94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
mushroom-shaped. A passage has been cut for boats through the shal- 
low opening connecting the lagoon with the inner sound, which receives 
the drainage of the interior of the depressed basin. The elevated lime- 
stone bluffs extend from the northern horn of the rim, along the coast 
line, towards the landing place on the north face of the island. To the 
MANGO, SEEN FROM THE NORTHEAST. 
east of this, on the north shore, are fine blufis of elevated limestone, 
weathered into dome-like lamellar masses, with rounded masses and 
mushroom-shaped heads at the base (Plate 86). A little farther west, 
on the beach beyond the bluffs, the underlying volcanic rocks crop out 
again, and from there south the negro-heads on the western reef flats 
are all of volcanic origin. 
The island of Mango is almost circular, about three miles in diameter, 
surrounded by a barrier reef, which passes into a fringing reef off the 
northern coast near the landing, 
and off a part of the southeastern 
face of the island. The inner 
lagoon is very narrow, not more 
than a quarter of a mile in width, 
and also very shallow. Its great- 
est depth is about two fathoms, 
and it is further studded with 
negro-heads and coral patches, 
thus protecting the lagoon to a 
certain extent from the invasion 
of the sea. The ridge of elevated 
coralliferous limestones on the 
west side of the island extends 
as far as the first islet on the 
south face of Mango. There the 
central volcanic ridge which has raised the reef joins the western edge 
of the rim of the island. The ridge runs in a southerly direction, at a 
MANGO LANDING, VOLCANIC SUBSTRATUM. 
