108 , THE ATLANTIC. [chap. ii. 



at right angles to one another. These clefts are filled up with 

 a hard flinty-looking substance, which appears from its struct- 

 ure to have been gradually deposited from water trickling 

 down the sides. Its mass is concretionary and sometimes foli- 

 ated; its color is white to yellowish white or brownish yellow. 

 It scratches glass with ease, and does not effervesce with acids. 

 Plates of two to three millimetres in thickness are quite trans- 

 lucent. Heated in the forceps, it does not fuse, but turns per- 

 fectly white, and is then easily crumbled between the fingers, 

 and in the closed tube it gives off alkaline reacting and em- 

 pyreumatic- smelling water. It was found to consist of phos- 

 phate of alumina and iron, with some silicate and sulphate of 

 lime. 



" Rat Island is the largest of the secondary islands, and the 

 most distant from the main island. It is composed on the 

 western side of massive basaltic rock, and on the eastern of 

 sandstone. The sandstone probably overlies the basalt, as, in 

 its structure, it bears the marks of having been deposited in 

 drifts, and the sand is calcareous, consisting of shell debris. 

 On the way to and from Rat Island we had to pass along the 

 western side of Booby Island. The wave -worn cliffs showed 

 that the island was entirely formed of the above-mentioned cal- 

 careous sandstone ; no igneous rock was visible, and, as the pe- 

 culiar wind-blown stratification-marks are continued below the 

 level of the sea, it is probable that the land here is sinking, or, 

 at all events, has sunk. Phitform Island consists of a mass of 

 perfect basaltic columns rising out of the water and supporting 

 a covering of massive basalt, on which is spread out the j^lat- 

 form of calcareous rock on which are the ruins of a fort, and 

 from which the island doubtless takes its name." 



In the pinnace we went along the northern shore of the 

 main island, dredging nine times, in water from seven to twenty 

 fathoms deep. We got surprisingly little, only a few crusta- 

 ceans, one or two star-fishes, and a pretty little Cidaris. We 



