148 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
sort of dark ridge or backbone was visible from end to end, not unlike the moraine 
on the top of a glacier. This reminds me of a curious analogy which often struck 
me, between two objects so dissimilar as a glacier and a lava stream. They are 
both, more or less, frozen rivers ; they both obey the law of gravitation with great 
reluctance, being essentially so sluggish, that although they both move along the 
bottoms of valleys with a force well nigh irresistible, their motion is sometimes 
scarcely perceptible* * * § .” This remarkable passage, worded with the usual scrupulous 
care of the author, combined with his account of the mechanism of a glacier in the 
description of the glacier of Miage in the same work, show that he had arrived at more 
correct notions on the subject than any of his contemporaries ; notions which chiefly 
required careful observation to give them the force of demonstration. The allusion 
to moraines as characteristic of lava streams as well as glaciers, in the preceding ex- 
tract, is perfectly borne out by the view of the lava of 1831 given by Mr. Auldjo and 
already cited ; the same appearance is mentioned by M. Elie de Beaumont in his 
account of Etna in the following terms: “ Une des circonstances que les coulees de 
lava prfisentent le plus invariablement * * * * consiste en ce que chaque coulee est 
flanquee de part et d’autre par une digue de scories accumulees qui rapellent par sa 
forme la moraine d’un glacier, * * * souvent aussi les coulees presentent de pareilles 
digues vers leur milieu, lorsqu’elles sont partagees en plusieurs cou rants distincts 
coulant l’un a cote de I’autrcf'.” 
In another place the same author compares the movement of the upper crust of 
the lava to that of glaciers according to the then prevalent theory: — “L’6corce 
superieure d’une coulee separee de l’ecorce inferieure et du sol sousjacent par une 
certaine epaisseur de lave liquide, ou du moins visqueuse, se trouve dans un etat 
comparable a celui d’un glacier, qui,ne pouvant adherer au sol sousjacent a cause de 
la fusion continuelle de sa couche inferieure, se trouve contraint de glisser^.” 
Finally, M. Rendu, Bishop of Annecy, in his excellent Essay on Glaciers, refers 
in one passage (and I believe in one only) to the possible analogy with a lava stream, 
“ [le glacier] s’affaisse-t-il sur lui-meme pour couler le long des pentes comme le 
ferait une lave a la fois ductile et liquide § ?” 
The following considerations seem to show more than a general external analogy 
between lava streams and glaciers. 
Their velocities are sometimes equally slow. Although common lava is nearly as 
liquid as melted iron, when it issues from the orifice of the crater, its fluidity rapidly 
diminishes, and as it becomes more and more burdened by the consolidated slag 
through which it has to force its way, its velocity of motion diminishes in an almost 
* Patchwork, by Captain Hall, vol. iii. p. 118, published 1841. 
f Recherches sur le Mont Etna, p. 184. Published in the Memoires pour servir a une Description Geolo- 
gique de la France, tome iv. 1838. 
J Ibid. p. 177. 
§ Theorie des Glaciers, M6m. de l’Acaddmie de Savoie, tome x. p. 93, published 1841. 
