18 



PERCY SLADEN TRUST EXPEDITION 



II. DENNIS ISLAND. (Fig. 3.) 



Dennis island lies about 25 miles east of Bird island and like it is situated on the 

 edge of the Seychelles bank. It is slightly larger than Bird, which broadly speaking 

 it i-esembles in topography and composition. The basis is calcareous sand, over which a 

 layer of guano has been deposited with the result that the sand has been phosphatised in 

 varying degrees. The greater part of the surface is a very soft brown sandstone, but in 

 places the sand has not been affected at all. In comparison with Bird the cementation 

 process has been carried less deeply. The changes of level over the island are not 

 conspicuous, but perhaps the highest portion (12 feet above low water mark) is in the 

 north and west. Wells dug in this latter part penetrate through nine or ten feet of pure 

 sand before reaching water (brackish or fresh). In various parts of the island are small 

 marshes : in some of these the water is almost fresh, but in others, especially those nearer 

 the sea, it is brackish and fluctuates with the tide. 



The coast is more varied than the land, and is of interest as showing the various 

 conditions in a calcareous sand island being removed by wave action. These may be 

 outlined as follows : 



(1) Sand beach sloping steeply, with a small sandflat on the top, bearing a little 

 vegetation (rushes, Ipomcea, etc.). The 

 whole of the beach scrub growing luxuri- 

 antly. 



(2) Sand beach uniformly gently 

 sloping ; beach scrub (Sccevola, Tourne- 

 fortia), with the outer bushes killed by 

 the sea. 



(3) Beach formed almost entirely of 

 beach sandstone, having to the landward 

 only a few feet of sand and rubble followed 

 by the phosphatic sandstone of the island. 

 Scrub, containing much Pemphis acidula, 

 growing right down to the beach sandstone 

 but in process of being killed. (Plate 1, 

 fig. 2.) 



(4) Beach sandy with honeycombed 

 cliffs of phosphatic sandstone; they over- 

 hang and are crumbling fast ; dead coconuts 

 and other trees can be seen with their roots 

 exposed on the cliff face. (Plate 2, fig. 2.) 



All these formations with the exception 

 of (1) point to rapid erosion, (l) is found 



on the north-west opposite the settlement Flg ' 3 ' 



and on part of the west coast, (2) occurs just south of west point and all round north 

 point, (3) is found from east point to south-east point; it is worth noting that the beach 



Dennis Island. 



