372 W. B. Clark — Potomac River Section 



Paleontological Criteria. — Although life-zones are frequently 

 of great extent and may be accepted as the most trustworthy 

 evidence of geological contemporaniety, yet the subdivisions of 

 a fauna recognized in one area under one set of physical con- 

 ditions may not be found in another area, distant from the first, 

 where the conditions are wholly different. It is scarcely to be 

 expected that the vertical range of the species will be the same 

 in the two regions, while the time occupied in migration is a 

 factor that cannot be ignored in most classes of organisms. 

 Forms, likewise, which, from their persistence under one set of 

 physical conditions, may be regarded as typical, are often entirely 

 wanting in an adjacent province. The presence also of a large 

 number of new species is of itself evidence of change in physi- 

 cal surroundings, and renders it necessary to proceed with great 

 caution when detailed correlations of the strata are attempted. 

 Especially is this true when the areas are widely separated in 

 latitude so that temperature differences occur. 



When we come now to compare the faunal charactistics of 

 the Eocene of the middle Atlantic slope with those of the Gulf 

 we find first of all that the assemblage of forms is very 

 different in the two areas. The great majority of the species 

 is unlike, while the identical forms are mainly of wide vertical 

 range. Most of those regarded as the same also show certain dif- 

 ferences, as the result of the dissimilar conditions under which 

 they lived, so that the determination of the middle Atlantic 

 coast forms often involves certain doubts as to their identity 

 with Gulf species. The sequence of forms is likewise different, 

 a differentiation into the great number of subdivisions recog- 

 nized in the Gulf, not occurring in Maryland and Virginia. 



The Aquia Creek fauna which is typically developed in zones 

 2 to 9 in the Potomac area occupies, so far as can be with 

 certainty determined, only about 70 feet of strata some 60 feet 

 from the base of the formation and contains among other Gulf 

 species Turritella mortoni, T. humerosa, Tornatella oella 

 Volutilithes (Athleta) tuomeyi, Fusus (Strejpsidura) perlatus, 

 Dosiniopsis lenticularis, Venericardia planicosta, Cuctdlcea 

 giga?itea, and Ostrea compreasirostra. The general aspect 

 of this assemblage is Lignitic, some of the forms being found 

 in the Gulf area in the middle, or in the middle and lower mem- 

 bers of that division, whileothers range into its upper portions, 

 and are also found at higher horizons. At the same time quite 

 60 feet of strata are found beneath the Aquia Creek f ossif- 

 erous beds in which as yet only a few indistinct casts of Turri- 

 tella sp. have been observed. If the fossiliferous zones represent 

 approximately the middle, or the middle and upper Lignitic, this 

 lower zone (1) may he regarded in a general way as the equiva- 

 lent of the lower Lignitic. 



The Woodstock fauna, which is typically represented in 



