ITS GRADUAL GROWTH. 473 



about rapidly in all directions, probably in quest of food. 

 Others are so sluggish that they might be mistaken for 

 pieces of the rock ; these are generally of a dark colour, 

 from four to five inches in length, and two to three inches 

 in circumference. On breaking the coral mass from a 

 spot near high-water mark, it was found to be a hard 

 solid stone ; but if any part were detached at a level 

 which the tide regularly washed, it proved to be full of 

 worms, all of different lengths and colours, some as fine 

 as a thread, and several feet long, generally of a very 

 bright yellow, but sometimes of a blue colour; while 

 others resembled snails, and some were not unlike 

 lobsters and prawns in shape, but soft, and not above 

 a couple of inches long. 



The coral ceases growing when the worm creating it is 

 no longer exposed to the influence of the tide. A reef 

 rises in the form of "a gigantic cauliflower," until its 

 crest has gained the level of the highest waters, above 

 which the corallite has no power to carry its operations, 

 and the reef, consequently, extends no further. The 

 surrounding parts, however, successively mount upward 

 until they also reach the surface, and stop. Thus, as the 

 level of the highest tide is the eventual limit to every 

 part of the reef, a horizontal field is duly formed co- 

 incident with that plane, and perpendicular on all sides. 

 But though the upward extension of the reef is at an end, 

 not so with its lateral expansion ; and this expansion 

 being apparently as rapid at the upper edge as it is lower 

 down, the face of the reef everywhere preserves its steep- 

 ness ; a circumstance which renders this class of rocks 

 exceedingly dangerous to navigation. In the first place, 

 they are seldom visible above water ; and, in the second, 



