48 THE POTOMAC OK YOUXGEK MESOZOIO FLORA. 



gradually or abruptly into souietliing quite different. The individual 

 layers may be variously constituted, and show in their parts on a minor 

 scale the variable features discerned iu the composition and the grouping 

 of the layers taken as a whole. Then on various horizons matter pre- 

 viously deposited seems to have been torn up, transported to varying 

 distances, and deposited in a confused manner. These last-named features 

 mark what, for the sake of distinction, I call erosion horizons. They add 

 much to the irregularity of the formation. When one has before him any 

 considerable exposure of the lower Potomac he may well hesitate to 

 attempt an accurate description of it, feeling that this would require an 

 examination of each yard of the material. 



The laj-ers of sand are often marked by undulating lines across the 

 direction of their length, forming a marked current bedding. This feature 

 is conspicuous in the cuts on the narrow-gauge railroad west of Freder- 

 icksburg. We may often find gravel, coarse sand, and clay pellets 

 confusedly mixed together. This is to be seen at the top of the quarry 

 on Acquia Creek on a large scale. While no regular dip could be made 

 out from the tracing of individual strata, the formation as a whole sinks 

 gently to the southeast. 



Two principal kinds of material make u\) the lower Potomac beds; 

 namely, sand and clay. The sand is by far the most abundant. It is the 

 typical material, while clay occurs in limited local layers, either inclosed in 

 the sand or replacing portions of it, owing to the graduation of the sand 

 into it. 



The sand may be loose, or cemented into a pretty firm sandstone, 

 which has been used as a building stone. This is its nature to a consider- 

 able extent on Acquia Creek and around Stafford Court House. It is 

 more generally friable and incoherent, especially in outcrops. All the 

 material near the Potomac River and north of it has this character, but 

 the larger portion of the sand of all the areas does not form a coherent 

 rock. The indurated sand or sandstone generally forms irregular masses, 

 inclosed in the moi-e friable material, or some irregular portion of a friable 

 stratum may show this induration. The crumbling sandstone or sand is 

 - often quite fine in texture, and has more or less of a light-colored to white 



