GEOLOGY OF FEBNANDO NORONHA. 93 



worms, Crustacea, and mollusca being very abundant ; and over 

 and round tlie edges grow many calcareous Algae and Poraminifera. 

 Although it is natural to talk of it as coral-reef, corals contribute 

 but a small share of its structure. A broken piece of reef shows 

 that layers of Lithothamnion, and other calcareous Algse, with 

 the tubes of Serpulce, JPolytrema ruhrum, and calcareous debris 

 form the greater part of the mass. These, however, are only 

 distinguishable at the top of the living reef. About an inch 

 •below the surface the remains become indistinguishable. 



With respect to the distribution of the reef, it may be noted 

 that it apparently never forms where the cliffs descend directly 

 into the sea, nor on the shores covered with large loose boulders, 

 nor in the sandy bays, but it seems to me that its formation here 

 is to a certain extent dependent on the streams which pour into 

 the sea at different points. Thus, there are streams at almost if 

 not all the living reefs, viz., at Chaloupe Bay, Sponge Bay, and 

 Sambaquichaba. 



The Eecife, too, at Pernambuco is at the mouth of the river, and 

 at Itamaraca, further along the coast to the north, Mr. Eamage, 

 who visited it, reports very extensive reef; and here, again, rivers 

 enter into the sea. 



I imagine that the nullipores, corals, and other plants and 

 animals which make the reef cannot grow upon sand which is 

 always shifting, nor upon irregular boulders ; but where the sand 

 becomes mixed a little with silt, or the gravel consolidated by 

 it, they can grow and thrive. 



The reef grows only in water just below high-water mark, 

 and abruptly terminates in ledges, beneath which are usually 

 hollows and caverns. The chief growth is along the edges exposed 

 to the waves. Sometimes, as at Sponge Bay, the outer edge of 

 the reef is much higher than the inner portion, probably owing to 

 the more rapid growth here of the nullipores. 



The shore of the neighbouring bays consists of sand or basalt 

 pebbles, and there is no more reef till Sambaquichaba, where a 

 narrow spit or two runs out into the sea. We saw no more reef on 

 this side of the island. On the south side, beginning again at the 

 east, there is a very extensive reef in Sponge Bay, of considerable 

 thickness and covered at high water. Beyond the reef, exposed 

 at low water, there seems to be a lower ledge of great extent. 

 Ear out at low water can be seen two rocks just raised above 

 high water, over which the sea constantly breaks. These are, I 



