Crosby.] 138 [February 5, 
fine gravel, a half mile south of the cliff from which the large speci- 
mens were obtained. The scarcity of this peculiar rock has been in- 
dicated by the statement that a search of three or four hours was 
required to find a dozen pieces of it, where the whole bank one hun- 
dred and fifty feet high was plentifully strewn with pebbles. It is 
not improbable, therefore, that fragments of it may occur sparingly 
throughout this whole series of cliffs, although I detected it only in 
the places mentioned. 
The rock is mainly a firm, massive, white, caleareous sandstone, 
effervescing freely with acid, and yet, to the eye, principally com- 
posed of rounded grains of transparent quartz. The fossil shells are 
always abundant, and in many cases form a larze part, nearly the 
entire mass, of the rock, which then becomes a shelly limestone; but 
the contained fossils are not the source of all the caleareous material, 
for Mr. Upham has found in this rock a distinct lime-stone pebble 
about half an inch in diameter. Minute grains of glauconite are 
freely disseminated; and this mineral not unfrequently fills the molds 
formed by the dissolving out of organic remains; but it never forms 
a prominent feature of the rock. Vestiges of lignite and grains of 
iron oxide are also sometimes observed. With the exception of the 
lignite, the organic remains are all animal, principally molluscan, 
bivalves predominating; and, although I have seen only a few small 
masses, probably less than fifteen pounds, of the rock, fully twenty- 
five species are represented, proving a varied as well as an abundant 
fauna. The large shells are usually in a fragmentary state, and this, 
together with the coarseness of much of the sand, is a plain indica- 
tion that the materials did not accumulate in tranquil water. In 
many other cases, too, the original shells have been removed by solu- 
tion, leaving nothing but imperfect molds or internal casts, though 
sometimes replaced, as above stated, by green sand; all of which adds 
to the difficulty of accurate identification. 
Ihave been able to determine satisfactorily only a small proportion 
of the species. None of these are recent, but appear to be mainly 
characteristic Eocene forms. One large Lamellibranch, although 
represented only by the mold of a single valve (the right), is cer- 
tainly Venericardia planicosta Lamk., now known from the Lower 
Eocene of Virginia. There appear to be two other species of this 
genus. One of these, a minute form, is probably J’. parva, de- 
scribed by Lea from the Eocene of Alabama; while the other, one 
specimen of which shows the valves united, is very near the young 
