60 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



only shallow agitated waters, and an abundant marine fauna flourished 

 whei'e hitherto no trace of marine organisms has been found. 



Judging, then, from the condition of things in Virginia, we must con- 

 clude that the Potomac is considerably older than the oldest Tei'tiary. In 

 the District of Columbia and in Maryland we have the Variegated Clays 

 superposed on the lower Potomac. At Relay Station, where the clays rest 

 on the lower Potomac sands, these last show the same marks of erosion and 

 the same pebble-bed as in Virginia, where the marine Eocene occupies a 

 similar place. At Federal Hill, where we find the Variegated Cla3's rest- 

 ing on an argillaceous memljer interposed between them and the sands, 

 the indications of a lost interval are not so decided, but still they may be 

 seen. We may then conclude with some degree of assurance that these 

 Variegated Clays are younger than the lower Potomac, although not yet 

 clearly separable as a distinct formation. So far as the evidence goes, it 

 indicates that the Variegated Cla3^s probably are of the same age as the 

 plant-bearing South Araboy clays of New Jersey. These have been held 

 to be nearly of the age of the Cenomanian of Europe. If this is correct, 

 then from the stratigraphy and general geologic features the lower Poto- 

 mac beds must be regarded as considerably younger than the Rhajtic and 

 somewhat older than the Cenomanian. Tliere is, at least in the evidence 

 above given, nothing which opposes the assumption that the age of these 

 beds is Neocomian. 



It is proper, in this connection to say something as to the origin of the 

 lower Potomac beds, or the conditions under which they were laid down ; 

 because, as it happens, they appear to be similar to those attending the 

 accumulation of a formation which in Europe has a number of plants 

 closely allied to those of the lower Potomac. In order that the reader 

 may appreciate the peculiarly irregular character of this formation, it may 

 be well to repeat in a summary the leading features that may be seen in it. 

 If the exposure of the beds at any locality is considerable, all of these 

 features might be seen there. 



The lower Potomac is usually a mass of sandy matter of varying 

 coai'seness, generally incoherent, and having some light-colored interstitial 

 clay. It has usually a light gray to white coloi*, but this is very often 



