are common. A narrow rim of beachrock is found on the SE side with no 
well-developed surge channels near the high tide line. The beach is com¬ 
posed of polished beachrock stines and the remains of clam shells formerly 
cemented in a matrix of coral sand and gravels. These fossil shells appear 
to wear less swiftly and have a "soapy" texture. The burrows of the wedge- 
tailed shearwaters and Christmas Island Shearwatersare found in the sandy 
soils along the rim of the island. Often these burrows are beneath clumps 
of Lepturus or under- slabs of half exposed beachrock. In July 19 64 a series 
of six areas on the/ E^ /SE)and/S 3ides of the lagoor were chosen along which 
between fifteen^stakea 3 dm. high were inserted into the powdery soil 
twenty-onei" approximately 4dm. apart. The stakes were orientated in such a way that 
a growing stem of the prostrate Sesuvium did not bisect a straight line 
drawn between any two stakes. It was hoped some idea of the growth rate 
of Sesuvium could be obtained. This species is usually found on thin layers 
of soil underlain by a well watered hardpan layer. It appeared that some 
stems which had grown onto the lagoon surface during a dry period had been 
killed by inundation. In November the sites were revisited and the length 
of stems which had grown beyond the line drawn between each stake were measured, 
and 
Description of Site 
I S edge of lagoon, 6 m. long 
II SE 
It 
,8m. long 
r ... 
m feKniuitflAfW mat 
V E side, evidence of stems 
killed.by inundation, 6.4 m long 
Average Growth of Stems 
9 cm. (8l) 
III - 5 CM. (17)* 
IV - 0 (2 )** 
11 cm. ( 37 ) 
11 cm. 
v. y 
- Ou 
- - 9 
VI E side, S of V, much guano, some 
stems extended . 75 m « from solid 
mat on hare crust, 6.4 m. long 
This information gives some idea of the rapidity of growth of the Sesuvium 
mat in a four month period during which no heavy inundation of water from 
high tides or rain filled the lagoon to a level where the peripheral mat 
was covered. In site IV several stems had died hack indicating that water 
during a high tide had seeped to a depth which covered a few stems or, 
possibly local /^disturbance at the edge of the mat had killed these stems. 
‘booby 
The soil undjeneath the Sesuvium mat is reddish with a large amount of 
humus derived from dead stems and leaves. This soil is augmented by the 
addition of material when the nesting activity of the bluefaced booby 
clears an area for nesting. The outer periphery of the nest site is covered 
with guano by the adults and nestlings. This fertilization doubtless speeds 
the regrowth of the mat after the birds have left. The depth of the soil 
layer varied being .5 to 4 dm. thick with a hardpan formed beneath perhaps 
through an exchange reaction between the layers of humus rich soil and 
the powdery saline soil beneath. The Sesuvium mat is probably trimmed 
periodically by waters seeping in duTlng the highest yearly tides. This 
plant species is important in the building of soils at the periphery of 
the lagoon basin, and as the seeps become clogged with less water pouring 
into the inner basin, the Sesuvium advances over the surface crusts. Even 
in those cases where the periodic trimming takes place the amounts 01 numus 
added to the detritus of the lagoon must over a period of time help raise 
the level of the lagoon. As this process continues the dry land area of 
the island is increased. 
* The number in parentheses indicates the number of stems measured. 
** See text. 
