384 MESOZOIC FLORAS OF UNITED STATES. 



more indurated and ferruginous sand or sandstone with iron crusts and 

 geodic lumps approaching bog ore. At some points, as, for example, 

 in the region of Bladensburg, there seems to be an imperceptible transition 

 from the basal clays to the ferruginous shales, which are finally overlain 

 loy purple clays, but it must be admitted that good exposures are rare. 

 The lower sand member is often wanting in Maryland, and the dull-red 

 clays often rest on the basal lilac clays. The san.ds do, however, appear 

 in places, to some extent in the railroad cutting through the divide 

 between the Potomac and Chesapeake drainages between Muirkirk and 

 Contee, but especially near Savage in the Little Patuxent Valley. On 

 the Patapsco close to the river at Relay there is a bed of typical arkose 

 sand not distinguishable from that of the James River. 



On July 20 we made a careful examination of the Federal Hill JDeds 

 in Baltimore, and measured the section, working out as far as possible 

 the homologies with the Virginia exposures and those in the District of 

 Columbia. 



A still further agreement in the conditions in both States is the 

 occurrence of lignite beds near the base of the series. This is seen in 

 Jessup's cut through the divide between the Patuxent and the Patapsco 

 on the Baltimore and Ohio Railroad, and at many points near Hanover 

 station, especially at the Reynolds mine a mile south of that point. The 

 close resemblance between the indurated sands and ferruginous shales 

 with casts and molds of stems and plants and the Rappahannock sand- 

 stones similarly affected seems to correlate these phases. About the only 

 difference is the reddish color of the Maryland beds, due to the iron which 

 colors nearly all the rocks of the Maryland Potomac. 



The expedition was continued southward and the banks of the Poto- 

 mac were thoroughly explored on both sides of the river. In Virginia the 

 entire belt was restudied to the Rappahannock River. Perhaps the most 

 important observation was that at many points below Mount Vernon, 

 notabl}^ at Cockpit Point, the basal clays have a decidedly greenish color, 

 so much so that they were at first thought to l)e glauconitic. Upon 

 analysis, however, the green color was found to be due to chlorite, and 

 this was doubtless derived from the serpentine and talcose schists that lie 

 to the west of this region. These greenish clays, however, where long 

 exposed to atmospheric conditions, change to a lively pink color. At 



