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 almost 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 varying 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 
Recife stands. These borings show that 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 or BORING on THE PERNAMBUCO REEFS. 
METRES. 
REL TOE HAND TU eI , у у, ‚у, у, 200 
White sand kcax ee е ee бор 
SU „у, шуо у ТО 
сп SUM > se oO 
Broken MOCK: 2 o у V 4 19 
DERA +... 4 o0 9 21535 a. 0 
MOMMA Save ca 180 
Yellow clay OA 
GUN said. + mot... roo, 800 
White sand ls PD ve de ut v 200 
Journal of a voyage to Brazil, by Maria Graham, London, 1824, р. 101. 
