THE VICTORIAN NATURALIST. 



159 



north of this the most continuous outcrop of the limestone 

 occurs. 



The island is near the middle of the lake, a little further from 

 the south-west side, allowing room for the abovementioned clear 

 water area. The length of the island, north to south, is about a 

 mile, greatest width about three-quarters of a mile. It consists, 

 except a small patch near the causeway, of pyroclastic (i.e., 

 volcanic fragmentary) materials, ash, scoriae, and ejected blocks of 

 basalt, or, more rarely, blocks of other material. A small outcrop 

 of the Tertiary rock occurs, which may be, however, detached 

 from the main mass of that rock. The height of the highest 

 point of the island, near the south end, was approximately 

 estimated at 270 feet above the lake level. 



Two very distinct craters occur, one near each end of the island 

 (C on map). The inner slopes are very steep, and where the rock 

 is seen it is scoriaceous, often more coarsely vesicular in the 

 interior, the vesicles being finer near the outside. Each of these 

 craters has a pool at the bottom, near, or probably at, the lake 

 level. The lowest part of the rim of the southern crater is about 

 150 feet above the pool. 



Near the middle of the island, and nearly at the centre of the 

 lake area, if the widening to the south-west be neglected, is a 

 hollow at a high level, its floor being about 200 feet above the 

 lake (V on map). On three sides a ridge surrounds it, but on the 

 south it falls away steeply to a valley. It may represent an early 

 crater on the island. To the south of this valley are several small 

 crateriform hollows, which may also have been centres of activity 

 (X on map). An arm of the lake cuts off a portion of the north- 

 east corner of the island, and another penetrates some distance 

 into the west side. 



The lake is, perhaps, better described as an explosion lake than 

 a crater lake. It occupies, not a crater in a volcanic hill, but the 

 hole in the underlying rocks formed by explosive action. Subsi- 

 dence may have contributed to the formation of the basin. The 

 level of the water in the two recent craters illustrates the small 

 likelihood of water standing in an ordinary crater. The per- 

 meability of the material would both maintain it at this level and 

 prevent its remaining higher. The later eruptions have built up 

 the island and almost filled in the lake. They have also added to 

 the rim, as is seen by the beds curving over into the lake basin. 

 These beds must be subsequent to the formation of the basin. 

 The clear water area may represent a site of activity of a late 

 date. The shoaling of the lake generally has been further 

 assisted by the waste of the cliffs and island. The whole 

 illustrates very well that a volcano is essentially a passage of 

 communication to the interior and that the hill is only a conse- 

 quence of its activity. 



