320 CORALS AND CORAL ISLANDS. 
one mile wide, and contains about 1,000 acres. Like Baker’s 
and Howland’s, it has the general features of a coral island, 
but it differs from them essentially in the fact that the 
lagoon, which it once contained, has gradually been filled up 
with sand and detritus, while the whole island has undergone 
some elevation. “It therefore presents a basin-like form, the 
surface being depressed from the outer edge toward the cen- 
tre. It is encircled by a fringing reef, or shore platform, 
about three hundred feet wide; from this a gradually sloping 
beach recedes, the crown of which is from eighteen to twenty- 
eight feet high, forming a ridge or border, of varying width, 
which surrounds the island like a wall, from the im-shore 
edge of which the surface of the island is gently depressed. 
Within this depression there are other ridges, parallel 
to the outer one, and old beach lines and water marks, the 
remaining traces of the waters of the lagoon, marking its 
gradual decrease and final disappearance. 
This flat depressed surface in the centre of the island is 
about seven or eight feet above the level of the sea. It bears 
but little vegetation, consisting of long, coarse grass, Mesem- 
bryanthemum, and Portulaca, and this is near the outer 
edges of the island, where the surface is formed of coral sand, 
mixed with more or less gtano. In the central and lower 
parts the surface is composed of sulphate of lime (gypsum), 
and it is on this foundation that the principal deposit of 
guano rests. 
In examining the foundation of the guano deposit on 
Baker's or Howland’s Island by sinking a shaft vertically, 
the hard conglomerate reef-rock is found directly underlying 
the guano. Resting on this foundation the guano has under- 
gone only such changes as the climate has produced. On 
Jarvis Island, however, after sinking through the guano, 
