ABT. 21 OEDOVICIAN TKILOBITES ULRICH 69 



discovery of overlaps of concerned formations furnishes the proof of 

 at least a part of its possibly very great importance. No more strik- 

 ingly illuminating example occurs to me that the slight difference in 

 the faunas of the Lebanon and Lowville limestones which are in con- 

 tact in central Tennessee but separated in east Tennessee by a maxi- 

 mum of 8,000 to 10,000 feet of limestone and shale deposits with en- 

 tirely different fossils. To further illustrate the uncertainties that 

 generic matching of fossil faunas may entail I would mention the 

 supposititious but easly conceivable and quite possibly actual con- 

 temporaneity of marine deposits with entirely different fossils. Such 

 a case could be explained only after we had learned that one of the 

 faunas had originated in and invaded from, say the Pacific, the other 

 in and from the Atlantic, and that both are directly overlain and 

 underlain — without sign of discontinuity of deposition — by the same 

 pair of formations. 



RECENT PROGRESS IN AMERICAN STRATIGRAPHY 



Old and new data indicating persistence of indigenous faunas. — • 

 The truth of preceding statements regarding the indigenous persis- 

 tence of faunal assemblages is clearly shown by previously published 

 information concerning the now undisputed recurrences of the Sper- 

 gen fauna in early, middle, and late Mississippian formations. Inter- 

 est in this case has been revived and emphasized by two still younger 

 recurrences of the same dwarfed fauna in Pennsylvania formations, 

 the highest only recently discovered by Charles Ryniker in Oklahoma. 

 Apparently these recurring hordes of small fossils are really to be 

 regarded as long persisting and very slowly modifj'ing dwarfed 

 descendants of a far southern Middle Devonian fauna of which 

 normally sized individuals reached New York State already in Hamil- 

 ton time. Whether this inferred derivation of the diminutive mollus- 

 can Spergen fauna is correct or not there certainly is less reason for 

 considering most of its constituents as dwarfed forms of species of 

 the same classes found in intervening Mississippian and Pennsyl- 

 vanian beds. The latter probably were produced in a nearer breeding 

 ground that supplied, or at least contributed to, most of the Paleo- 

 zoic faunas that invaded America from the south through the Missis- 

 sippi embayment. Another alternating sequence of invasions from 

 two distinct faunal centers through the same Lower Mississippi en- 

 trance comprises the simulating late Trenton Catheys fauna, the 

 Fairview fauna, and, finally, the early Richmond Arnheim fauna, on 

 the one hand, and the preceding Trenton and intervening Cincin- 

 natian faunas on the other. The latter also exhibit a more marked 

 general resemblance to each other than to the immediately succeeding 

 and preceding faunas of the first set. Manj^ other such instances of 



