29 



KINDS OF SHELLS WHICH ARE MOST SUITED FOR THE PURPOSE OF MARLING. 



The Chama, formerly mentioned as occurring in extensive beds 

 in some portions of the Miocene, and existing in considerable pro- 

 portion in nearly all the fossil strata, forms the principal component 

 of some of the most productive marls. This is a small two-valved 

 shell of rugged exterior, which readily breaks to pieces in the ground, 

 and is spread evenly over the land with great facility. Another 

 shell, (Serpula,) of which there are numerous rich beds in Surry 

 county, possesses these advantages in a still higher degree. From 

 its fragile texture, and irregular tubular structure, it is quickly min- 

 gled with the soil in a minutely subdivided state, and as like the 

 Chama, it frequently occupies the marl beds to the exclusion of other 

 fossils, it furnishes a marl of a very superior description. As a ge- 

 neral rule, the small shells are most likely to prove efficacious, as well 

 from the fact that, where they occur, the proportion of carbonate of 

 lime and green sand is usually great, as because they are most 

 easily reduced to the condition in which they become available in the 

 land. 



Of the Pulverulent white marl. — In many situations the marl pre- 

 sents an appearance not very unlike that of an impure chalk. The 

 mass of the stratum is chiefly made up of a white or yellowish fri- 

 able material, intermixed with fragments of the harder species of 

 shells. In such cases the shells are rarely found entire, and the con- 

 dition of the fragments is usually such as to render it difficult to re- 

 cognise the species of fossil to which they belonged. Many exten- 

 sive beds of marl of this description have been opened in the coun- 

 ties of Middlesex, New Kent, James City, York and Gloucester, all 

 of them largely abounding in calcareous matter. Even as much as 

 97 per cent, of this substance was found in a specimen from one of 

 these localities ; and it will appear from the table of calcareous marls 

 hereafter to be given, that in general the proportion exceeds 80 in 

 the 100. Occasionally, however, a mixture occurs in which the cal- 

 careous matter is blended with a large proportion of white clay and 

 sand, presenting in the mass an aspect so nearly resembling the for- 

 mer, that without chemical analysis it would be difficult to distin- 

 guish between them. 



Of the Blue marl. — In our general description of the arrangement 

 of the fossiliferous strata, mention has been made of the bluish green 

 or clayey marl which occurs low down in the ravines and near the 

 water's edge on the river banks. This is what is usually known 



