1832.] SINGULAR INCRUSTATIONS. 9 



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- 

 gamic plants (Marchantiaj) 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 in- 

 crustation 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 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 bones and shells, of all living animals, it is an 

 interesting physiological fact * to find substances harder than the 

 enamel of teeth, and coloured surfaces as well polished as those 

 of a fresh shell, reformed through inorganic means from dead 

 organic matter mocking, also, in shape some of the lower vegetable 

 productions. 



We found on St. Paul's only two kinds of birds the booby and 

 the noddy. The former is a species of gannet, and the latter a tern. 

 Both are of a tame and stupid disposition, and are so unaccustomed 

 to visitors, that I could have killed any number of them with my 

 geological hammer. The booby lays her eggs on the bare rock ; but 

 the tern makes a very simple nest with seaweed. By the side of 

 many of these nests a small flying-fish was placed ; which, I sup- 

 pose, had been brought by the male bird for its partner. It was 



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

 Transactions, 1836, p. G5) 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. 



