BAR OF SANDSTONE. 
277 
Supplement on a remarkable Bar of Sandstone off Per- 
nambuco, on the Coast of Brazil. (Originally published 
in the Philosophical Magazine, October 1841, p. 257.) 
In entering the harbour of Pernambuco, a vessel passes 
close round the point of a long reef, which, viewed at high 
water when the waves break heavily over it, would natu- 
rally be thought to be of coral formation, but when beheld 
at low water might be mistaken for an artificial breakwater, 
erected by cyclopean workmen. At low tide it shows itself 
as a smooth level-topped ridge, from 30 to 60 yards in width, 
with even sides, and extending in a perfectly straight line, 
for several miles parallel to the shore. Off the town it in- 
cludes a shallow lagoon or channel about half a mile in 
width, which further south decreases to scarcely more than 
a hundred yards. Close within the northern point, ships 
he moored to old guns let into the reef. Here, on the 
inner side, at low water spring-tides, a section of about 
seven feet in height is exhibited. This consists of hard 
pale-coloured sandstone breaking with a smooth fracture, 
and formed of siliceous grains, cemented by calcareous 
matter. Well-rounded quartz pebbles, from the size of a 
bean, rarely to that of an apple, are embedded in it, 
together with a very few fragments of shells. Traces of 
formed of the same material cemented ; and then, at a depth of 45 feet, 
through an old peat with land vegetation, shells of Helix bermudensis, 
and bones of birds, beneath which was the ordinary hard ‘ base rock.’ 
Serpul® are very abundant on the Bermuda reefs, and form evi- 
dently, by their mode of growth, miniature atolls from 2 to 20 feet in 
diameter, with little interior lagoons. It was found by soundings 
tha» on the S.E. edge of the bank the 100-fathom line was about 1J- 
mile from the rocks awash. Then a slope of about 20° led down to 
350 or 400 fathoms, after which it varied, from 7° to 15°, to 1,000 
fathoms. The 100-fathom line on the N.E. edge was about 3 miles 
away; on the S.W. still further, and the submarine slopes were more 
gentle. The rock of the island appears to be of asolian origin, but it 
is not said whether this also forms the highest ground, which is 256 
feet above the sea.l 
