BUANiXEU: THE STONE EEEFS OF BRAZIL. 2G3 



would therefore be regarded as fringing reefs. This relation has usually 

 been brought about, however, by small geographic changes, such as the 

 dnftuig of the shore sands behind the outer reefs. The channel between 

 the coral reefs and the shore is deepest between the Abrolhos and the 

 Parcel das Puredes. It is there only eleven fathoms. For the most part 

 the depth IS considerably le«s, and too small to admit the entry of ordi- 

 nary sailing vessels. 



With the exceptions noted below the coral reefs of Brazil have no 

 apparent connection with eruptive phenomena. There are probably in- 

 c.pient reefs about the volcanic island of Fernando de Xoronha and it is 

 possible or even probable that the Rocas reefs are built upon an erup'- 

 tive base. But the Rocas reef is two hundred and twenty-four kilo- 

 me res from the Brazilian mainland, with a channel of over two thousand 

 fathoms separating it from the barrier reefs of the coast. The island of 

 Santo Alaxo, just south of Cape Santo Agostinho, is likewise eruptive 

 but I IS only two or three kilometres from the shore, and therefore h.^.s 

 no relation with the coral reefs other than that of the sedimentary rocks 

 along the same coast. There are igneous rocks also in the Abrolhos 

 Islands, but they are sheets and dikes in the rocks that form the group. 



The Chemical Composition of the Brazilian Corals. 



I have had analyses made of a few of the skeletons of living corals 

 the results of wliich are given in table (A) on pa^e 264 



All samples were washed with boiling distilled water to remove sea 

 salt, and the complete removal was verified. The washed samples were 

 dried and of these the analyses were made. All the specimens contained 

 considerable organic matter. 



It should be noted that these analyses are of skeletons of polvps the 

 upper portions of which were living when the samples were cdlected 

 Samples were also taken of the dead reef rock at Ponta do Mangue,' 

 State of Alagoas, one hundred kilometres northeast of Maceio, and of 

 this rock an analysis has been made with the results as -ivon in table (B) 

 on the next pa"-e. "" ^ ■' 



If all the calcium present were there as carbonate, and tlie rest of the 

 carbon diox.de as magnesium carbonate, it would be equivalent to 



Carbonate of lime (CaCOg) go.lO 



Carbonate of magiK'siuui (MgCOg) , . . 12.93 

 It should be especially noted that the specimens of the reef rock rep- 

 resented by the last analyses were taken from an old reef that has Ion. 

 been dead. = 



