PLANT LOCALITIES. 17 



that no fossilifei'ous localities exist in a given region, he must have exam- 

 ined every clay layer in it. As these occur generally in small patches 

 which in the poverty of exposures must be often hidden, it will he easily 

 seen tliat inau}' fossiliferous spots may exist where nouL- liiive 1)l'Cmi found. 



Going north, the next plant locality that has been seen is on the farm 

 of Mr. Powhatan Moncure and in its vicinity. The spot may be l)est 

 located as follows: The Telegraph road crosses Potomac Run aljout seven 

 miles north of Fredericksburg, at the farm of Mr. Moncure. Near the run, 

 and just south of it, a rather steep hill descends, and here the road is much 

 washed, producing a deep gully on the right-hand side. The gully exposes 

 at the foot of the hill an interrupted stratum of gray clay, which rests 

 upon a gravel bed. The clay is mixed with sand, and was evidently torn 

 up from some pre-existing layer, the particles being deposited confusedly, 

 sometimes in masses of considerable size. Owing to the mixed character 

 of the material and the very limited amount of it, it was diiHcult, except 

 wlien the fragments were large, to get out good specimens. All the clay 

 visible was worked out. The spot is interesting for yielding one of the two 

 species of Sajioiopicris found in the Potomac beds and for the occurrence 

 of several well-inaiked i)laiits found also at Dutch Gap and Frederickslturg. 

 This locality may be designated as "Road-side near Potomac Run." 



It may be noted here that the tidal portions of the streams entering 

 the Potomac River from Virginia are known as " creeks," while from the 

 head of tide up the streams are called "runs." 



At the point on the Telegraph road above mentioned an obscure farm 

 road diverges to enter a field. At a distance of about two hundred and 

 fifty yards this road deflects to approach a wooded hill. Where the road 

 passes first along the base of the hill another fossiliferous stratum occurs. 

 This is found a few yards up the hill and within the woods. This .stratum 

 is composed of gray shale intin'cahited in the sand. The material is more 

 indurated than we usually hiul the clays to be, and it is well laminated. 

 Lithologically it is strikingly like the fossiliferous gray shale of the Poto- 

 mac found at Federal Hill, on Covington street, in Baltimore, which will 

 be noticed further on. The stratum is from two to three feet thick, and has 

 apparently considerable horizontal extent, although no traces of it appear 



MON XV 2 



