12 ST. Paul's rocks, 



face 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 gi'ey. 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 jioli-h, 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 in- 

 crustation. When we remember that lime, either 

 as a phosphate or carbonate, enters into the compo- 

 sition of the hard parts, such as bones and shells, 

 of all living animals, it is an interesting physiolo- 

 gical fact* to find substances harder than the enam- 

 el of teeth, and coloured surfaces as well polished 

 as those of a fresh shell, reformed through inorgan- 

 ic 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 spe- 

 cies of gannet, and the latter a tern. Both are of 

 a tame and stuj)id disposition, and are so unaccus- 

 tomed to visitors, that I could have killed any num- 

 ber of them with my geological hammer. The 



* Mr. Horner and Sir David Brewster have described (Philo- 

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

 resembling shell." It is deposited in fine, transparent, highly pol- 

 ished, brown-coloured laminae, possessing peculiar optical prop- 

 erties, 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 mat- 

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

 see the strong tendency which carbonate of lime and animal mat- 

 ter evince to form a solid substance allied to shell. 



