THE COMPLETED ATOLL. 295 



containing common salt, gypsum, and other salts found in the 

 waters of the ocean. From these the more soluble parts 

 would gradually be washed out again by the occasional rains, 

 leaving the less soluble sulphate of lime as we find it here. 



Some additional light is thrown on this matter by the dif- 

 ferent parts of the surface, which, though nearly flat, shows 

 some slight 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 evapor- 

 ated, 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 incrustations of crystallized gypsum and common salt, 

 ripple-marks, and similar evidences of the gradually disappear- 

 ing lake. The whole is composed of a crystalline deposit of 

 sulphate of lime, which, around the borders, as already ob- 

 served, is mixed with some common salt, while near the cen- 

 tre, 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 gen- 

 erally be found. 



These facts help us to understand the varying conditions 

 in which we now find the guano beds. The most impor- 

 tant part, and that from which the importations have thus far 

 come, rests on a bed of sulphate of lime, of an earlier but 



