TAXONOMY OF THE CHESAPEAKE FORMATION. 445 



Taxonomy. — The Chesapeake formation is abundantly fossiliferous through- 

 out, and the Miocene age of the fauna was recognized by Rogers and Conrad 

 half a century ago. 



It has been shown by Heilprin that the fauna of the upper part of the 

 formation differs materially from that of the lower, but I fiud that there is 

 less difference than is indicated by Conrad's lists, and that the transition is 

 a very gradual one. The formation can hardly, on these grounds, be sepa- 

 rated iuto Marylandian and Virginian, as proposed ; and there is every 

 reason to believe that the beds in Virginia in their entirety are of precisely 

 the same age as their extension into Maryland. The paleontologic evidence 

 is fully in accord with this, at least in a general way, and the structural 

 evidence indicates complete continuity. 



APPOMATTOX FORMATION. 



Distribution and Characteristics. — The differentiation of the Appomattox 

 formation in the southern states by McGee is one of the most valuable con- 

 tributions ever made to American geologic science. The great extent and 

 prominence of the formation and its significant bearing on the geologic his- 

 tory of North America give it an importance second to that of no other 

 formation on the Atlantic slope. 



The northern termination of the deposits was supposed to be near Potomac 

 creek, a few miles north of Fredericksburg ; but I have found that while 

 there is a break in its continuity in the region east of the Potomac river, it 

 soon begins again and thence continues northward probably through Mary- 

 laud, and in attenuated scattered outcrops, through Delaware and iuto 

 Pennsylvania and New Jersey. 



It is displayed in the. high terraces about Washington, and it caps nearly 

 all the higher terrace levels of the "western shore" of Maryland north- 

 ward to the latitude of Baltimore. Still farther northward it is confined 

 to outliers on the divides along the western margin of the coastal plain 

 region ; but at the head of Chesapeake bay it extends farther eastward and, 

 in the high Elk ridge, caps the Cretaceous and Potomac formations over a 

 considerable area. 



The formation was no doubt originally continuous throughout the Atlantic 

 coastal plain, but it has suffered great erosion. Southward it caps all the 

 high terraces, but northward from the Mattaponi the drainage ways have 

 invaded it more widely until north of the latitude of Washington its remain- 

 ing areas are relatively small isolated outliers. 



The Appomattox formation in eastern Virginia consists of light-colored 

 loams of buff and orange tints, contaiuing streaks and beds of pebbles and 

 coarse sand in varying proportions and irregular deposition. Northward 



