500 C. SCHU CHERT CHRONOLOGY ON BASIS OF PALEOGEOGRAPHY 



3,000 miles, while at least 5 per cent have a coastal distribution greater 

 than 5,000 miles. Smith also points out that many of the species and 

 nearly all of the genera of the Upper Trias are common to California and 

 the Alps, regions that are 6,000 miles apart in a straight line and 12,000 

 miles by the route of faunal migration. 



It is this wide range of most of the marine invertebrates and the addi- 

 tional fact that all organisms, living and extinct, are changing, some very 

 slowly and others faster, that enable paleontologists to work out not only 

 their genealogies, but also the times of their geologic origin, duration, 

 and vanishing. The evolution is in the main progressive in the intro- 

 duction of new characters; a small per cent of the species are almost 

 stationary in their make-up ; more are regressive in the losing of inherited 

 parts ; and yet, in all this change, no species is repeated in time. These 

 facts indicate that fossils can be depended on in the correlation of forma- 

 tions, and that the greater part of the faunas will radiate as far as the 

 marine overlaps can spread. While it takes time for the sea to trans- 

 gress the land, to the stratigrapher the faunas appear simultaneously in 

 widely separated places on the same continent. As an example may be 

 cited the fauna of the Galena and equivalent formations of the Middle 

 Ordovician, which are common to Illinois, Iowa, Minnesota, and Arctic 

 America. A later and easily recognized Ordovician fauna, that of the 

 Eichmondian, is known from Texas to Alaska and from Nevada to Mari- 

 time Canada. Again, the Upper Devonian Lime Creek fauna occurs in 

 New York, Iowa, Arizona, and California, and parts of it are widely dis- 

 tributed in the Euro-Asiatic region. Smith has pointed out similar wide 

 occurrences for the Triassic and Jurassic faunas of the Pacific States, 

 and that the Triassic assemblage of Bosnia is duplicated in Nevada ; 

 therefore even intercontinental correlations can be made through the 

 fossils, though here the difficulties are greater than for intracontinenta] 

 time determinations. 



THE PALEOGEOGRAPHIC METHOD 



Paleogeography treats of the ever-changing geography of geologic time. 

 It seeks not only to map the configurations of lands and seas and their 

 relationship to one another, but to determine as well the topography, 

 something of the structure and volcanic activity of the lands, the depths 

 and circulation of the marine waters overflowing the lands, and the 

 physical and chemical actions of the varying climates on the sediments. 

 The constantly changing physical environment reacts on the organic 

 world and causes it to alter variously in its parts; and as in one place or 

 another sediments and organisms have been at all times adding them- 

 selves to the stratigraphic sequence, it follows that the interpretation of 



