DEV^ELOPMENT OF THE REEF 249 



of the colony, therefore, becomes increasingly flattened. When 

 this process has gone on for some time the flattening of the 

 rock enables sediment to find a resting-place upon its upper 



Fig. 60. 



Photograph of a Coral Colony that has Living Zooids 

 over its entire surface. 



No zooids are as yet killed either by pressure or by sedimentation. 



surface ; and the activity of the uppermost zooids declines 

 as this aspect of the dome becomes increasingly favourable to 

 the settlement of silt. The flattening becomes increased by 

 the margins rising, till finally the central portions are entirely 

 killed by sediment, and a flat sand-strewn area is produced, 

 surrounded by the raised rolled edges of actively dividing 

 zooids which are untouched by sediment. Upon the barrier 

 there is no doubt that injury of the upper zooids, caused by 

 moving particles sweeping backwards and forwards in the 



