THE COMPLETED ATOLL. 319 
its bed has a gradual slope from the borders toward the cen- 
tre, or, perhaps more properly, from northwest to southeast, 
giving the guano a variable depth from six inches at the 
edges to several feet. 
Howland’s Island is situated in lat. 0° 51’ north, and long. 
176° 32’ west from Greenwich. It is about a mile and a half 
long by half a mile wide, containing, above the crown of the 
beach, an area of some four hundred acres. The highest 
point is seventeen feet above the reef, and ten or twelve feet 
above the level of high tide. The general features of the 
island resemble those of Baker’s. 
The main deposit of guano occupies the middle part of 
the island, and stretches, with some interruptions of inter- 
vening sand, nearly from the north to the south end. Its 
surface is even, and in many places covered by a thick 
growth of purslane, whose thread-like roots abound in the 
guano where it grows. The deposit rests on a hard coral 
bottom, and varies in depth from six inches to four feet. 
The fact observed at Baker’s that vegetation flourishes most 
where the guano is shallow, is also quite apparent here, and 
the consequent characteristic difference between the 2uano 
of the deep and shallow parts is distinctly marked. 
Some interesting pseudomorphs occur buried in the guano 
of this island. Coral fragments of various species were found 
that had long been covered up under the deposit, and in some 
of which the carbonic acid had been almost entirely replaced 
by phosphoric acid. In such I have found seventy per cent 
of phosphate of lime. In many others the change was only 
partial, and, on breaking some of these, in the centre was 
usually found a nucleus or core of coral. 
Jarvis Island is situated in lat. 0° 22’ south, and long. 
159° 58’ west from Greenwich. It isnearly two miles long by 
