1(5 



varieties of the soil. Many highly valuable marls extensively in use 

 are of this description. 



The very general existence of the lower stratum, above described, 

 forms an interesting and prominent feature in the geology of the 

 Miocene Tertiary districts, as well of eastern Virginia as of 

 Maryland. Throughout all the upper fossiliferous strata, as well 

 as in the argillaceous beds just mentioned, will be found disseminated, 

 greenish black grains of Silicate of iron and potash, identical with 

 those already described as existing in the stratum immediately 

 overlying the shells, and having the same form and composition 

 with the granules contained very abundantly in an older formation, 

 both in this country and in Europe. In some beds of the marl or 

 shells, these particles so abound as to give a very decked colour to 

 the whole mass. In specimens from James City and York counties, 

 as much as thirty-five per cent, of the green sand has been found, 

 and occasionally shells are seen filled with this substance almost 

 alone. 



The surface of the strata containing shells is usually. irregular. 

 Sometimes it rises abruptly, in the form of a hillock,, then it is 

 scooped out into depressions of a few feet in depth. These irre- 

 gularities, however, are apparently of two kinds ; the one the origi- 

 nal form of the deposite, the other produced by denuding action upon 

 the surface. Thus in many places the same stratum may be re- 

 marked, rising with more or less abruptness, then again descend- 

 ing, and perhaps preserving a nearly horizontal line for some dis- 

 tance, marked at its upper surface by a clear and unbroken outline, 

 and presenting no indication of violent abrasion from above. In 

 other places, and this is a very frequent occurrence, deep and irre- 

 gular furrows and cavities are seen, such as would naturally arise 

 from the action of the currents and eddies of a large mass of water 

 in rapid movement. Whether from this cause, or from the gradual 

 dissolving action of percolating water, sinks exist in this region in 

 many places, though they are by no means as numerous or ex- 

 tensive as in the limestone districts. 



Having thus given an account of the nature and arrangement of 

 the strata overlying the shells, as well as those in which they are 

 embedded, we will now describe the general condition and disposi- 

 tion in which the shells occur. 



