166 THE VICTORIAN NATURALIST. 



be based on three points — general disposition of material, dip of 

 beds, and distribution of large blocks. None of these give very 

 satisfactory conclusions. The source of the lava is from under 

 Mt. Callender. No dyke appears in the cliffs near here, but it 

 may come most likely by direct overflow. Its surface, too, is 

 inclined away from Mt. Callender. The scoria cone may be 

 simply a cone built over a vent here. A subsidence in the area 

 occupied by the bay would be the easiest explanation of the dips 

 of the tuffs as a whole, though the dips to the north of the bay 

 would be well satisfied by a centre between the little hill and 

 Stuart's Cliff, and near to the largest blocks. Perhaps a number of 

 vents were in existence. The simplest explanation, as a whole, is 

 a centre in Callender Bay around which the tuffs were formed, 

 and at which there has been later subsidence, followed by a new 

 centre on the west of its rim on which has been formed the scoria 

 cone of Mt. Callender. 



The area and thickness of the tuffs must have been to some 

 extent, perhaps considerable, lessened by denudation. Where 

 tuff or scoria rested on lava they might easily be removed 

 completely without the surface of the lava being much affected, 

 for the formation of springs on the surface of the less pervious 

 rock would tend to cause slips, if the slopes allowed it, and the 

 difference in resistance to the denuding agencies would favour 

 complete removal, with little sign left either of the material 

 removed or of extensive denudation. Except for the springs 

 and slight slipping at Mt. Callender nothing of this was noticed. 

 There is no likelihood that in the bay the tuffs have been 

 exposed to much action of moving water in currents, for no 

 stream enters it ; nor are the cliffs directly exposed to much 

 wave action — Stuart's Cliff is much more exposed. The dis- 

 position of the streams feeding the lake may have been such 

 at some time as to cause easy removal of tuff at this place 

 by a current of water. I have met with no direct evidence of 

 this. 



The tuffs were probably deposited in shallow water, which 

 seems the readiest way to account for the original low dips of 

 part at least, and possibly a great part, and for the false bedding 

 and other irregularities. Scouring out of beds just formed could 

 take place as easily in shallow water as on land, and the other 

 features seem more easily explicable on this hypothesis. Lacus- 

 trine conditions might be produced in a variety of ways, the 

 simplest being blocking of streams by products of other vents. 

 Unequal movements might have occurred also at the time of the 

 volcanic activity of so many vents, and may have caused forma- 

 tion of a lake. 



There appears no likelihood that any of the basalt along the 

 north of the lake should be referred to these centres of eruption. 



