124 BULLETIN : MUSEUM OF COMPAEATIYE ZOOLOGY. 



stratum of ancient limestone of greater or less thickness, according to 

 the height to which the particular i-eef has been elevated. 



Au 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 difi"erent 

 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 ax-eas 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 gi'eat 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 

 to a 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 



