124 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
stratum of ancient limestone of greater or less thickness, according to 
the height to which the particular reef has been elevated. 
An atoll formed from the disintegration, erosion, and denudation of an 
island of volcanic structure, like Komo, for instance, would not differ in 
structure from an atoll like Oneata, formed from the disintegration of an 
island composed of elevated coralliferous limestone; but their origin 
would at once be detected, if the islands had disappeared, from the exist- 
ence in one case of negro-heads and rocks of volcanic origin, in the other 
of coralliferous limestone scattered within the lagoon and on the surface 
of the outer reef flats. The steep slopes of these two lagoons, of different 
geological structure, being in no way due to the thin veneer of coral 
patches growing upon their respective flats and slopes, but to the fact 
that they are a continuation of the slopes of the islands once covering 
the areas of Komo and of Oneata Islands, which have been elevated 
and since their elevation have disappeared in great part, and have been 
denuded and eroded to form the platform surrounding the islands now 
remaining to attest their former greater extent. The denudation and 
erosion of these islands, being accompanied by the scouring out of the 
lagoons from the submarine platform through the action of the sea, 
caused by the incessant pouring into the lagoon of a mass of water 
which can only find its way out through the entrances into the lagoon 
or over the reef flats, where it passes out with considerable velocity, 
a velocity obtained in part from the hydraulic head, and in great part 
also from the drift due to the constant driving of the trade winds in a 
westerly direction. 
North of the Exploring Isles there are some small banks, in from 
eight to twelve fathoms least water. They are Alacrity, Jeffreys, and 
Lewis Banks. Reid Reef (Plate 20, 20°, Figs. 1-4) is a narrow encir- 
cling reef, enclosing a large lagoon with three islets, rising respectively 
toa height of sixty, ten, and twenty feet, probably of elevated limestone. 
The outer reef flat is narrow on the western side, bordered by a belt of 
heads along the inner edge. There are two passages into the lagoon on 
the western face. The length of the lagoon is about eight miles, and its 
width five. The greatest depth is twenty fathoms. 
Mbukata tanoa or Argo Reefs. 
Plates 20, 20°, Figs. 5-8, and Plate 21. 
These reefs are irregularly triangular, about twenty miles in greatest 
length. A narrow continuous outer reef flat extends along the eastern 
