Haedcastle. — On the Cause of Volcanic Action. 341 



I think that by the expansion of uuremoved rock by heat, and 

 still more by its expansion by escape from the elastic compres- 

 sion it was previously subjected to, the place of the rock removed 

 has been occupied. Were it not so, the extensive fissurings 

 that have occurred in the bed of Eotomahana must have allowed 

 the whole of its waters to sink into the cavity on the subsidence 

 of the grand eruption. Possibly there has been some slight 

 sinking of the ground immediately above the locality whence 

 the rock was removed, (I have read something about the southern 

 end of Lake Tarawera having subsided 18 inches,) but the 

 other means of filling the gap may have been sufficient for 

 the purpose at present. As the heated rock cools and contracts 

 further sinkage must occur, of which the deepening the existing 

 lakes would be one indication. The second set of eruptions has 

 been spoken of as hydrothermal, as distinguished from volcanic. 

 I confess I do not understand the distinction — that is, if by the 

 second eruption so much solid matter was ejected as I understand 

 there was. It would seem to be a proper distinction to call that 

 action hydrothermal which seemed to arise from access of 

 water to heated beds ; but, (as coiiteuded above,) no con- 

 siderable eruption could be originated in this way. There 

 could be no solid ejections worth speaking of. For true volcanic 

 action the water must be in the rock when heated, or, must 

 have time to permeate a heated rock before a fissure of escape is 

 provided, when the same results would follow. Yet it must be 

 more difficult for steam to break up a solid rock, than one that 

 from the effects of recent mechanical action upon it has lost 

 much of its cohesion. 



It has been remarked that there were no "warnings" of the 

 eruption. There never are other warnings of a new outbreak 

 than such as were given to those living in the neighbour- 

 hood. There were numerous earthquakes which indicated 

 that movements were going on below. The springs were 

 affected, being more copious, without meteorological cause, in- 

 dicating that the movements were compressive — the water being 

 squeezed out of the fissures in the strata. But it was impos- 

 sible to gauge the extent of those movements, or foresee their 

 actual effect. 



