40 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The Pirangy rock reef. — At Pirangy, south of Rio Grande do Norte, 
is the next reef that attracts attention. This reef does not, however, 
belong in the same category as the sandstone reefs of the Brazilian 
coast, neither is it a coral reef. It is mentioned here chiefly for the 
purpose of calling attention to a kind of reefs which are found occa- 
sionally on this coast and which are liable to be mistaken for either 
sandstone or coral reefs. The accompanying map shows the position 
and form of the Pirangy reef. It is, however, only the more resisting 
parts of the Tertiary (?) rocks that form the mainland in the vicinity. 
The stone reefs of the Cunhahi and Sibaúma. — The Rio Cunhahú in 
the State of Rio Grande do Norte enters the ocean sixty-seven КПо- 
metres (in a line) south of the Natal lighthouse, and nine kilometres 
north of Cape Bacopary. It descends through a wide-mouthed, flat- 
bottomed valley that extends from Bahia Formosa to the hills immedi- 
ately north of the river, — a distance of seven kilometres. The hills 
south of the valley are put down on the hydrographic charts as being 
ninety metres high, and those on the north as being one hundred 
metres. A single isolated, round-topped hill stands out in the middle 
of this valley south of the river. It is shown in Plate 22 on the right. 
This valley is a large and a long one, and retains these characteristics 
in the main for many kilometres, even above where it is crossed by the 
Natal and Nova Cruz Railway. 
The immediate shores between Bahia Formosa and the mouth of the 
river are covered with sand dunes almost the entire distance. These 
dunes are at least fifteen metres high (a. t.), and on the west side their 
sands fall upon the sandy soil of a caatinga forest. On the oceanward 
side there are exposed beneath these dunes beds of snuff-colored to 
black false-bedded sandstones. These dark sandstones are in places 
from two to four metres thick, and lie unconformably against the red 
and mottled Tertiary (1) beds exposed at Bahia Formosa. The contact 
between these two series of rocks was not seen, but both sets of beds 
are horizontal, and the dark beds of a later age do not appear in the 
beautifully exposed Bahia Formosa section as they would do if they 
formed a part of it. 
Here and there springs of amber-colored fresh water emerge beneath 
these dunes and upon the surface of the dark sandstones. At one place 
a large spring comes from the sandstone itself. In places the snuff- 
colored beds are below high-water level; in others they are somewhat 
higher. The geological relations of the beds are shown in the cut on 
page 28. 
