SING ULAR I NCR US TA TIONS 



found to consist of numerous exceedingly thin layers, its total 

 thickness being about the tenth of an inch. It contains much 

 animal matter, and its origin, no doubt, is due to the action of 

 the rain or spray on the birds' dung. Below some small 

 masses of guano at Ascension, and on the Abrolhos Islets, I 

 found certain stalactitic branching bodies, formed apparently in 

 the same manner as 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 

 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 accu- 

 mulation 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 (Marchantia;) 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 gray. 

 I have shown specimens of this incrustation 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 

 tlie 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 



