386 MESOZOIC FLORAS OF UNITED STATES. 



clavs are present iieneath the white cross-bedded sands. These relations 

 are clearly .seen in Pi. LXXIX. 



In view of the excellent fresh exposure existing that season on 

 Sixteenth street above Florida avenue, which as was expected, was soon 

 after obsciu'ed and will never be presented again, I made a special effort to 

 work out the relations of the beds, and on several occasions in May and 

 June, 1894, I carefully measured the strata, recorded the results, and 

 drew a diagrammatic section to scale, which, as typical of much of the 

 Potomac formation, I consider worth introducing here. The strata con- 

 sist of basal Potomac cross-l^edded sands below, overlain l^y irreguhu'ly 

 stratified clays that hold plants characteristic of the Rappahannock 

 series. Upon these latter lies a very definite bed of stratified gravels and 

 clays which may perhaps be referred to the Lafayette formation, and on 

 the crest of the hill is a small cap of Columbia bowlder clay. The section 

 was measured on the east side of the street, beginning about 200 feet north 

 of Florida avenue and extending to the top of the hill, a distance of 660 

 feet. The beds all dip slightly to the north, and as a consequence the 

 several formations appear to plunge into the hill and pass under the road- 

 bed. This, as then graded, rose 33 feet in the 660 feet measured at the 

 base, or one foot in 20. At the bottom of the section the basal sands 

 occupy 360 feet, the clays 200 feet, and the gravel bed 100 feet. But the 

 clays begin at a point 150 feet north of the origin of the sands, and are thus 

 exposed for a distance of 310 feet, while the gravels begin 210 feet north of 

 the origin of the sands, and are exposed for a distance of 450 feet. The 

 sands have a maximum thickness of 20 feet, the clays of 10 feet, and the 

 gravels of 20 feet . The cap of Columbia is 5 feet thick. At this maximum 

 jioint the exposure measured 45 feet in thickness. The northward dip 

 gives an additional 20 feet, making the section 65 feet. The clays over- 

 lap the sands a distance of 210 feet, and the gravels the clays 350 feet. 

 The following is the section (compare PI. LXXVI). 



Description of the section shown in Jig. 10. 



Feet. 



1 . Cross-bedded Rappahannock sands 25 



2. Stratified clays (Rappahannock ) 15 



3. Lafayette ( V) : 



a Lower gravel bed 5 



b Clay and loam 10 



c Upper gravel bed 5 



Total 20 



4. Columbia bowlder clay 5 



Total exposure 65 



