18 PROFESSOR W. J SOLLAS. 



The outer ridge and the lagoon mound are supposed to be consolidated within the 

 area marked by vertical lines. After depression, the mean sea-level may be repre- 

 sented by the horizontal line 2 (fig. 10). The outward ridge is now brought within 

 the destructive action of the breakers ; its loose material will consequently be washed 

 away, and may be driven over the reef into the lagoon, while the hard core will 

 remain as the ridges and pinnacles of the ocean glacis. A fresh outer ridge will 

 then be formed further inland behind the pinnacles left by the first (fig. 11). 



In this way not only would the relation of the pinnacles to the outer ridge be 

 fairly explained, but also one very singular character of the pinnacles, viz. : that 

 they frequently occur as a linear series running parallel with the coast or even as a 

 continuous ridge. It mig-ht be difficult to understand why denuded reef- rock 

 should assume this form, but it readily follows if the pinnacles are merely remnants 

 of the core of the outer ridge. If now we cast a glance over the section taken 

 through the islet of Pava (fig. 12) we shall discover some very suggestive relations. 

 The pinnacles form a ridge, which rises behind the nullipore rim, in much the usual 

 way, but behind the pinnacles where we should expect to find the outer ridge, there 

 is simply a broad, gently sloping platform of consolidated coral breccia, some 

 80 yards across, which ends against an accumulation of sand and small fragments 

 of coral corresponding in composition and position with the lagoon-mound of other 

 islets. Pava thus has the appearance that we might expect to be presented by 

 an islet M'hich had lost its former outer ridge without acquiring a new one. At 

 the same time we must not omit to observe that the narrow ridge-like form of 

 the piiniacles might also be explained by the action of erosion upon a stratified 

 rock dipping seawards, as it does in this case ; the outer face looking towards the 

 ocean is a sea-cliff, the inner face looking in the opposite direction is a scarp, cut at 

 right angles to the di}). Here the cliff and scarp have closely approached each other, 

 hence the narrow ridge-like form ; but instances may be observed in this northern 

 region of the atoll where they still stand far more remote, ljo\mding a comparatively 

 broad platform. Sometimes also the scarp exists without a corresponding cliff. 



If the features of the sea-ward face of the reef could be explained by a positive 

 movement of the strand, we should next have to inquire whether the same explana- 

 tion would apply to those bordering the lagoon. If, after submergence, any trace 

 remained of the lagoon-mound we should expect to find its constituents, or the 

 coarser part of them, arranged with a slope conformable to the ancient lagoon beach, 

 i.e., dipping lagoon wards, while as we have already seen the fragments in the 

 consolidated platform and beach of the lagoon slope in the opposite direction. 

 This, however, only proves that the lagoon cliffs and platform of hard rock are 

 not the remains of an ancient lagoon mound, and this was a result to which we 

 had previously been led ; for we had already concluded that they represent the 

 remains of a deposit, formed at a time when the waves washed debris from 

 the ocean into the lagoon. They are indeed the oldest members of this deposit 



