8 BULLETIN; MUSEUM OF COMPARATIVE ZOOLOGY. 
sitie 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 
