The Badland Formations of the Black Hills Region 



ditions attendant upon such migration. Mammalian life since 

 the beginning of Tertiary time has passed through various faunal 

 phases, the nature of which has been controlled to no little de- 

 gree by the presence or absence of opportunity for faunal inter- 

 change between the several continents.* The accompanying 

 Figure 10, adapted from Matthew, is an attempt to indicate 



Fig. 10— Hypothetical Continental outlines— Middle Oligocene. Matthew, 1906 



in a general way the relations of all the continents during the 

 Middle Oligocene timet 



*For a brief, important, recent statement of the conditions see 

 Osborn's "Correlation of the Cenozoic Through Its Mammalian Life," 

 Journ. of Geol., Vol. 18, 1910, pp. 201-215. 



tiMlatthew, W. D. Hypothetical .Outlines of the Continents in 

 Tertiary Times. Am. iMus. Nat. Hist., Bull. Vol. 22, 1906, pp. 353-383. 



