S/.YGC/LAR /A'CRUSTA T/OXS 



foiiiul lo consist of luimcrDiis cxcccdiiiLjl)' lliin l,i\crs, its total 

 thickness bcini^" about the tenth of an inch. It contains much 

 animal matter, and its orig-in, no doubt, is due to the action of 

 the rain or spra)- on the birds' duiiL^r I'elfjw some small 

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

 foimd certain stalactitic branching br>dics, formed a])[)arcntl}' in 

 the same manner as tJTc thin white coaling on these rocks. 

 The branching bodies so closcl)' resembled in general appearance 

 certain nulli[3or<e fa famil)- of hard calcareous sea-plantsj, that in 

 latel\- looking hastil)- <)\x-r my collection I did not percci\e the 

 difference. The 



globular extremities 



of the branches are d^I^ ^^^ j^^^e^W^MVlJK Ifc^f^ % 



of a pearl)- texture, 

 like the enamel of 

 teeth, but so hard as 

 just to scratch plate- 

 glass. I ma)- here 

 mention, that on a 

 part of the coast of 

 Ascension, where 

 there is a \-ast accu- 

 mulation of shell)- sand, an incrustation is deposited on the tidal 

 rocks, b)- the water of the sea, resembling, as represented in the 

 woodcut, certain cr)-ptogamic plants (Marchantiae) often seen on 

 damp walls. The surface of the fronds is beautifulh- gloss)- ; and 

 those parts formed where fully exposed to the light, arc of a 

 jet-black colour, but those shaded under ledges are onl)- gra)-. 

 I have shown specimens of this incrustation to sc\-eral geologists, 

 and the)' all thought that the)- were of \-olcanic or igneous origin ! 

 In its hardness and translucenc)- — in its polish, ccjual to that of 

 the finest oliva-shcll — in the bad smell gi\en out, and loss of 

 colour under the blowpipe — it shows a close similarit\- with 

 li\-ing sea-shells. Moreo\er in sea-shells, it is known that the 

 parts habituall)' co\'ered and shaded b)- the mantle of the 

 animal, are of a jjaler colour th,-m tliose full)- exposed to the 

 light, just as is the case with this incrustation. When we 

 remember that lime, either as .-i phosphate or carbon.-ite, enters 

 into the composition of the hard parts, such as liones .-md 

 .shells, of all li\-ing animals, it is ,in interesting plnsiological 



