352 Pi'of. Hartt on the Brazilian Sandstone Reefs. [June, 



Whether the process by which the boring is accompHshed, in 

 this special case, is chemical or mechanical, it is very certain that 

 this sea-urchin is able to resort to the latter means at times, for 

 its excavations are common in the trap and gneiss rocks of many 

 parts of the coast. 



The sea-urchin holes, enlarged by the action of the surf, give 

 rise to cavities and irregular pools on the surface of the reef, and 

 these are deepened and widened by the dissolving action of the 

 water left stagnant in them at low tide. But nullipores, barnacles 

 and tube-building worms also live upon the surface, in immense 

 numbers, and construct a very hard and durable substance which, 

 to a certain extent, compensates for the loss of material caused 

 by the sea-urchins. The barnacles and worms build up little 

 walls of limestone around the mouths of the holes and between 

 them, keeping them for a time more or less separated and per- 

 fect; but these walls are being constantly broken away, to- 

 gether with much of the surrounding material, resulting in the 

 formation, over most of the outer slope of the surface, of a mul- 

 titude of irregular, shallow pools, whose edges are protected by 

 the same growth of barnacles and worm tubes. Many project- 

 ing masses, also capped with the hard limestone coating, are left 

 standing in the pools, and wearing most rapidly below, they pro- 

 duce mushroom-like structures, which are very characteristic of 

 the outer surface. They reach to the same height as the margins 

 of the pools. 



Ordinary sea-weeds grow luxuriantly on the outer edge of the 

 reef, but they are of slight importance compared with the calca- 

 reous nullipores, which live in the full face of the surf and form, 

 over large areas, a crust of a foot or more in thickness. Thus 

 the barnacles, worm-tubes and nullipores combined, aid very 

 materially in protecting the reef from wear, but the good they 

 do is more than counterbalanced by the harmful action of the 

 waves and sea-urchins, and, unless the latter are in some way 

 exterminated, they will eventually work much mischief to the 

 harbor of Pernambuco. 



The inner slope of the surface of the reef differs from the 

 outer, in not possessing those features which result from the 

 actions of the sea-urchins, and also in being without the nullipore 

 crust. It is generally characterized by a series of shallow basins, 

 lying one below the other along the slope, the rim of each being 



