branner: the stone reefs of brazil. 249 



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, whatever they were, are 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 color- 

 ing ; 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 complete 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 in- 

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

 has been the result. 



" It is evident that the serpulse and nullipores were at one time living together 

 over the surface of the reef, and by their combined action has been formed most, 

 if not all, of its outer raised portion, which is sometimes over four feet high and 

 twenty-five feet across. The barnacles are generally broken from the reef when 

 dead, but are sometimes overgrown by worm tubes and thus become imbedded, 



" Here and there, the slaves, in procuring lime, have quarried into the low 

 inner part of the reef, and even into the high wall-like portion. Good sections 

 for study are thus formed, and they tell us of what the reef consists. Many 

 large heads of Orbicella, Acanthastraea, and Siderastraea stand there exposed in 

 their original positions, and when cut through show their structure to be aa 

 open and perfect as though they were still living. With them are many 

 large millepores and nullipores, and all the intervening spaces are filled in with 

 a compact calcareous substance. 



" Our structure began as a true coral reef, stretching along the submerged 

 rocky ledge. The water was very shallow, however, and the reef soon reached 

 a level above which its corals could not live. Over them nullipores be^an to 

 grow, but probably while the reef was being raised by other causes than those 

 of growth, large numbers of these dead and partly entombed corals were swept 

 inward by the waves. Nullipores continued to thrive and serpulae came in to 

 aid them, but with these forms we are already familiar." 



Reefs between Itaparica and Caravellas. — South of Itaparica the 

 coral reefs have been but littlo studied, probably on account of the 

 difficulties of transportation along this part of the coast.* 



1 One can get an approximate idea of these difficulties when I say that the 

 Bahia company wliose steamers run as far soutli as Viyosa and the Abrolhos have 



