22 A NATURALIST ON THE "CHALLENGER." 



one another by extremely sharp projecting points and edges of 

 thin laminae, which break with a crackling noise under the feet. 

 In some places on the coast the rock has been left by denudation 

 projecting in isolated pinnacles and peaks of fantastic form. 



The surface of the rock is not only honeycombed by the 

 action of rain, but hardened by re-deposit of carbonate of lime ; 

 and a fresh surface exposed to the weather soon becomes 

 covered with a hard film. Extensive caverns exist all over the 

 islands, undermining the rock in all directions, and filled at the 

 bottom with water, which, in caves near the sea, rises and falls 

 with the tide and is salt. At Paynter's Yale Cave the water is 

 only brackish, so that the communication underground with the 

 sea must be slight. Such caves must necessarily result from the 

 consolidation of masses of loose sands by means of the percola- 

 tion of rain water. The carbonate of lime taken up must leave 

 cavities unless the whole mass were to shrink gradually ; but as 

 the outer or upper layers receive the water first, they become 

 consolidated, and hardened more thoroughly than the inner. 

 Subsequently, these outer layers being hardened, the water 

 ceases to take up so much lime from them, but passes through 

 cracks and clunks, to dissolve away the softer interior, which 

 sinks and falls in. A cave is the result, on the roof of which 

 stalactites form at once. 



The falling in of the roofs of ancient caves oives rise to manv 

 peculiar features in the landscape of Bermuda. The stalagmites 

 at Walsingham Cave are far under water, proving a sinking of 

 the floor of the cave which might possibly be supposed to be 

 local, due to the giving way of some hollow beneath ; but since 

 the same condition is to be seen in nearly all the caves, and 

 there is the further evidence of the sunken bed of lignite, there 

 seems no doubt that there has been a general sinking of the 

 island in comparatively recent times. In some places on the 

 coast of Bermuda are reefs composed by Serpulse, which were 

 called by Nelson Serpuline reefs. These often form regular 

 circles or tiny atolls, as it were, about 20 to 30 feet in diameter. 

 The form evidently results from the fact that the most externally 

 placed animals have a great advantage in procuring food over 

 those placed behind them or in the centre of the area, 



