220 CORALS 



There seems to be nothing more fatal to the growth of 

 corals than this deposit of silt. The delicate polyps have 

 some power of removing a few light foreign particles that 

 fall upon them, but a continuous shower of grains of sand or 

 mud hinders their powers of expansion, interferes with their 

 capacity to capture and ingest their food, and by shutting 

 off the light from the canal systems checks the photo- 

 synthetic action of the zoochlorellae. Any change in the 

 set of the tides and currents that drives the silt on to a 

 vigorous part of a reef, or causes stagnation and a fresh 

 deposit of silt elsewhere, may be regarded as among the most 

 destructive of the agents which check the growth of the reefs. 



The study of the existing conditions on the reefs leads, 

 then, to the conclusion that, in addition to the great con- 

 structive factors of coral growth, there are also destructive 

 agencies at work which may check and destroy what has 

 been built up when environmental circumstances change. 

 There are probably no examples of homogeneous reefs that 

 have shown continuous progress for long periods of time. 

 The growth of a reef is a process of stages of active increase, 

 of comparative stability, and in some cases of considerable 

 reduction, the sequence and duration of these stages varying 

 enormously in different parts of the tropical world. 



It has been shown that in the building of the tropical 

 reefs a great many varieties of corals take part. It is not 

 the work of one genus or of one order of corals. There are 

 perforate and imperforate Zoantharia, Millepores, Alcyonaria, 

 and coral Algae in varying proportions contributing their 

 quota to the formation of the great masses of coral rock. A 

 critical examination of these corals proves that they are 

 not the same as those found in more isolated patches in deep 

 water or in the Mediterranean Sea, the Norwegian fjords, 

 or other extra-tropical regions of the world. 



It becomes a matter of some importance, therefore, in 

 the consideration of the problems of coral reef formation, 

 to collect the evidence that is available concerning the dis- 

 tribution in depth of those that can be roughly classified 

 as reef-building corals as distinct from those that do not 

 enter into the composition of the reefs. 



