18 bulletin: museum of comparative zoology. 



rising above the general level, and almost equal in area to the base, 

 owing to the wall-like front the island presents to the sea. The island 

 is small, being eleven or twelve miles in circuit and three and a half 

 miles in diameter. A strip of fertile flat about 100 yards in width runs 

 round the greater part of its circumference and relieves the ruggedness 

 of what would otherwise be an iron-bound coast. In one place this 

 feature is found where the huge cliffs plunge abruptly into the water. 

 Mango possesses a great number of vertical cliffs, some of them as high 

 as 400 feet (Plate 9), and where the limestone ring or girdle of the 

 island persists, this cliff structure is varied only with the " sugar-loaf" 

 formation. 



On a closer examination traces of a " terrace " up to 200 feet above 

 high-water mark, will be found on the northern and southern aspects of 

 the island. Splendid patches of raised reef, with Pontes and Astraean 

 corals in abundance in situ, lie scattered over the sand fiats and up the 

 lower slopes of the cliffs. These are subsequent to the first upheaval. 



The practised eye soon detects a want of uniformity in the limestone 

 ring (Plates 12, 15, 16, 17). The wall structure is replaced by lava 

 flows, and at the centre of flow the coralline rock is absent. Glancing 

 aloiiu the line of cliffs from some point where they still preserve then- 

 original shape, the escarpment is seen to dwindle away towards the 

 centre of flow of the disrupting lava, passing from great cliffs to in- 

 significant wedge-shape, still preserving, however, the general level of 

 the ring as it vanishes beneath the lava. Continuing the line of sight, 

 the limestone is picked up again at the same level a little further ou, 

 now, however, merely a few limestoue islands rising above the lava 

 sheet. The volcanic mass pours through the gap made in the old lime- 

 stone ring and spreads out, fan-shaped, away from the cliffs, carrying the 

 island structure farther out to sea than formerly by some 400 yards 

 (Plate 11). This is the case on both the north and south of the island. 

 On the northeast a vast natural break occurs in the old ring. This gap 

 represents a former channel of communication between the outer sea and 

 the central lagoon. 



A coil of rope laid on the floor, sand poured gradually over any two 

 opposite sides of the rope-ring till the cone-shaped sand masses rise con- 

 siderably above the sub-level of the ring and hide the rope at that spot, 

 a V-shaped gap cut in the rope at a spot bisecting either of the rope 

 semicircles left, — this is the contour of Mango, with the rope substituted 

 for the limestone girdle, the sand-cones for the andesite masses, and the 

 V gap for the old passage from the sea to the central lagoon. 



