19 



casts lie loosely in the cavities produced by the removal of the 

 shells, entirely distinct from each other, and covered by a film of 

 oxide of iron. The layer beneath, consisting of bluish green 

 silicious clay, is full of well preserved Pectens, Pernas, and a variety 

 of other shells. 



In general, the various species of shells are found associated in 

 colonies or groups, but, as in the case of banks of recent shells, 

 these colonies contain many scattered specimens, differing from the 

 general contents of the group. The two species of Chama, the C. 

 congregata and C. corticosa, which are found in almost every de- 

 posite of shells in this region, in many cases form extensive beds, 

 with a very small admixture of other genera. The best agricultural 

 marl, of a purely calcareous nature, which is used in lower Virginia, 

 is derived from these beds of Chama, the friable texture of the shell 

 upon exposure to the air, rendering this species of marl more easy 

 of application to land, and more prompt in its ameliorating effects. 

 Crassatellae often form an extensive deposite, and the large Pectens 

 occur in continuous layers of considerable depth and extent. The 

 different species of Area, Artemis Crepidula, &c. present a similar 

 arrangement. Even those shells which are of comparatively rare 

 occurrence, are usually found in little groups. Thus the Isocardia 

 fraterna is found, to the extent of a dozen or twenty, closely packed 

 together. This gregarious assemblage of shells of the same species 

 is what would naturally be anticipated in the absence of violent 

 agencies during or after their deposition, and furnishes another very 

 striking proof of the comparatively tranquil condition of the sea or 

 estuary in which they were allowed to accumulate. 



DISPOSITION OF THE FOSSILS. 



In nearly all the vertical sections of the deposite we are now de- 

 scribing, a series of beds or strata may be observed, each dis- 

 tinguished by the predominance of one or more species, and the 

 order of superposition of these beds frequently continues without 

 interruption for some distance. It docs not appear, however, that 

 in localities remote from each other, the arrangement of the 

 shells is always alike, although in many instances there appears to 

 be a striking correspondence. In a majority of cases in the neigh- 

 bourhood of Williamsburg, the upper layer is composed principally 

 of Chama congregata. In many localities also, the large Pectens 



