the Middle Atlantic Slope. 125 



dally defending, the term " formation " as employed in this 

 article. It is applied to a naturally defined series of deposits, 

 evidently formed by a definite set of agencies within a deter- 

 minate area during a definitely limited period — an easily rec- 

 ognizable structural, chronologic, and taxonomic unit. 



The formations described are : (1) the Potomac, consisting 

 of irregularly bedded and heterogeneous sand, clay, arkose, 

 gravel, etc., of Mesozoic age and forming the base of the 

 series of unaltered deposits of the Coastal plain, named from 

 the river on which it is best developed; (2) the Appomattox, 

 a series of predominently orange-colored sands and clays of 

 later Tertiary age, also named from the river on which it is 

 typically exposed ; and (3) the Columbia, a series of delta and 

 associated littoral deposits formed during a brief period of 

 submergence in early Quaternary time, named from the dis- 

 trict in which it is typically developed and in which it is first 

 systematically studied. Certain deposits of each of the for- 

 mations have already been investigated by different geologists, 

 but they have not hitherto been correlated and systematically 

 combined. The Potomac formation has already found place 

 in geologic literature and taxonomy ; the Columbia has been 

 defined and briefly described in print ;* but the Appomattox 

 is here defined and named for the first time. 



Professor William M. Fontaine began his elaborate investi- 

 gations of the later Mesozoic formation and its flora in Vir- 

 ginia some time before the study of the contemporaneous 

 deposits of the District of Columbia was taken up by the 

 writer ; but a meeting was soon arranged, notes were compared 

 and exchanged, and a detailed examination of the principal 

 exposures in Virginia, in the District, and in Maryland south 

 of Baltimore, in which Mr. Lester F. Ward participated, was 

 undertaken in the summer of 1885 ; during this joint study 

 the characters of the Potomac formation were worked out and 

 its name (previously proposed by the writer) adopted, the 

 Appomattox formation was discriminated, and many features 

 of the Columbia formation were brought to light ; and it is a 

 pleasure not only to acknowledge indebtedness to these gentle- 

 men, and especially the former, for the greater share of our 

 knowledge of the Potomac and Appomattox formations, but 

 to record their substantial concurrence in the following state- 

 ments and inferences concerning them. It is an equal pleas- 

 ure to add that the phenomena of the Columbia formation 

 have been more or less carefully examined in the vicinity of 

 Washington by the same gentlemen and several other geolo- 



* Rep. Health Officer of the District of Columbia, 1884-85 (1886), 20; this 

 Journal, III, xxxi, 1886, 473; Proc. A. A. A. S., xxxvi, 1888, 221; etc. 



