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NATURAL HISTORY .OF A GUANO ISLAND. 167 
- fissures in the coral rock. The. im mense evaporation from the 
surface of the lagoon in aisha dry weather causes a c t 
influx of water, which is only influenced in rapidity by the state 
of the tide, slenesh <i at low-water, and flowing outwards 
ing 
only at low-water near S amachiglidan as far as I could o 
During heavy tropical rains, to which the island is at times 
subjected, the rain-water soaking from the higher surrounding 
lands washes out the lagoon as it were, and reduces the amount 
ormed around the shallow margin. The specific gravity of the 
water inthe open sea I found was 1:026, and on different occa- 
sions I eta) that of the lagoon water to be 1:090 and 1:120, 
ere tha seitane: was only three or four ives hae salt 
c 
This alternate evapora and removal of the more — 
salts has caused Gas 5: ‘of eh of calcium with m 
or less carbonat te to be formed, ia 2 at vee west ak ects 
which the wind is generally blowing, and w is furt est from 
being bound together by a mat of vegetable matter 
apparently the same or similar to the iar marine ogen 
described by M. Aimé Girard as covering niche bottom of the salt 
gardens of Portugal*. This forms a tremulous crust which may 
be walked upon, whilst underneath, the hapowks is a white mud 
which may be readily probed to a ‘de epth of several feet. The 
wind occasionally removes portions of this crust and carries it 
with ee deposit farther west, so that it forms a long slope, 
which as it rises above the water level is being luall 
= upon by a thick-leaved plant having the appearance 
a mesembrianthemum. This, collecting the dust blown from 
the drier portions below, forms a ridge about 18 inches high in 
which the aga birds burrow holes, altering i its appearance by 
panep 
charging it with organic matter and 
The sails of the island, although lying near the equator, 
and sometimes having the north-east and sometimes the south- 
east trades, is generall characterized i extreme Go agon Z 
* Compt. Rend., Lxx1, 1193. 
a 
