1832.] SINGULAR INCliUSTATIOKS. 9 



the thin white coating on these rocks. The branching bodies so 

 closely resembled in general appearance certain nulliporae (a 

 family of hard calcareous sea-plants), that in lately looking hastily 

 over my collection I did not perceive the difference. The glo- 

 bular extremities of the branches are of a pearly texture, like the 

 enamel of teeth, but so hard as just to scratch plate -glass. 1 

 may here mention, that on a part of the coast of Ascension, 

 where there is a vast accumulation of shelly sand, an incrustation 

 is deposited on the tidal rocks, by the water of the sea, resembling, 

 as represented in the woodcut, certain cryptogamic plants (Mar- 

 chantiae) often seen on damp walls. The surface of the fronds is 



beautifully glossy ; and t-hose parts formed where fully exposed 

 to the light, are of a jet black colour, but those shaded under 

 ledges are only grey. I have shown specimens of this incrusta- 

 tion to several geologists, and they all thought that they were of 

 volcanic or igneous origin ! In its hardness and translucency — 

 in its polish, equal to that of the finest oliva-shell — in the bad 

 smell given out, and loss of colour under the blowpipe — it shows 

 a close similarity with living sea-shells. Moreover in sea-shells, 

 it is known that the parts habitually covered and shaded by the 

 mantle of the animal, are of a paler colour than those fully ex- 

 posed to the light, just as is the case with this incrustation. 

 When we remember that lime, either as a phosphate or carbonate, 

 enters into the comoosition of the hard parts, such as bones and 



