1845.] TERMINATION OF A LAVA STREAM INTUMESCENCE. 91 



stiffness of the matter ; such is a beautiful glacier, named, as 

 far as I can learn, La Gria, or Glacier de Bourget, which de- 

 scends from the Aiguille de Goute towards the valley of Cha- 

 mourii. See Plate IV., fig. 9.* 



Many, perhaps most, lava streams, where they have well- 

 determined hanks, are concave during the longer part of their 

 course, but towards their termination they become convex as 

 their viscosity increases. Nevertheless, I have seen portions of 

 well-bounded streams decidedly convex. 



The appearance of the termination of a lava stream ap- 

 proaches strikingly that of a glacier. But this is much more 

 than a vague analogy, and the accounts of faithful eye-witnesses 

 prove the resisted motion of the doughty stream to be such as 

 I anticipated. We find it explicitly stated over and over again 

 in the writings of Dolomieu f and Delia Torre f (and more par- 

 ticularly by the latter), that when a lava stream meets with any 

 obstacle in front which checks its course, or when its course is 

 checked by its own sluggishness, the stream swells, and gains 

 gradually in thickness by the fluid pressure from behind urging 

 its particles forwards and upwards. So striking was this natu- 

 ral effect of semi-fluid pressure, that these old observers attri- 

 buted it to a peculiar force developed in the lava, of the nature 

 of " fermentation," producing intumescence, the only way by 

 which they could account for the vertical rise of the fluid, 

 although it was very evident that the result was only what 

 might be expected from the nature of the lava. It was also 

 observed that when the lava stream had thus attained a certain 

 height, it began to move on again, the necessary result of the 

 increased hydrostatic pressure, although attributed by the authors 

 named to the heat developed by chemical action. The tenacity 

 with which the idea was long adhered to, that the residual 

 fluidity of a nearly cooled lava stream was insufficient to account 

 for its progress, without attributing to it the qualities of a 



* [Of the Philosophical Transactions for 1846. It has not been thought essential 

 to reproduce this figure.] 



f Papers in the Journal de Physique. 



\ Histoire du Vesuve. Naples, 1771, 8vo, p. 207-9, and several other places. 



