432 PERCY SLADEN TRUST EXPEDITION 



ridge running from the north to the south near the west shore measuring some 18 — 20 feet 

 above sea level, assuming that the rest of the island is about 12 feet (above sea level). 

 In the south-west this ridge sinks gradually, but in the north-west the slope is steep and 

 even forms low cliffs, leaving a sort of terrace or flat near the shore, which is piled with 

 blown sand. 



The pits, before referred to, are most common in the interior of the island, but 

 also occur within a few yards of the sea. As at Aldabra they probably owe their origin 

 to the imperfect consolidation of the reef before elevation. The pits vary exceedingly 

 both in area, depth and contents; the largest measured 18 yards by 14 yards wide 

 and 9 feet deep ; the deepest was 45 feet. A considerable number contained water 

 always salt and fluctuating tidally ; others contained guano and mud, but even in these 

 salt water could be reached within a few feet of the surface and usually showed tidal 

 fluctuations ; as at Aldabra therefore sea water can penetrate completely through the 

 island. 



The sides of the pits were always much overhanging and erosion was everywhere 

 obvious in those which contained water, though even in the dry pits weathering seemed 

 to cause an increase in size. The pits are gradually connecting, and will eventually 

 divide the island into a number of islets or rocks, which will of course be rapidly removed 

 by wave abrasion. It is important to notice that, if the reef had been basin-shaped 

 before elevation, then the pits in the interior would naturally have connected before those 

 near the outside, and thus a lagoon would have been formed and the island would have 

 been an atoll. 



Large sand dunes (90 feet high) occur in the south-east of the island ; the soundings 

 taken by Captain Parcou, previously mentioned, demonstrate much loss of land in the 

 south-east and the sand resulting from the process of erosion is driven on to the 

 coasts and forms the material from which the wind has built up these large dunes. 

 The shore of the island is chiefly rocky, though there is a long stretch of sand in the west, 

 and small sand "lances" are not uncommon elsewhere. Erosion is everywhere obvious. 



Owing to the visit occurring during the neap tides the reef was imperfectly examined. 

 It seemed to have a very ill-defined edge, to be very sandy and indeed hardly existant 

 to the south-east. It is probably of the same nature as the Astove and Aldabra 

 fringing reefs, but is less definite on account of the sand acting as a check to reef- 

 building organisms. 



The two chief points of theoretical interest in connection with Assumption are : 



(1) The fact that it is not an atoll, and 



(2) The bad consolidation of the rock as a whole. 



Both are probably due to the same cause. The base on which the island is built 

 has probably a narrow summit, and the reef was enabled to build vertically more quickly 

 than it could form a talus slope on which to extend horizontally. Supposing it was 

 elevated when it became nearly awash it presumably had not had sufficient time to form 

 a reef of large area or of solid composition ; there is therefore but little difference in 

 primary rock structure throughout the island. Secondary rock changes again show the 

 importance of phosphate in metamorphosis, and give evidence of great denudation. The 



