.SINGL'LAU INCKUriTATlONri. 11 



ly to a coating ot" a hard, glossy substance, with a 

 pearly lustre, which is intimately united to the sur- 

 face of the rocks. This, when examined with a 

 lens, is 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 fovmd certain stalactitic branching liodics, 

 formed apparently in the same manner as the thin 

 white coating on these rocks. The branching bod- 

 ies so closely resembled in general appearance cer- 

 tain nuUiporae (a family of hard calcareous sea- 

 plants), that in lately looking hastily over my col- 

 lection I did not jierceive the difference. The glob- 

 ular exti'emities of the branches are of a pearly tex- 

 ture, 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 incrusta- 

 tion is deposited on the tidal rocks, by the water of 

 the sea, resembling certain cryptogamic plants 

 (Marchantia^) often seen on damp walls. The sur- 



