38 St. Paul's Rocks. part 1. 



large. Of these vein-like masses, some calcareous ones, 

 which contain minute fragments of shells, are clearly 

 of subsequent origin to the others, 



A glossy incrustation. — Extensive portions of these 

 rocks are coated by a layer of a glossy polished sub- 

 stance, with a pearly lustre and of a grayish white 

 colour ; it follows all the inequalities of the surface, to 

 which it is firmly attached. TVhen examined with a 

 lens, it is found to consist of numerous exceedingly thin 

 layers, their aggregate thickness being about the tenth 

 of an inch. It is considerably harder than calcareous 

 spar, but can be scratched with a knife ; under the 

 blowpipe it scales off, decrepitates, slightly blackens, 

 emits a fetid odour, and becomes strongly alkaline : it 

 does not effervesce in acids. 1 I presume this substance 

 has been deposited by water draining from the birds' 

 dung, with which the rocks are covered. At Ascension, 

 near a cavity in the rocks which was filled with a 

 laminated mass of infiltrated birds' dung, I found some 

 irregularly-formed, stalactitical masses of apparently 

 the same nature. These masses, when broken, had an 

 earthy texture ; but on their outsides, and especially at 

 their extremities, they were formed of a pearly sub- 

 stance, generally in little globules, like the enamel of 

 teeth, but more translucent, and so hard as just to 

 scratch plate-glass. This substance slightly blackens 

 under the blowpipe, emits a bad smell, then becomes 

 quite white, swelling a little, and fuses into a dull white 

 enamel ; it does not become alkaline ; nor does it 

 effervesce in acids. The whole mass had a collapsed 

 appearance, as if in the formation of the hard glossy 

 crust the whole had shrunk much. At the Abrolhos 

 Islands on the coast of Brazil, where also there is much 



1 In my Journal I have described this sibs ance ; I then believed 

 that it was an impure phosphate of lime. 



