1 879.] Prof. Hartt on the Brazilian Sandstone Reefs. 351 



From this belt the surface slopes more or less strongly on both 

 sides, but presents quite different characters, resulting from 

 differences in exposure. The outer side of the reef has become 

 very irregular from the constant beating of the surf, and is pierced 

 with innumerable cavities of sea-urchins and thickly overgrown 

 with sea-weeds and calcareous incrustations. The inner side, 

 after a more or less rapid landward slope, breaks down abruptly 

 and irregularly, and often presents an overhanging edge. 



The reef is cut up into large blocks by joints or cracks, which, 

 though quite variable in their courses, may be reduced to two 

 general series, one parallel to the axis of the reef, the other trans- 

 verse to it ; but many run obliquely or radiate from a common 

 center, as though the reef at that point had settled down upon a 

 hard underlying spot ; sometimes they form a tangled maze. 

 These joints are vertical or highly inclined, and the angular 

 masses resulting from them are likely to be detached, on the 

 outer side of the reef by the force of the waves, and on the inner 

 side by the undermining action of the currents in the harbor. In 

 this manner the margins of the reef have been made very jagged, 

 the outer being the most irregular. On the upper surface of the 

 reef, where there has been no dislocation, the joints tend to widen 

 by the action of the surf and by chemical decomposition. There 

 are thus formed open passages, a foot to a yard or more in width, 

 and with a considerable depth of water. In these we always find 

 a rich collection of marine animals, corals and other polyps pre- 

 dominating. 



Now let us inspect more minutely the character of the surface, 

 and the many foreign objects living or growing upon it, which 

 tend either to protect it from wear, or to gradually and surely 

 effect its destruction. As stated above, sea-urchins are burrowins 

 into its outer edge. There is only a single species on the Brazilian 

 coast that is able to excavate in the solid rock ; it is the Ecluno- 

 vietra subangnlaris, everywhere abundant, and possessed of stout, 

 sharply-pointed spines. On abrupt slopes of the reef, this dili- 

 gent worker forms rounded holes, having only a slight depth, but 

 where the slope is gradual, the holes are much longer, running 

 either directly inwards, or in a more or less winding way, being 

 sometimes curved or bent upon themselves. They have often a 

 length of four or five feet, and a width of three or four inches, the 

 urchin apparently occupying the lower end of the elongate 

 holes. 



