280 



gvacliially to the interior of the island between the lake and the sea, and 

 occiapying two-thivds of the space within the amphitheatre. The interior por- 

 tion of the space is occupied by a hot lake, the water of which is strongly 

 acid. The form and general appearance of this lake as observed by Dr. Eolston 

 and Lieut. Edwin, of H.M.S. "Brisk," has already been described in our 

 Transactions (Yol. i., p. 58), and by tlieir survey it was found to have an area 

 of fifteen acres. From tlieir account it appears that the level of the water in 

 this lake above the sea, undergoes great variations. On the occasion of my visit 

 the water appeared to be at its extreme height of 25 to 30 feet above tide mark, 

 there being no beach exposed on which it would be possible to travel round the 

 margin of the lake as has been frequently done. Thus, Dr. Rolston states in the 

 paper above referred to, that on March 16, 1868, "There was very much more 

 water in the lake than when last visited, in November, 1866, which precluded 

 the possibility of reaching the largest steam jets at the extreme north-west 

 corner of the crater ; but it wa,s observed that these steam jets were not nearly 

 so active as in 1866." 



A c\\&, 10 to 30 feet high, bordei-s the lake where it is bounded by the 

 tufa plain, but on each side cliffs rise perpendicularly from its margin to the 

 height of several hundred feet. A small level area at the iniier end of the 

 lake, only slightly above the water level, is where the active geysers are 

 situated. The general form of the island is thus a horse-shoe shaped range of 

 hills, about 800 feet high, enclosing, on rather more than three sides, a level 

 area occupied in part by the hot lake, and in part by the deposits from the hot 

 springs which supj^ly it. 



As far as I was able to determine, the structure of the island is that shown 

 in the accompanying section. The exterior layer of the island, so to speak, 

 consists of a laminated deposit of incoherent breccia, or conglomerate (a), 

 evidently the sub-aerial slope deposit that had, at an early pei'iod, been formed 

 on the flank of a volcanic cone. This deposit rests on layers of lava and 

 trachyte tufa (b), which are intersected by dykes (c). These rocks form the 

 external wall of the crater, but, resting on ledges to a height of 200 feet 

 above the present level of the lake, are laminated deposits (d) of a similar nature 

 to the tufaceous sinter that is now forming round its margin, thus indicating 

 that at a former period the crater wall was more complete, and that 

 the water of the lake stood at a higher level relatively to the mass of the 

 island. 



The manner in which this sinter is deposited on the shore of the lake 

 appears to be as follows : — the water, which is of a deep green colour and has 

 an average temperature of 110° F., deposits crystals of sulphate of lime when 

 exposed to the air ; these form a crust around the margin of the water*, and as 

 the level is continually fluctuating, this crust attains a considerable thickness 

 and projects from the shore like a shelf, until it breaks down in large masses 



