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, whatever 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 
ormer 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 serpule 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 
