290 
Lewes, Del. The shells are in very perfect preservation, 
though the epidermis is nearly gone, and the texture is be- 
coming fragile and chalky. The principal species are San- 
guinolaria fusca, Nassa obsoleta, and Modiola plicatula, of which 
the first two are now living on the beaches outside, and 
probably the last also. These specimens give an excellent 
illustration of the mode of formation of many of our fos- 
siliferous clays and marls. The deposit may, perhaps, have 
value as a fertilizer. 
He also gave some description of the very remarkable sand- 
dunes or moving hills at Cape Henlopen, a mile or two east 
of Lewes. The sand brought down by the Delaware River 
accumulates at this point, and when thrown up on the beach, 
is taken in charge by the heavy east winds, and carried in- 
land in a great line of drifting hills, which rises in a very 
long and gentle slope on the winlward side, and falls off 
abruptly from the crest on the leeward, as is usual in wind- 
drifts. The whole surface of the windward side is studded 
with the tops of dead tree trunks,—the remnants of a pine 
forest, overwhelmed by the advance of the hill. The crest 
seems steadily approaching the light-house keeper’s dwelling, 
and will, probably, necessitate its removal in the course of 
some few years. 
Pror. A. M. Epwarps said the specimens just exhib- 
ited are of considerable interest, as they show very nicely 
the mode in which certain stratified rocks containing fossils 
are evidently formed. Under certain circumstances, say 
when formed in a locality like the Tropics, where animal 
life abounded and the Mollusca especially occurred in large 
quantities, so that Caleareous matter would accumulate, such 
a deposit might become, in time, converted into a Limestone 
in which the forms of the enclosed shells and other organic 
remains would be preserved in a more or less perfect manner. 
Tf, on the other hand, Calcium compounds were not present 
in abundance, but the particles of the deposit thrown down 
should consist of coarse and for the most part Siliceous sand, 
Sandstone, also enclosing fossils, would eventuate. But to 
