Say CORALS AND CORAL ISLANDS. 
edge toward the middle. Around the borders are incrusta- 
tions of crystallized gypsum and common salt, ripple-marks, 
and similar evidences of the gradually disappearing lake. 
The whole is composed of a crystalline deposit of sulphate 
of lime, which, around the borders, as already observed, is 
mixed with some common salt, while near the centre, where 
rain water sometimes collects after a heavy shower, the salt 
is almost entirely washed out, leaving the gypsum by itself. 
It is closely, but not hard, packed, and is still very wet.- By 
digging eighteen to twenty-four inches down, salt water may 
generally be found. 
These facts help us to understand the varying conditions 
in which we now find the guano beds. The most important 
part, and that from which the importations have thus far 
come, rests on a bed of sulphate of lime, of an earlier but 
similar origin to that just described above; part rests on a 
coral formation; while still another part, covering a large 
tract, has been by the action of water mixed with coral mud. 
The first named deposit, lying on the sulphate of lime 
bed, has a peculiar character. It is covered by, or consists 
of, a hard crust, that is from one-fourth of an inch to an 
inch and a half in thickness, beneath which hes a stratum of 
~ guano, varying in depth from one inch to a foot. In many 
places where the guano was originally shallow, the whole is 
taken up and formed into the hard crust which then lies im- 
mediately on the sulphate. This crust, when pure, is snow- 
white, with an appearance somewhat resembling porcelain, 
but is usually colored more or less by organic matter. Gen- 
erally it is very hard, and strongly cohesive, though some- 
times friable, and it lies unevenly on the surface in rough 
fragments that are warped and curved by the heat of the 
sun. It consists chiefly of phosphoric acid and lime, but, 
