148 
THEORY OF TIIE FORMATION 
Ch. V, 
the rate of upward growth of the polypifers, the death 
of the reef must ensue, and it would have been strange 
had we found no evidence of this. It is, then, not 
at all improbable that the corals should sometimes 
perish either on the whole or on part of a reef. 
If only on a part, the dead portion, after a small 
amount of subsidence, would still retain its proper 
outline and position beneath the water. After a more 
prolonged subsidence, it would form, owing to the 
accumulation of sediment, a more or less level bank 
marking the limits of the former lagoon. Such dead 
portions of a reef would generally lie on the leeward 
side , 1 for the impure water and fine sediment are 
driven out from the lagoon over this side of the reef, 
where the force of the breakers is less than to ■wind- 
ward, and where the corals are, in consequence, less 
vigorous and less able to resist any destroying agency. 
It is owing to this same cause that reefs are fre- 
quently breached to leeward by channels which serve 
1 Sir C. Lyell, in the first edition of his Principles of Geology, 
offered a somewhat different explanation of this structure. He sup- 
poses that there has been subsidence ; but he was not aware that the 
submerged portions of reef were in most cases, if not in all, dead ; 
and he attributes the difference in height in the two sides of most 
atolls chiefly to the greater accumulation of detritus to windward 
than to leeward. But as matter is accumulated only on the back- 
ward part of the reef, the front part would remain of the same 
height on both sides. I may here observe that in most cases (for 
instance at Peros Banhos, the Gambier group and the Great Chagos 
bank), and I suspect in all eases, the dead and submerged portions 
do not blend or slope into the living and perfect parts, but are sepa- 
rated from them by an abrupt line. In some instances small patches 
of living reef rise to the surface from the middle of the submerged 
and dead parts. 
