50 THE rOTOMAO OR YOUNGER MESOZOIC FLORA. 



mon in some of the gray clays of the Petersburg area, but it is not there 

 abundant. This is not what we would expect if we attribute the origin of 

 the material to the crystalline rocks lying to the west of the Potomac. In 

 most of these mica forms a large proportion of the rock, as in the granite, 

 gneiss, mica-schist, hornblende-schist, etc. The sandy matter of the Poto- 

 mac is by most persons called a sandstone, but it is more properly a sand 

 in most cases. The gradation in the fineness of the material and the 

 change in the proportion of clay and sand, as has already been stated, are 

 not what we should expect if the sediment came from the west, the mate- 

 rials being coarser and having more sand in the exposui'es farthest east. 



The color of the sand, especially of the lower beds, when they con- 

 tain much diffused clay, is light gray or white ; but this material, owing 

 apparently to the oxidation of diffused pyrite, is sometimes colored yellow- 

 ish, reddish-brown, etc., the colors being in irregular patches and seams. 

 This coloration often follows, and makes much more distinct the planes of 

 current-bedding. 



Towards the upper part of the formation there is a considerable ad- 

 mixture of colored and impure clays with the sand, which then tends to 

 assume browni.sh-gray, yellowish-gray, and other kindred colors. The 

 colors are in such cases often arranged so as to produce a motlling on a 

 small scale of gray with other colors, or a blotching on a large scale. 

 Towards the western margin of the Fredericksburg area, and especially 

 between Acquia Creek and Occoquan, there is a large proportion of sandy 

 claj's of various colors — gray, reddish, yellowish, etc. The colors here are 

 often arranged irregidarly, producing mottling and blotching. These 

 colored impure clays are in considerable force along the route of the 

 Atlantic Coast Line Railroad between Alexandria and Telegraph Station. 

 Possibl}' some of the Variegated Clay members may be found with these 

 clays. 



Pebbles and cobbles, as will appear from what has already been said, 

 are not uncommon in the lower Potomac. They occur at different hori- 

 zons within the beds and are common towards the top. Very generally in 

 Virginia, where the Eocene rests on the Potomac sand, the upper portion of 

 the latter is a layer of cobbles packed in sand. This is the case at Rich- 



