1832.] SINGULAR INCRUSTATIONS. f 



so closely resembled in general appearance certain nulliporse (a family 

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

 collection I did not perceive the difference. The globular extremities 

 of the branches are of a pearly texture, like the enamel of teeth, but so 

 hard as just to scratch plate-glass. I 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 crypto^amic 

 plants (Marchantiae) often seen on damp walls. The surface of the 

 fronds is beautifully glossy; and those 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 incrustation to several 

 geologists, and they all thought that they were of volcanic or igneous 

 origin I 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 exposed 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 composition of the hard parts, such as bohes and 

 shells, of all living animals, it is an interesting physiological fact * to 



* Mr. Homer and Sir David Brewster have described (Philosophical 

 Transactions, 1836, p. 65) a singular " artificial substance resembling shell." 

 It is deposited in fine, transparent, highly polished, brown-coloured laminae, 

 possessing peculiar optical properties, on the inside of a vessel, in which 

 cloth, first prepared with glue and then with lime, is made to revolve rapidly 

 in water. It is much softer, more transparent, and contains more animal 

 matter, than the natural incrustation at Ascension ; but we here again see 

 the strong tendency which carbonate of lime and animal matter evince to 

 form a solid substance allied to shell. 



