40 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The Pirangy rock reef. — At Pirangy, south of Eio 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 Cunhahii and Sibaiiuia. — The Eio Cunhahii in 

 the State of Eio Grande do Xorte enters the ocean sixty-seven kilo- 

 metres (in a line) south of the Natal lighthouse, and nine kilometres 

 north of Cape Bacopary. It descends through a wide-mouthed, flat- 

 bottomed A-alley 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 tliis 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 ^jahia Formosa and the mouth of the 

 river are covered with sand dunes almost the entire distance. These 

 dunes ai-e 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 thei-e are exposed beneath these dunes beds of snuft-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 (?) 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 snufi- 

 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. 



