1879] Brazilian Corals and Coral Reefs. 549 



most conspicuous, and they are sometimes nearly perfect, but 

 most often broken into irregular masses, large and small. The 

 majority are also coated over with a thin nullipore crust, as 

 though they had been dead a long time before they were swept 

 from their proper dwelling places. This coral deposit has con- 

 siderable thickness near the middle of the channel and thins out 

 gradually toward the beach. 



The extreme southern end of the reef is very low, and near to 

 the beach. It breaks down abruptly on the outer side, but on the 

 inner is bordered by a thick, consolidated layer, which reaches so 

 nearly its own level that it is often difficult to make out the 

 dividing line between the two. A close examination, however, 

 discloses the upright corals in the one and the prostrate fragments 

 in the other. 



A great difficulty stands in the way of our determining the 

 intimate structure of this nearly extinct reef, whose outward 

 appearance and surroundings we have so fully discussed. It has 

 evidently not been formed entirely by those agents at present 

 occupying its upper and outer surfaces ; but the remains of the 

 real builders, whazever they were, are now entirely covered up 

 and hidden from view, excepting at the one point at the southern 

 end just mentioned. We must resort to artificial sections, no 

 easy undertaking in a coral reef. 



Breaking with hammer and chisel into the higher part of the 

 reef, we obtain specimens of a very hard, compact limestone, 

 partly of a nearly homogeneous structure, partly marked by 

 straight or wavy lines of lighter and darker coloring ; these two 

 kinds of structure are intermingled with one another without 

 order, sometimes one, sometimes the other predominating. The 

 former has resulted from the masses of serpula tubes — by the com- 

 plete filling in of their winding cavities and the spaces between 

 them by carbonate of lime, until no trace of the original structure 

 remains. The latter is due to the growth of incrusting nullipores, 

 one thin layer upon another, until quite a thickness of rock has 

 been the result. 



It is evident that serpulse and nullipores were at one time liv- 

 ing together over the surface of the reef, and by their combined 

 action has been formed most, if not all, of its outer raised por- 

 tion, which is sometimes over four feet high and twenty-five feet 

 across. The barnacles are generally broken from the reef when 



