134 GEOLOGY [CHAP. IX 



Letter 488 of his papers discusses such cases, and urgently wishes the 

 height of the fluid lava was known in adjoining volcanoes 

 when in contemporaneous action ; he argues vehemently 

 against (as far as I remember) volcanoes in action of different 

 heights being connected with one common source of liquefied 

 rock. If lava was as fluid as water, the case would indeed 

 be hopeless ; and 1 fancy we should be led to look at the 

 deep-seated rock as solid though intensely hot, and becoming 

 fluid as soon as a crack lessened the tension of the super- 

 incumbent strata. But don't you think that viscid lava 

 might be very slow in communicating its pressure equally in 

 all directions ? I remember thinking strongly that Dana's 

 case within the one crater of Kilauea proved too much ; it 

 really seems monstrous to suppose that the lava within the 

 same crater is not connected at no very great depth. 



When one reflects on (and still better sees) the enormous 

 masses of lava apparently shot miles high up, like cannon- 

 balls, the force seems out of all proportion to the mere gravity 

 of the liquefied lava ; I should think that a channel a little 

 straightly or more open would determine the line of explosion, 

 like the mouth of a cannon compared to the touch-hole. If a 

 high-pressure boiler was cracked across, no one would think 

 for a moment that the quantity of water and steam expelled 

 at different points depended on the less or greater height of 

 the water within the boiler above these points, but on the 

 size of the crack at these points ; and steam and water might 

 be driven out both at top and bottom. May not a volcano 

 be likened to a protruding and cracked portion on a vast 

 natural high-pressure boiler, formed by the surrounding area 

 of country? In fact, I think my simile would be truer if the 

 difference consisted only in the cracked case of the boiler 

 being much thicker in some parts than in others, and there- 

 fore having to expel a greater thickness or depth of water in 

 the thicker cracks or parts a difference of course absolutely 

 as nothing. 



I have seen an old boiler in action, with steam and drops 

 of water spurting out of some of the rivet-holes. No one 

 would think whether the rivet-holes passed through a greater 

 or less thickness of iron, or were connected with the water 

 higher or lower within the boiler, so small would the gravity 

 be compared with the force of the steam. If the boiler had 



