408 Ridley — Raised Reefs of Fernando de Noronha. 



very different in appearance and constituents from the reef- 

 rock. I noted some points with respect to the reef -rocks. This 

 rock is not at all like the coral reef of the open ocean atoll, 

 such as Cocos island. It is not built to any large extent of the 

 larger corals, but the nullipore seaweeds take the most impor- 

 tant part, broken shells, echinus tests, worm tubes, crabs all go 

 to make it up. The growth is chiefly produced by the nulli- 

 pores. Xow the reef does not occur everywhere. It cannot 

 grow on sand because of the fluctuation of the bottom nor on 

 bowlders which are too irregular and constantly shifting. In 

 every spot, where I noticed reef in Fernando de Noronha, I 

 found a small stream trickling into the sea, bringing down silt 

 enough to make a firm bottom for the nullipores. The same 

 observation was made at the Recife (I prefer to keep the old 

 name for the reef itself) and at Itamaraca, And even here I 

 see that where streams debouch into the sea the silty banks 

 they form on one side are the best places for corals. This fact 

 would account for not only all the silica which is deposited in 

 the "reef rock," but for in some cases a larger proportion. 

 If Mr. Branner will examine the Pernambuco Recife at low 

 tide, he will see how compact it is and how it rings to the ham- 

 mer. It contains much silt, but it is not an seolian sandstone, 

 but true reef rock still growing. If he will walk along the 

 coast to Olinda he will pass plenty of sand banks but though 

 these have probably been growing for centuries there is not a 

 bit of the rock which he classes as aeolian sandstone, simply 

 because no silt comes down, and the sand is too shifting for 

 any growth of nullipores, etc. 



I noticed too that where the base of the supposed sandstone 

 could be seen it always overlay, neither sand, nor bowlders but 

 silt. One spot at first puzzled me. It is in Ilha Rapta at the 

 landing place, here is a stratum of black basalt bowlders which 

 at one part is covered with about ten feet of reef rock. At 

 first sight the reef appeared in contact with the bowlders, but 

 closer examination showed that the bowlders had been ce- 

 mented together (with gypsum probably) and then covered 

 with silty stuff on which the reef had grown. 



Both at Fernando de Noronha and at Christmas Island 

 raised reefs occurred in strata lying flat upon each other. In 

 some spots the upper part of each reef seemed distinct in tex- 

 ture and coloring, suggesting a frame before the deposit of the 

 next reef upon it. I do not see how sand dunes could be 

 formed into flat regular strata like this. When I was recently 

 at Anjar point in Java opposite Krakatao I was shown an 

 immense block of stone said to have been thrown there from 

 the volcano. On close examination, however, it proved to be 

 a block of coral-limestone, a block of reef rock, just like those 



