142 
THEORY OF THE FORMATION 
Ch. V., 
also evident that the central rings, although broader 
than the knolls or reefs which commonly occur within 
lagoons, occupy the same relative position. The ring- 
like structure has been shown to he contingent on 
the breaches into the lagoon being wide and numerous, 
thus causing the inner side of the marginal reef and 
the central reefs to be placed under nearly the same 
conditions with the outside of an ordinary atoll which 
is exposed to the open sea. Hence the margins of these 
reefs have been favourably circumstanced for growing 
outwards and increasing beyond their usual breadth ; 
and the conditions have likewise been favourable for 
their growing vigorously upwards, during that subsi- 
ding movement to which by our theory the whole 
archipelago has been subjected ; and subsidence toge- 
ther with the upward growth of the margin would 
convert the central space of each little reef into a 
small lagoon. This, however, could only take place 
with reefs which had increased in breadth sufficiently 
to prevent their central spaces from being almost im- 
mediately filled up with the sand and detritus driven 
inwards by the waves from all sides. We can thus 
understand how it is that few reefs less than half 
a mile in diameter, even in the atolls where perfect 
ring-formed reefs are found, include lagoons. This 
remark, I may add, applies to all coral-formations. 
The basin-formed reefs of the Maldiva Archipelago 
may, in fact, be briefly described as small atolls 
formed during subsidence over separate portions of a 
large and broken atoll, in the same manner as the 
