146 CORALS AND CORAL ISLANDS. 



finally cemented and so made solid. At Tongatabu and 

 among the Feejee Islands, reefs thus formed of corals standing 

 in their growing positions are common. Though now mere 

 dead rock, and exceedingly firm and compact, the limits of 

 the several constituent coral masses may be distinctly made 

 out. Some individual specimens of Porites in the rock of the 

 inner reef of Tongatabu are twenty-five feet in diameter ; and 

 Astraeas and Mseandrinas, both there and in the Feejees, meas- 

 ure twelve to fifteen feet. These corals, when growing be- 

 neath the water, form, as has been stated, solid hemispheres, 

 or rounded hillocks ; but on reaching the surface, the top dies, 

 and enlargement takes place only on the sides ; and in this 

 manner the hemisphere is finally changed to a broad cylinder 

 with a flat top. This was the condition of the Astraeas and 

 Porites in the reef-rock referred to. Such a platform looks 

 like a Cyclopean pavement, except that the calcareous ce- 

 menting material, filling in between the huge masses, is more 

 solid than in any work of art : it even exceeds in compactness 

 the corals themselves. Other portions of reefs consist of 

 branching corals, with the intervals filled in by sand and small 

 fragments ; for even in the stiller waters fragments are to some 

 extent produced. A rock of this kind is often used for build- 

 ings a nd for walls on the island of Oahu. It consists mainlv 

 of Porites, and in many parts is still cavernous, or but imper- 

 fectly cemented. 



There is also to be found about inner reefs, over large 

 areas, the solid white limestone already described, showing 

 internally no evidence of its coral origin, and containing rarely 

 a shell or other imbedded fossil. It is a result of the consoli- 

 dation of the fine coral sand or mud that is made and accu- 

 mulated through the action of the light waves that work over 

 the inner reefs. It has been said that large regions of barren 



