436 Marsh — Jurassic formation on the Atlantic Coast. 



It cannot, of course, be positively asserted at present that the 

 entire series now known as Potomac is all Jurassic, or represents 

 the whole Jurassic. The Lias appears to be wanting, and some 

 of the upper strata may possibly prove to belong to the Dakota. 



The latter formation in the West often lies apparently con- 

 formably on the Atlantosaurus beds, and besides its many 

 fossil plants contains fragments of bones, but these may have 

 washed out of the Jurassic clays below. Footprints resembling 

 those of birds have also been found. 



The Potomac Formation. 



The Maryland Potomac, as we know it to-day, is the key- 

 stone to the arch. If this is Jurassic, as now seems certain, it 

 is a fair conclusion that the same series of deposits, north and 

 south, are essentially of the same age. The only region along 

 this line of a thousand miles or more, where a systematic 

 search for vertebrate fossils has been made, is in Maryland, and 

 here a rich fauna has been found. Doubtless in many out- 

 crops of this formation, animal remains may be rare or absent, 

 as they appear to be in the Triassic below, but vertebrate life 

 we know was abundant during the Jurassic, and characteristic 

 remains will sooner or later come to light. 



Taking, then, the Potomac formation as it is developed in 

 Maryland as an eastern representative of the Jurassic, let us 

 see what follows. The authorities on this formation^ — McGree, 

 Ward, Fontaine, Uhler, and others, agree that it extends south 

 along the Atlantic border as far as North Carolina, holding the 

 same relative position, and the same general characteristics. 

 That it also extends west around the G-ulf border has been 

 asserted by those most familiar with its southern development, 

 but on this point I cannot speak from personal observation. 



From the Potomac River northward, however, I have made 

 sufficient explorations along its outcrops through Maryland, Dela- 

 ware, and Pennsylvania, to the Delaware River, to ascertain its 

 distinctive features, essentially the same throughout, with its 

 geological position still maintained. In New Jersey, I have 

 likewise followed its equivalent strata across the state in the 

 great series of variegated plastic clays, to the Raritan River, 

 and again in their exposure on Staten Island, . everywhere 

 seemingly the same series of strata and of the same age. The 

 position is a definite one, always along the line where the 

 Jurassic must lie, if present. 



Along the northern shore of Long Island, the same forma- 

 tion extends, and at many outcrops it may be seen with its. 

 characteristic features well displayed. I have recently exam- 

 ined these exposures at many points, and all tell the same story. 

 At Montauk Point and on Gardiner's Island, I found apparently 

 the same deposits, but with local variations that need not now 

 be discussed. 



