44 SIXTH LETTER ON GLACIERS. [1844. 



were very frequently flowing from mouths within the crater of 

 Vesuvius. On the 30th November, in particular, I descended 

 to the bottom of the crater, in order to examine a current of 

 very liquid lava, fifteen or twenty feet wide, which issued from 

 a cavity near the foot of the small cone which .occupied the 

 centre of the crater, and from whose top (in the shape of an 

 inverted funnel, or of a blast furnace) there issued smoke and 

 flames,* occasionally accompanied by a discharge of volcanic 

 projectiles. The lava issued in a very steady rapid stream, and 

 spread itself over a gentle declivity with a velocity of not less, 

 I think, than a foot per second. 



Admitting the plastic or viscous theory of glaciers, the 

 resemblance to lava fails (1.) In respect of the great liquidity 

 of the lava near its source ; (2.) From its very unequal rate of 

 consolidation ; a crust being very soon formed upon the surface, 

 which becoming more and more massive, the principle of fluidity 

 is not uniformly distributed throughout the mass, as in the 

 glacier, but a tolerably perfect fluid struggles with the increasing 

 load of its ponderous crust, which it tears and rends by the 

 mighty energy of hydrostatic pressure ; and here and there 

 finding a freer exit far removed from its source, tosses high 

 those mighty fragments of the stony arch which confined it into 

 the wild shapes which strike the eye in crossing the wastes of 

 a lava stream, and which seem at first incompatible with the 

 fluid or semifluid principle of motion. This second circum- 

 stance, then, the very unequal and rapid superficial consoli- 

 dation of the lava near its source, has no analogy in a glacier, 

 nor even in a river, unless when breaking up a ponderous crust 

 of ice after a sudden thaw. The regulated progression of the 

 glacier, swiftest in its centre, and with a graduated retardation 

 towards the sides, has a much more precise analogy to that of 

 a river than the lava stream has, which is subdivided (when it 



* I am able to add my distinct testimony to that of M. Pilla, as to the emission 

 of flames by the crater of Vesuvius. I spent part of the evening of the 1st January 

 on the top, and had not the least doubt that what I saw were actual flames, which 

 issued from time to time from the orifices of the small cone, and which were of a 

 pale colour, often inclining to blue. 



