1879. | Prof. Harit 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. 
e 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 de 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 burrowing 
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 Achino- 
metra subangularis, 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 Ave feet, and a width of three or four inches, the 
urchin apparently occupying the lower end of the see 
holes. 
