64 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



At extreme high tide when the wind is high (neap tide at Pernambuco 

 is less than one metre ; spring is 2.2 metres), the surf breaks over the 

 top of the reef ahnost its entire length, though not with force enough to 

 disturb the shipping anchored in the narrow harbor behind the reef. 



The upper surface is approximately flat, but somewhat rough, owing to 

 the A'arying hardness of the rock and its uneven wearing. To protect 

 the reef, and to prevent the surf from disturbing the shipping inside the 

 harbor, an artificial stone wall was built during the Dutch occupancy 

 along the northern end of it.-^ In width it varies from twenty to sixty 

 metres. 



The inner or landward face of the reef is slightly irregular, as is shown 

 in the accompanying illustrations. The scour of the ebbing tides sweeps 

 out seawards the silts brought down from the land, so that the inner face 

 of the reef is abrupt, and the water close alongside is usually deep. 



The reef rock is composed mostly of siliceous sand grains cemented by 

 carbonate of lime. It contains besides many shells of such mollusks as 

 live in the sea along the coast, and more or less calcareous matter from 

 broken Serpulae tubes, mollusks, gorgonias, and the like. The shells 

 retain their original bright colors. 



The structure of the stone reefs was never certainly known until 1874, 

 when Sir John Hawkshaw, the English engineer employed by the Bra- 

 zilian government to report upon the harbors of that country, made a 

 series of borings upon the Pernambuco reef and in the spit upon which 

 Kecife stands. These borings show tliat the hard rock is only three or 

 four metres thick, and that beneath this are beds of sands, clays, marls, 

 and shells. The deepest boring on the reef was made nearly opposite 

 the landing-place at Recife, and was seventeen metres in depth. The 

 record is as follows : — 



Record of Boring on the Pernambuco Reefs. 



Metres. 



Reef rock, hard 2.95 



White sand 1-22 



Shells 1-10 



Gray sand 0.65 



Broken rock 1'22 



Dark sand 2.10 



Mottled clay 1-80 



Yellow clay 0.70 



Gray sand 3.00 



White sand 2.20 



Journal of a voyage to Brazil, by Maria Graham, London, 1824, p. 101. 



