1879.] Prof. Hartt on the Brazilian Sandstone Reefs. 353 
protected by barnacles or worm-tubes. These basins are some- 
times several feet in diameter and a foot or more deep, and are 
often very irregular. Prof. Hartt has compared them in appear- 
ance to the basins formed by the hot springs of the Yellowstone 
valley; but while the latter have been formed by deposition, the 
former result from wear. 
Having now finished our rather detailed study of the reef 
where it is most perfect, we must proceed farther south and see 
how it terminates. Nearly opposite the mouth of the river 
described in the first part of this paper, we find a small channel 
running underneath the sandstone of the reef, and through this 
there is a constant movement of water. At the sides large 
blocks have been dislodged, and, tumbling down, have reduced 
the width of the upper surface to about thirty or thirty-five feet. 
It thus becomes evident, as has been otherwise proven, that the 
sandstone reef rests on a very insecure foundation of soft mate- 
rial, which the water has washed out in places, forming covered 
passage-ways through which there is a strong current, varying in 
direction according to the time of the tide. Engineers, in boring 
through the rock, came to loose sand underneath, demonstrating 
that the structure we are dealing with is only the consolidated 
capping of a long bar of sand. 
To the south of the channel just described, the reef has been 
much excavated, not only by the waves but by quarrying; this 
unwarranted destruction of the only object that gives to Pernam- 
buco its prominence as a commercial city has, however, been 
stopped, and a breakwater has been constructed at this point, 
but it is now much out of repair. Around the Ilha dos Pinhos 
the reef curves slightly outwards, and then, bending westward, 
approaches gradually to the shore, which it skirts for some dis- 
tance as a narrow line of rocks, almost lying upon the beach. 
Another small reef begins to the south, and running obliquely, 
finally joins the beach. 
This closes our imperfect description of the reef as it appears 
at low water; at high tide, on account of the shallow water with- 
out, it is played upon by a very heavy surf, which sometimes 
‘rolls completely over it, and wave after wave, rising above the 
outer edge, bursts high into air on striking the artificial wall at 
the north. During spring tides the water is much agitated within 
the reef, but not enough to endanger the shipping. 
