3 so 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 

 a nearly straight and unbroken Hne to near the latter place, where, 

 however, it is cut through so as to afford a passage-way for small 

 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. 



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. 



