252 CORALS AND' CORAL LSLANDS. 



different parts of the surface, which, though nearly flat, show 

 some sHght variety of level. The higher parts, particularly 

 around the outer edges, are composed chiefly of coral sand, 

 either mixed With or underlying guano. Nearer the centre is 

 a- large tract, rather more depressed, forming a shallow basin, 

 in which the bulk of the sea-water must have been evaporated, 

 and whose surface (now partly covered with guano) is a bed of 

 sulphate of lime, while, further, there is a still lower point, 

 the least elevated of the whole, where the lagoon waters were 

 without doubt most recently concentrated. This latter locality 

 is a crescent-shaped bed, about 600 feet long by 200 or 300 

 feet wide, having a surface very slightly depressed from the 

 outer edge toward the middle. Around the borders are incrus- 

 tations 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 18 or 

 24 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 be 1, 

 has a peculiar character. It is covered by, or consists of, a 

 hard crust, that is froni one-fourth of an inch to an inch and 

 a half in thickness, beneath which lies 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 Hes immediately 

 on the sulphate. This crust, when pure, is snow-white, with an 



