FAUNAL MIGRATIONS AND DIASTROPHISM 397 



Faunal Migrations and Diastrophism 



In the foregoing discussion of the vertebrate faunas of the late Cretacic 

 and early Tertiary the differences in faume have been ascribed to three 

 factors: (1) Lapse of time; (2) Difference of facies; (3) Wide-spread 

 migration movements. The first affords a measure of the time interval 

 between two formations. Tlie second, when occurring in superposed for- 

 mations, indicates a change in local conditions, often accompanied by a 

 stratigraphic break or unconformity. The third, occurring often inde- 

 ])endently of any local cJumges in environment, points to changes in the 

 conditions in some other region, usually in the center of dispersal, where 

 the migration movemenls originated. 



It is usually assumed by paleogeographers that these changes consisted 

 in the union of regions formerly isolated, permitting land animals to 

 invade areas hitherto isolated. But it has been yery conclusively shown 

 by C. Hart Merriam that the range of land mammals is limited not so 

 much by mountain barriers or even oceanic barriers as it is by climatic 

 zones. This is also true of land reptiles, and presumably of the land 

 fauna generally. A change in range is therefore conditioned not merely 

 by the land connection which permits or facilitates the migration, but by 

 climatic change. which forces the movement through the changed environ- 

 ment. This climatic change will be largely dependent on great and wide- 

 spread movements of elevation or submergence. A wide-ranging migra- 

 tion movement resulting in the simultaneous appearance in Europe and 

 North America of identical new types is, therefore, to be ascribed not 

 merely to such slight changes as might serve to make a land connection, 

 but to a great moxement of upheaval of the land, affecting a large part, 

 if not the whole, of the intervening region from which the new types are 

 presumably derived. 



These great migration movements therefore I regard as caused by 

 diastrophism. If the evidence is properly interpreted and the migrations 

 adequately proved, they afford, it seems to me, tlie most reliable and in 

 some sense the only evidence of diastrophism ; for it is not possible, save 

 through the evidence of the paleontologic record, to prove that t • move- 

 ments of which the stratigraphy gives evidence were simultaneous or to 

 correlate them exactly in different regions. The extent and an.junt of 

 the stratigraphic break between the Lance and the formations of the 

 Montana group is a matter of disjuite. But were it not, I fail to see how 

 we could correlate it with the break between the Euro])ean Cretacic and 

 Tertiary series save through the faunal evidence. 



XXVIII— Bull. (Jkol. Sue. Am., Vol. 26, 1913 



