or- 



76 MESOZOIC FLORAS OF UNITED STATES. 



to run under it to the south, but even this is much older than the beds 

 on the Severn River in Maryland in which occur his type sections of 

 the Albirupean. 



My own studies in the formation were resumed with the opening 

 of the field season of 1892. Early in May Mr. David White accorn- 

 panied me on an excursion to Fredericksburg and the region adjacent. 

 We first studied the contact of the Potomac with the underlying crys- 

 tallines on the Rappahannock, on Fall Run above Falmouth, and on 

 Hazel Run to the south of Fredericksburg, and usually found a bed 

 of clay underlying the freestone. A number of instructive sections 

 were made. Above the freestone occur heavy beds of loose sand. We 

 then followed the Rappahannock down in a rowboat from Fredericks- 

 burg to the Eocene contact at the Marl Mill, 6 miles below. This affords 

 a fine section. The dip to the southeast is about 50 feet to the mile 

 and the distance in a straight line from the crystalline contact to the 

 Eocene contact is about 6 miles, giving the Potomac a thickness of 300 

 feet. We were able to measure only a little over 200 feet, JDut there 

 was evidence of erosion at several points. The 200 feet measured were 

 as follows: 



'Section of the Rappahannock River at and helow Ffederichshurg. 



Feet. 

 3. Loose argillaceous, mostly white or 3'ellowish, sand with thin clay seams, becoming darker and ligni- 



ciferous above, and unconformably overlain by the Eocene (Pamunkey) marl .50 



2. Coarse, feldspathic, conglomeratic sandstone with lenses, nodules, and pellets of fine white claj', and 



with casts and molds of stems, logs, and indeterminable plants i 100 



1. Red, pink, and purple to white claj', carrying lignite and (on Fall Run) lignitized logs; resting uncon- 

 formably upon the crystalline rocks 50 



Total exposure 200 



The clay disappeared beneath the water of the river opposite Pratts 

 Rock, the sandstone 3 m.iles below that point, the sand at Travelers 

 Reach, and the last of the darker lignitic beds half a mile above the 

 Marl Mill. Back from the river the higher country_ in all directions 

 from Fredericksburg is covered with a relatively modern deposit, prob- 

 ably the Lafayette, while the bottom lands usually show a bed of Colum- 

 bia brick clay. For these reasons the geological map of this region is 

 colored for these formations only, giving no proper idea of the geology. 



Leaving Fredericksburg on the 5th we explored the general region 

 to the north as far as Stafford Court House along the western margin 

 and found the elays almost everywhere underlving the sandstone. We 



