic types 

 out of the small sized and less specialized stocks, and by the 

 development of hordes of new species. 



The last or Cascadian revolution is so recent that the record 

 of it is not lost, and a study of this enables us better to com- 

 prehend the changes wrought by the earlier revolutions. 

 LeConte regards it " as the type, as the best proof of the fact 

 of critical periods, and as throwing abundant light on the true 

 character of such periods, and especially on the causes of the 

 enormous changes in organic forms during such times " 

 (1895). 



The periods usually take their names from the geographic 

 area where the system of rocks was first considered to be of 

 period value. Thus the Cambrian, Ordovician, Silurian, and 

 Devonian systems were first discerned in England and Wales 

 and take their names from ancient peoples living in these 

 countries, or from the district itself in which the rocks 

 are best developed. Permian is from the Province of Perm 

 in the Urals of Russia, while Jurassic comes from the Jura 

 mountains. Mississippian directs attention to the Mississippi 

 valley, where these rocks are well developed; Pennsylvanian, 

 to the greatest coal state in North America ; and Comanchian 

 (1887), to the home of the Comanche Indians in Texas. With 

 regard to the last division, how T ever, Shastan (1869) is an older 

 name, having reference to the Shasta mountains of California. 

 Triassic has reference to the tripartite development of these 

 rocks in Germany, and is an heirloom from the days of geology 

 when the science had not worked out the principle that forma- 

 tions and periods must be based upon type areas. Cretaceous 

 is a still older inheritance from the days of mineral geology, 

 before there was much stratigraphy, the name being based 

 upon the chalk deposits of western Europe. Should this petro- 

 graphic term be objectionable, the geographic name Platte 



