350 Prof. Hartt on the Brazilian Sandstone Reefs. June, 
situated to the north of the city, to the Ilha dos Pinhos, at the 
south. Its course is south, a few degrees west, and it runs in 
_ anearly straight and unbroken line to near the latter place, where, 
however, it is cut through so as to afford a passage-way for smail 
boats and jangadas. From this point to its southern end it is 
much broken up and generally irregular. 
But to examine it the more closely we must land upon it at 
low tide, and walk along its nearly level surface, stopping at times 
to investigate any unusual appearance that may attract our 
attention. 
The reef rock is a sandstone, of a light brownish tint when 
wet by the waves, and is formed of siliceous sand and rounded 
pebbles, mingled with which are many perfect and broken shells, 
all firmly cemented together by carbonate of lime. The most 
abundant shell in the sandstone is a species of Venus, still very 
common on the neighboring shores, and preserving in the rock 
its natural colors. 2 
At its northern end the reef is very narrow and has, apparently, 
been much undermined and worn by the waves, for it is lower 
here than to the south, and the sandstone lies in immense blocks, 
inclined toward the sea. To increase its height and better pro- 
tect the harbor at this place, an artificial wall, partly of brick and 
mortar, partly of reef stone clamped together, has been con- 
structed. That the reef formerly extended much farther north- 
ward is evident from the existence of an isolated mass of reef 
rock just off the northern end, and of a stretch of submerged 
reef beyond the channel. Upon the extreme point of the reef 
stands the Pernambuco light-house, and a short distance from it 
is a little old octagonal fort, called Picao, built of reef stone, and 
occupying the entire width of the reef. 
Following southward, the reef becomes broader and more level 
on top, but is still very irregular at the sides. In front of the 
city it has an average width of about two hundred and fifty feet, 
and a height equal to about that of high tide, though on account 
of the great commotion made by the waves at such times, it is 
impossible to exactly determine this fact. As to the thickness of 
the reef, it must reach down to below the level of low tide, 
because the rock is never entirely uncovered by the water. 
The highest part of the reef forms a broad belt of varying — 
width, whose axis ‘lies a little to the inside of that of the reef. 
