278 Trayisactions. — Geolopy. 



Mount Tarawera, should have been indicated by other and dis- 

 tinctive signs than mere earthquakes, as by changes of levels, 

 disturbance of springs, and so forth. No such signs are men- 

 tioned. It will scarcely be suggested that there existed an 

 enormous cavity beneath the mountain, void even of water, of 

 which the supposed molten rock took possession, and where it 

 remained quietly stowed away while performing the heating 

 work attributed to it. But if not, where was it bestowed, that 

 no unmistakeable sign of its presence was manifested ? And 

 where is it now ? Professor Hutton says all the matter ejected 

 was re-heated surface-rocks. Then the supposed mass of molten 

 rock must still be a source of danger. It must have retained at 

 the close of the eruption at least as high a temperature as it had 

 imparted to the rocks heated by it, and that was of volcanic 

 intensity. Or has it retreated to the abyss whence it came ? 

 This explanation is prolific of puzzles, and more are provided by 

 the modification made in it to account for the different nature of 

 the eruption from the craters on the plains. 



Not only is there no evidence that any uprise of molten rock 

 took place, but both the modes by which it is suggested such an 

 uprise could take place are, I believe, inefficient. One involves 

 the admission of a thin crust over a fluid or plastic interior. 

 The physico-astronomers disallow that. They threaten to stop 

 tides, and twist and wrench the crust about every day unless 

 that idea is given up, so the sooner it is given up the better. 

 The alternative, that the molten rock is forced up by water 

 expanding into steam, or by the expansive force of other gases 

 imprisoned down below among the molten rock, must, I believe, 

 be also given up. I hope to have opportunity to show on a 

 future occasion, in some detail, that at no depth at which 

 molten rock can reasonably be expected to be met with, accord- 

 ing to this hypothesis, can steam or expansive gas of any effec- 

 tive force be formed at all. The familiarity we possess with the 

 power of steam, and with the tremendous work it does in active 

 volcanoes, has caused inquirers to neglect inquiry, and to 

 attribute to high-pressure steam powers which it does not 

 possess, and to credit it with work which it is really incapable 

 of. Hot water, however hot, is not omnipotent. There are 

 fixed relations between the temperature, density, and pressure 

 of steam ; and just as our boiler-makers can, and do, shut it 

 down at a temperature of 400° or so, by means of a few quarter- 

 inches of steel plate, so gravitation, working through a few 

 miles of rock, can keep it in subjection at 2,01)0°. Steam plays 

 an important part in most (not in all) volcanic eruptions ; bit 

 it is not the prime cause. 



I now turn to the explanation which Professor Hutton dis- 

 cards as " very improbable," the production of volcanic heat by 

 rock-crushing. 



