INTRODUCTION. 9 



these particles are small, and are intimately mixed with sand ; in otliers 

 they take the shape of rolled masses or balls of considerable size. These 

 have been seen five or six feet in diameter, but more commonly they are 

 only a foot or less. The balls rarely contain good impressions, for they 

 have generally been much crushed and contorted. They are often associ- 

 ated with large cobbles. The presence of these balls, and of the disturl)ed 

 clays, may be taken as indicating that some change has occurred which 

 increased the agitation of the waters and caused erosion to take place for a 

 short time. Hence they indicate erosion-horizons. It is clear that the 

 normal deposit of the Potomac of Virginia was sand, and that the clay 

 layers were formed only in eddies and still portions of the waters. Any 

 renewal of the more violent movement, continuing the transport and depo- 

 sition of sand, might erode the lately deposited clay. 



Lignite is very abundant in the Potomac formation, and its mode of 

 occurrence is worthy of notice. As stated before, it occurs always imbedded 

 in dark gray clay. The usual form in which this material is found is that 

 of single logs and limbs, or aggregations of these. The single logs, and 

 some of the aggregations, were plainly drift material. It is quite a common 

 thing to find imbedded in the sand, and surrounded immediately by a patch 

 of clay, logs of lignite wliich now lie where they quietly settled to the bot- 

 tom after floating about for a jieriod. These represent entire trees, and the 

 lignite does not occur in the fragmentarv condition in which it is always 

 found in the so-called "iron-ore clays." The logs occasionally have great 

 length. In the cutting of the Dutch Gap Canal there was removed, as I am 

 informed, a lignite log forty feet long. An end of this log may still be seen 

 in the bank of the canal, indicating a tree of the diameter of ten inches. 

 At least twent)' feet in addition to the forty feet have been removed from it. 



In several localities we find aggregations of lignite logs which appear 

 to have been formed by the prostration of forests, the trees falling where 

 they grew. In these cases the lignite logs occur lying one over the other 

 and nearly all having the same direction. 



No plants that can ])e identified have been found south of James River. 

 In the banks of tlie Appomattox clay occurs that seems favorable for the 

 preservation of plant-impressions; only small fragments, however, have 



