234 /. D. Hague on the Guano Islands of the Pacific Ocean. 



ticallj the section shows a series of concentric layers above and 

 around this central mass. The exterior is almost pure phosphate, 

 and, proceeding from the outside towards the centre, each suc- 

 cessive layer has less phosphate and more sulphate until the 

 central mass is reached, which is almost pure sulphate. It is 

 worthy of note that this hydrated sulphate of lime, which inva- 

 riably fills the centre of a " hummock," is amorphous and ex- 

 ceedmgly fine and soft, even when the underlying gypsum is 

 crystalline. These hummocks are scattered over certain parts of 

 the deposits and occur in close proximity to each other. la 

 these places the deposit is invariably damp, and, usually, be- 

 neath each one may be found, mixed' with the underlying sul- 

 phate, a black, earthy and damp substance containing much 

 phosphate and some carbonate of lime. This black substance 

 was, probably, coral mud, in which, as in the coral pseudomorphs 

 of Howland's, the carbonic acid has been ex{)elled and replaced 

 by phosphoric acid, and this affords the only explanation that I 

 can offer for this remarkable formation, namely, that in the 

 chemical interchange that must have taken place between the 

 so uble salts washed down from the guano on the surface, the 

 sulphate of lime and the coral mud, there may have been an ex- 

 cess of carbonic acid liberated from the latter and replaced by 

 phosphoric acid. The surface guano was probably wet and in a 

 plastic state like thick mud, and the ascending carbonic acid, 

 fandmg no other means of escape, and exerting an upward 

 torce, produced these hummocks, which have since become dry 

 and hard. 



In those parts of the crusted deposit where there are no "hnm- 

 mocks ' the surface is usually a little higher and the deposit be- 

 low drier than where the hummocks occur, and this would fur- 

 nish a reason for their absence, since the hummocks could hardly 

 be formed, as above explained, if the surface, for want of moist- 

 ure^ were not sufficiently plastic and yielding. 



Thus this guano has not only been deprived of its ammoniacal 

 salts, uric acid, etc., as have the deposits of Baker's and How- 

 land s but by its immediate contact with the gvpsum has under- 

 gone further chemical and phvsicai changes. Probably, too, the 

 direct action of sea water has effected much by bringing together 

 and mixing the guano with the bed on which it lay, and, by oc- 

 casional inundations, exposing the whole alternately to the ac- 

 tion of water and to the intense heat of the sun.* "Thus it bas 

 been baked into a thick and hard crust whose chemical conapo- 



terially from the guano in its usual form 



and 1 

 . high 



ye*e]^vat?d so hi-h^"*^'"'''-^'^*"- described bel. 



Thus we may suppose that Jarvi* in an earlier stag* 

 (fas subjected to occasional floods, keeping in nHod tb* 

 by digging now in the lower parts of the island salt «»- 



