64 PEOCEEDINGS OF THE NATIONAL MUSEUM vol.76 



All these varying sequences of the deposits of a given pe- 

 riod — and the Silurian as developed in southeastern North Amer- 

 ica is no exception — may occur in a distance of 100 to 200 miles. 

 It makes a lot of difference, therefore, in the final systemic classi- 

 fication of formational units where the type section of a system, 

 series, or group may be. And in the case of the Silurian, provided 

 world-wide application of divisions of the stratigraphic column of 

 its grade is contemplated, I see no reasonable ground for insisting 

 that the system must be limited below by the base of the Llandovery. 

 As we know from the work of Jones and others; and as I know 

 from personal observations in the concerned areas, the base of the 

 LlandoA'ery in Wales and Shropshire is unconformable by overlap 

 so that tlie stratigraphic significance of the hiatus between it and 

 the underlying Orclovician formation varies from place to place. 

 Doubtless if this hiatus could be pursued to its minimum in Britain 

 the unconformity would pass beneath beds that are not present in 

 Wales and Shropshire and Avhich, despite their inherited Ordovician 

 faunal types, would be more naturally classified as early Silurian 

 than late Ordovician. 



Age of the Keisley limestoTie and other Ev/ropean fonnations. — 

 In 1926,^^ in discussing the persistence of important Ordovician 

 generic types of the Middle Atlantic fauna to apparently early 

 Silurian time in certain European formations, I pointed out that the 

 Drummuck of Scotland and the Keisley of England, also their 

 generally accepted Scandinavian and Baltic equivalents, all contain 

 the first appearances in their respective countries of genera that 

 occur in North America only ahove Richmond in beds that are uni- 

 versally admitted to be of Silurian age. A composite list of the 

 better known of these genera includes, of brachiopods, Atrypa, 

 Atrypina^ Bilohites^ Ghonetes^ DictyoneUa^ MerUtelJa^ Mimulus (or 

 Streptis), Rhipidodomella, Schuchertella, Stropheodonta^ Stropho- 

 nella^ W hitfleidella^ and varieties of Dalrmmella elegcmtula and 

 Rhynchotreta cuneata; and of trilobites, Cheir-urm s. s., Deiophon^ 

 Dicranogmus, Dicranopeltls, Lichas s. s., Arctinu?nts, Staurocephalas, 

 and Trochurus. In my opinion, none of these genera originated in the 

 middle Atlantic realm. They are migrants from the southern At- 

 lantic basin or from some other marine breeding ground that lay 

 to the south of the present Gulf of Mexico and which supplied the 

 greater part of most of the Silurian faunas that invaded America 

 through the Mississippi embayment. 



A fact of considerable importance in this connection — important 

 because it supplements and greatly strengthens the previously avail- 

 able evidence on which I based my view as to the time when these 



=' Geol. Soc. America Bull., vol. 37, p. 322. 



