86 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



We struck the central part of the western face of Tahanea, and 

 found the interior of the lagoon dotted all over with islands or ledges, 

 the islands flanked with spurs of old ledge outcrops, and the ledges running 

 diagonally across the middle and southern part of the lagoon. Parallel 

 with the south face runs a remarkable secondary lagoon, about one mile 

 wide at its widest part, formed on the sea face by the broad reef 

 platform, fully 1000 feet wide, and on the other side by a series of 

 islands rising from an inner reef flat as wide, if not wider, than the outer 

 flat, and separated by a deep channel, in places from two to three fathoms 

 in depth (Pis. 54, figs. 2, 3 ; 55, figs. 2-4). The depth of the channel 

 greatly varies ; it shallows both at the eastern and western ends, where the 

 secondary lagoon narrows, and eventually, at the northern end, terminates 

 in an island (PI. 54, fig. 1), which blocks the cul-de-sac of that part of 

 the lagoon where the outer reef flat is not more than from 50 to 100 

 feet wide. The islands to the east of the secondary lagoon are gener- 

 ally well wooded (Pis. 54, fig. 3 ; 55, figs. 1, 2, 4). From the topsail 

 yards no extensive clear space could be seen in the lagoon, the islets 

 and islands occurring apparently throughout the whole lagoon. The old 

 ledge seems to be exposed all along the south face, and is flanked by a 

 comparatively narrow reef platform, edged by Pocillipore and Nullipore 

 knolls (Pis. 54, fig. 3; 55, figs. 3, 4). 



The secondary lagoon is cut up by ledges in all sorts of ways ; these 

 same ledges send spurs into the main lagoon, where there are innu- 

 merable single rocks and ledges and islands and islets making irregular 

 divisions of the lagoon, more or less shut off from one another (Pis. 54, 

 55). Numerous sand bars form low islets and islands on the inner side 

 of the reef platform. The outer reef flat is from 800 to 1000 feet 

 wide (Pis. 54, fig. 3 ; 55, figs. 2-4). At the southwestern horn, where 

 the secondary lagoon ends, islands and sand bars again appear on the 

 lagoon side of the outer edge of the reef platform of the atoll. These 

 islets are from eight to nine feet high, with coral sand beaches on the inner 

 or lagoon side (PI. 55, fig. 3). A few boulders are thrown up on the 

 reef flat, of which tlie outer edge is well gouged out in the finger-like 

 channels and scollops running at right angles to the sea face of the reef 



