8 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



sitic volcanic rocks, the highest level of the junction plane being about 

 400 feet above the sea. As no trace of this calcareous volcanic con- 

 glomerate can be seen in the sea-cliff less than half a mile distant, and 

 about 400 feet high, it follows that the coral rock must thin out con- 

 siderably against the inland foundation of the island, while it thickens 

 rapidly seawards, as shown on Mr. Andrews' section. 



This, I think, points either to stable equilibrium of the earth's crust 

 in that neighborhood for a sufficiently long period of time to have 

 enabled the reef to grow out seawards on a rock formed partly of its 

 own talus, partly of foraminifera and nullipores, or it points to upward 

 growth of the reef during a slow subsidence. Mr. Andrews inclines to 

 the former view. Had either elevation or subsidence, as the case may 

 have been, taken place fairly rapidly and continuously, the result would 

 probably have been the formation of merely a veneer of coral rock lying 

 on the foundation rock. 



It seems to be very difficult to obtain a reliable natural section show- 

 ing the internal structure of this raised limestone, for two reasons : — 



(i.) As the elevation progresses, the tendency is for each newer ring 

 of coral and talus formed at successively lower levels to hide from view 

 the base of the previously formed ring. See the description of Vatu 

 Leile by Mr. Andrews. This masking of the older limestone formation 

 by newer formations and thick soils is more conspicuous on the leeward 

 side of the islands than on, the windward, for while narrow terraces of 

 coralline limestone, mostly now covered by deep soil, were added to the 

 western side during the elevation of the atoll, not only was coral growth 

 checked, but the sea even made some inroads into the earlier limestone 

 rock, as proved by the wave-worn beach lines up to over fifty feet above 

 sea-level. 



(ii.) Chemical solution is constantly forming stalagmitic or tufaceous 

 crusts over the cliff faces, sides of ravines, and even sides of caves in 

 the limestone, and this material, of course, either hides the original 

 rock from view, or by infiltration changes its original structure con- 

 siderably. The limestones are much dolomitized in places, so that 

 almost all original structure of the organisms composing them is 

 obliterated. 



5. Andesitic rocks, later than the raised limestones. 



(a) Andesite and coral agglomerates. These are well developed at 

 Mango. The inclusion of blocks of angular raised limestone, up to four 

 or five feet in diameter, in this agglomerate proves that it is newer than 

 the raised limestone, as Mr. Andrews is convinced that these included 



