OLDER POTOMAC OF VIRGINIA AND MARYLAND. 509 



that the Aquia Creek fossils occur in a stratum only a few feet above 

 that canying Mount Vernon plants. In any case it is not prol^able that 

 the Hell Hole beds are as old as the Rappahannock strata. 



FOSSIL I'LAMS FKOM OHIXKAPIX HOLLOW. 



The locality called Chinkapin Hollow is situated between Alexandria 

 and the Episcopal high school known as Fairfax Seminary, being near 

 the latter. It is on the Leesburg pike. The exposures are on a drain 

 worn into the Lower Potomac beds, and just below a spring that dis- 

 charges into the drain. There are two exposures of fossiliferous strata 

 here. One, which is much the richer in fossils, is a stratum of lignitif- 

 erous sand and clay that is, in its top, about on a level with the stream 

 bed. This may be called exposure No. 1. The other is a few yards 

 lower down the stream in a bluff bank made by the cutting down of 

 the stream l^ed. This may be called exposure No. 2. The plants in the 

 latter are found in thin clay seams of lenticular form, inclosed in the 

 predominant partly indurated sand of the lower Potomac. They lie 10 

 feet or more above the water level, and hence are above the level of 

 .exposure No. 1. The plants in these clays are much more limited in 

 kinds than those in exposure No. 1, while the species are few. The 

 number of individuals of the kinds present is considerable in the case of 

 some. The fossils are reduced to small bits, but these are often remark- 

 ably well preserved. They are mostly different species from those of 

 No. 1. The clay of exposure No. 1 has poor cleavage and is very prone 

 to break up into small fragments, so that it is difficult to get good spec- 

 imens from the exposure as now presented. It is possible that if an 

 excavation were made to reach material not affected by the atmosphere 

 and water better specimens could be found. • The bed is so rich in varied 

 forms that it is desirable that it should be explored more fully. As 

 there is 'a considerable difference in the plants occurring on the two hori- 

 zons, although they are so near together, they will be distinguished in 

 speaking of the fossils. There is a good deal of difference in the rock 

 material also. The lower exposure. No. 1, is composed of a bed contain- 

 ing layers of poorly indurated sand with much thinner onQS of a dark- 

 ash-gray highly plastic clay. The clay layers contain the recognizable 

 fossils. Part of the bed is below the level of the stream. Both the 

 sand and the clay are full of small fragments of plants and' l^its of lignite, 



