102 THE SHORE 



to which it gives entrance.* The latter depth is appar- 

 ently fixed by the first seaward slope, a gradual one to 

 40 fathoms or thereabouts, this approximately being 

 about the depth to which lagoons generally attain. 

 Observations on these matters are urgently needed ; 

 they should be accompanied by precise information as 

 to the conditions in different parts of the lagoon of coral 

 growth, sand deposition (exemplified by samples), rate 

 of currents, etc. Estimations of the amount of solid 

 material carried into and removed from the lagoon by 

 the tides are wanted, and may be made by suspending 

 plankton nets of fine mesh we suggest two kinds, 

 60 and 1 80 meshes to the inch in passages leading into 

 the lagoon for one, two, or more hours at ebb and flood 

 tides both in calm and stormy weather, and at different 

 depths. A good current meter (Ekman) might be 

 suspended from the same anchor chain between two of 

 the nets. 



The above theory of the formation of lagoons by 

 solution and erosion offers no explanation to account 

 for the submarine foundations on which atoll reefs are 

 built up. It is conceivable that islands, which were 

 originally surrounded by fringing reefs, may have been 

 completely removed by abrasion and solution, the reef 

 becoming successively of barrier and atoll type. This 

 mode of formation is seen in the windward group of 

 Fiji (Fig. 30), where the islands are mostly formed of 

 raised coral rock ; but atoll formation by this means is 

 probably not of wide occurrence. Secondly, an island 

 may be cut down by the action of wind, waves, and 



* Sometimes rounded growths of Lithothamnia are common 

 in such a position. Often the sides of such channels are rich 

 dredging-grounds for sedentary organisms, especially Gor- 

 gonians, Polytrema, and Hydrocorallines. Where the lagoon 

 is large and there are several channels, this question of depth 

 becomes complicated . It should be re-investigated in each atoll . 



