MARYLA]SfD GEOLOGICAL SURVEY 29 



because of tlie supposed contradictory evidence presented by the 

 fossils. Marsh stoutly contended that the Potomac strata were 

 Jurassic in age (i. e. Wealden) while the paleohotanists have as 

 firmly held that they were Cretaceous, the whole question hinging 

 on the Cretaceous or Jurassic age of the Wealden deposits of Europe. 

 In 1897 Clark and Bibbins published an article in which, from 

 stratigraphic evidence, they advanced the idea that the lower beds 

 from which Marsh secured the dinosaurian remains probably belong 

 to the Jurassic, while the upper members which have yielded most 

 of the fossil plants are Cretaceous in age. In the same article the 

 differentiation of the strata into four fonnations was first proposed. 

 In a later article by the same authors on the "Geology of the Potomac 

 Group in the Middle Atlantic Slope," an elaboration of the previous 

 article accompanied by many maps, sections, and illustrations is 

 given. 



An important contribution to the literature of the Potomac Group 

 is contained in a monograph of the IT. S. Geological Survey 

 entitled "Status of the Mesozoic Floras of the United States," by 

 Ward and others. In this work many plants from the Potomac 

 strata of Prince George's County are described and the age of the 

 beds is discussed. 



The Potomac is discussed by Clark in 1910, the Raritan formation 

 being referred to the Upper Cretaceous. The balance of the Poto- 

 mac group, on the evidence of the vertebrates which have been re- 

 studied by Lull and of the plants which have been restudied by 

 Berry, is referred to the Lower Cretaceous. 



Tlie Upper Cretaceous.- — In ISGO Tyson mentioned the presence 

 of Cretaceous strata in Prince George's County, but gave no descrip- 

 tions nor definite localities where they occur. Uhler in 1883, in 

 a short article described the quartzitic sandstone near Collington, 

 which he classed with the Potomac beds, but which is now considered 

 a part of the Earitan formation. He correlated the sandstone with 

 the Wealden of Europe. 



In 1889 Clark discovered Cretaceous fossils in the bluffs at Fort 

 Washington in strata that had previously been confused with the 



