MAini.WD (ii;o|.()(;ic \l. SI l!\ i:v (Jl 



The l'isc;il;i\\;iy >iilistii;i(' is liiiiitcil In Zniics 1 ttj 7. The foHHils 



restricted tn i h i- liiii-iziui arc: 



TliirilclKinilixil mricoilmi i'f) Cojn-. Tnilirla sp. 



7'hvfHfliiiiiipmi foiitilsor (.\>\n\ Plu'itiiriiiiiijii pftrnnii ((.'((liliiil i. 



KiirliisU'x sp. J'/i<iliitli)inyii iiiiinjlitit<lirii Coiiriid. 



Triii>ii/.r I'iri/hiiinni Cluik. /Jt/ii>//hti(/a iiiiirijlniiilicit Clark iiiiil Martin. 



Sijinrliodiia chtrkii Eustuiiii). Oxtrvn roniprexiiiroxlrd <•<<»•. alepidnln Dull. 



()di)ii((txpix mnvrodi (Agassi/,). (rnjphned rcxScnlnrix Lamurck. 



Od(>}itaspis eleffaun (Agassiz). Terebrnlnl'i hurlinii Morton. 



Odotiis obliijKHS (Ai^assiz). Textularia .•iithiinyulatii d'Orbiiriiy. 



Sphyrna prixca Atjassiz. Vuginnlinti legumcn (Linue). 



Xi//hiii.s rii'liiifii Clariv. 



Tlio various zones of this division in the Potomac rcrrion are charac- 

 terized as follows: 



Zone 1. — The thickness of ihe ]M3cene deposits beneath the hjwest beds 

 holdiii*:' the typical A(piia lanna has l>een variously estimated up to GO 

 feet. Some exposures are seen in the ravines ti> tlic west of the bluff, 

 but no complete sequence of the l)eds has been found. At some points 

 the strata scarcely exceed 10 feet, and at Glymont the beds are only 

 8 feet in thickness. They, however, gradually increase in thickness sea- 

 ward. The almost entire absence of fossils renders the faunal relations 

 obscure. The poorly preserved animal and plant remains are of prac- 

 tically no value, and provisionally this bed is left as an indeterminate 

 basal zone of the Aquia. The materials are greensands, fre([Uently 

 quite argillaceous and with a basal pebble bed overlying the Cretaceous 

 at some points. 



Zone 2. — This zone is characteristically developed both at the base of 

 the Aquia Creek section and also on the opposite bank of the Potomac 

 river at Glymont. The beds are from 13 to 20 feet in thickness, and 

 entirely disappear below the water-line a little beyond the middle of the 

 Aquia Creek bluff. The dark greensand of which the zone is mainly, 

 composed is packed with the shells of Crassatellifes alaeformis and 

 Dosiniopsis knticuJaris, while TtirrilcUn morfcvii. T. Inimerosa, Crassa- 

 tellites aquiana. Meretri.v ornfa var. pi/ga, CucuIIaea gigantea, and Osfrea 

 com pressi rostra also occur. 



Zone 3. — The limestone ledge composing this bed is much less per- 

 sistent than in the overlying limestones. Zone o, and at times it nearly 

 or quite disappears. It is from 2 to 3 feet in thickness, highly glau- 



