MARYLAND GEOLOGICAL SURVEY 43 



preliminary paper setting forth the results of these investigations was 

 published in 1895 under the joint authorship of Clark and Bibbins. 

 In this contribution, the Potomac was, for the first time, separated into 

 four well-marked formations which were natural in their sequence and 

 capable of being cartographically represented. These were called, 

 beginning with the oldest, the Patuxent, Arundel, Patapsco and 

 Earitan formations. All of these formations, with the exception of 

 the Arundel, are developed in Cecil county. 



Although the stratigraphy of the Potomac group seems now to be 

 established on a firm basis, yet the age to which its various formations 

 belong has not been definitely settled. Professor H. Carvill Lewis, as 

 early as 1880, had assigned a white sand at the base of the Potomac 

 beds near Elkton to the Jurassic and had correlated it with the 

 " Hastings Sand." A few years later, in 1889, Fontaine assigned the 

 Potomac to the younger Mesozoic, and Marsh the same year main- 

 tained that the Potomac beds in Maryland were Upper Jurassic. 

 McGee and others, however, believed that the series belonged to the 

 Cretaceous. This question was in rather an unsettled state when it 

 was revived by Marsh in 1896, who again claimed that the Potomac 

 should be assigned to the Jurassic on the evidence derived from 

 certain vertebrate remains found within it in the State of Maryland 

 which were regarded as Jurassic in age. This announcement by 

 Marsh precipitated a lively discussion, which was participated in by 

 Messrs. Gilbert, Marcou, Hollick, Hill and Ward. The question was 

 still under discussion when Clark and Bibbins brought out the above- 

 mentioned paper. It was found on careful examination that the 

 lower two members, the Patuxent and Arundel formations, carried 

 mainly monocotyledons with a few dicotyledons of somewhat primitive 

 type. It was in these beds that Marsh had discovered the vertebrate 

 remains, which he referred to the Jurassic. Above, in the Patapsco 

 and the Earitan, dicotyledonous leaves appeared more abundantly, 

 and the flora had a much more modern aspect. It then seemed 

 probable that the Potomac beds belonged to two distinct ages, and 

 accordingly the Patuxent and Arundel have been referred question- 

 ably to the Jurassic while the Patapsco and Earitan are believed to be 

 Lower Cretaceous. 



