18 



of the Crepidula, the minute and sharp angles of the Fusus Tetricus, 

 the thin and hollow Fissurella — are found In perfect preservation 

 in many places. The state of the shells seems to depend chiefly 

 upon the mechanical texture and chemical character of the materials 

 with which they are mixed, and of which the overlying stratum is 

 composed. In the moist blue clay, the shells are generally found in 

 a very soft condition. In a highly ferruginous clayey bed they are 

 found either partially or entirely dissolved, and beautiful casts re- 

 main in their stead. 



In many places entire banks occur, composed of casts of Chama 

 and other shells, sometimes separate, sometimes cemented together 

 so as to form a species of rock. These appearances occur chiefly 

 near the surface, and when the soil is porous and ferruginous. 

 The casts thus formed, often consist chiefly of carbonate of lime, 

 and in many specimens as much as eighty per cent, of this sub- 

 stance is found. Casts of this kind belong mostly to the smaller 

 shells, and by far the most common are of the Chama congregata. 

 These, as already stated, are often found nearly or quite empty, 

 and we may, therefore, conceive, that as the matter of the shell in 

 an extensive bank of Chatnas is gradually dissolved, the water 

 charged with carbonate of lime enters the cavities, and slowly de- 

 posites the carbonate mixed with fine particles of clay and sand. 

 Thus by degrees the cavities are filled. In the mean time the shell 

 disappears, frequently leaving on the surface of the cast a chalky 

 covering, like the decomposed inner film of shelly matter. In sup- 

 port of this explanation it may be added, that in many casts beauti- 

 ful crystals of carbonate of lime are found, forming a portion of the 

 cast, and having the appearance of Dog-tooth Spar. In some cases 

 the shelly matter appears to have been dissolved, and its place sup- 

 plied by the crystallized carbonate, encrusting the earth formerly 

 contained within the shell. Sometimes, too, a thin film of oxide of 

 iron surrounds the cast, showing very distinctly all the markings 

 of the inner surface of the shell. In many localities, presenting a 

 series of beds differing in composition, the shells will be found per- 

 fect in some of them, while in others immediately above or below, 

 only casts remain. Thus at the College mill, about one mile from 

 Williamsburg, the upper fossiliferous layer is a yellow silicious sand, 

 containing perfect shells. Below this is a brown ferruginous clay, 

 filled with the mo6t beautiful casts of Chama, Pectunculus, Turri- 

 tella, &c. The shelly matter has entirely disappeared, and the 



