872 Proceedings of the Boy al Society 
by a bar over which at low water neaps there is less than a fathom 
of water ; in the northern lagoon there is only a depth of 3 or 4 
feet at low tide. Outside the entrance of each lagoon, there is a 
second lagoon-like basin of more recent origin : the southern of 
these basins has a depth of from 20 to 26 fathoms, and affords a very 
sheltered anchorage ; while the northern is shallower, and more 
obstructed. Both the inner lagoons are lined by mangroves, and 
they communicate with the extensive mangrove swamps in their 
vicinity. Some of these swamps, when exposed at low water, 
present extensive tracts of a white chalk-like mud dotted here and 
there with clumps of mangroves. On the surface lie shells and 
coral fragments, and large masses of Meandrina and Coeloria occur 
as they grew, their summits being at present raised 1 to 2 feet 
above high water. 
In remarking on the history of the formation of this island, I 
will first refer to its condition at high tide. If a bird’s-eye view 
could then be obtained, the larger portion of its surface would 
present the appearance of an expanse of water in which, were it not 
for the mangroves, the eye would be unable to distinguish between 
the swamps temporarily overflown by the sea and the permanent 
inner lagoons. The two low hills would stand out from the water 
nearly surrounding them as imperfect islands; and such a panoramic 
view would carry us back to the time when they existed as two 
submerged reefs lying, like Lark Shoal, a few fathoms beneath the 
surface. Then ensued a period of upheaval, during the latter part 
of which the permanent land surface was formed on the eastern or 
weather side, and the island assumed the character of a double 
atoll. After a pause in this movement, there was a more modern 
elevation of a few feet, since which the outer lagoons have been 
formed, and the inner lagoons have been gradually filling up. The 
interesting point in connection with the formation of this island is 
one which is of importance in connection with the early history of 
small atolls;* for it is evident that it commenced as two flat-topped 
submerged reefs, and that the atoll form has been assumed since 
these reefs have been upheaved. In the neighbouring Lark 
Shoal we have the first stage in the production of an atoll; but the 
characteristic form, as I hold, will not be assumed until it has 
* Atolls only a mile or less in width. 
