154 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
appears to be that they have been stretched whilst flowing slowly onwards in a pasty 
condition, in precisely the same manner as Professor Forbes believes that the ice of 
moving glaciers is stretched and fissured. In both cases the zones may be compared 
to the finest agates ; in both they extend in the direction in which the mass has 
flowed, and those exposed on the surface are generally vertical 
The other illustration is contained in a communication with which I have been 
favoured by Mr. Gordon, Professor of Civil Engineering in Glasgow, and which has 
been printed in the Philosophical Magazine for March 1845, to which therefore I 
may refer. I need only state at present that it demonstrates, from observations on 
the flow of Stockholm pitch with a speed wholly insensible, and which requires some 
months for its accomplishment even in small masses, that a motion, of the nature of 
fluid motion, takes place at temperatures at which the pitch remains so hard as to 
be fragile throughout, and presents angular fragments with a conchoidal fracture. 
Mr. Gordon adds, that the resistance of the pitch to its own forward motion produces 
bands of .differential velocity and having the frontal dip. 
Edinburgh , February 26, 1845. 
Note on the Velocity of Lava, referred to in p. 149. 
The following are a few facts which I have collected on the velocity of lava. That 
of Vesuvius in 1805 appears to be the most fluid on record. Von Buch, who was in 
company with MM. de Humboldt and Gay-Lussac, describes it as shooting suddenly 
before their eyes from top to bottom of the cone in one single instant-^, which must cor- 
respond to a velocity of many hundred feet in a few seconds without interpreting it lite- 
rally. Melogrami, quoted by Breislak;};, says it described three miles in four minutes, 
or about seventy-five feet per second at a mean. The same lava, when it reached 
the level road at Torre del Greco, moved at the rate of only eighteen inches per minute, 
or three-tenths of an inch per second §. The lava of 1794 (Vesuvius) reached the sea, 
a distance of 12,961 feet, in six hours, or passed over one-third of a mile per hour, or 
eight inches per second || ; whilst the lava of Etna, in 1651, described sixteen miles 
in twenty-four hours, or above a foot per second the whole way. That of 1669 (Etna), 
which destroyed Catania, described the first thirteen miles of its course in twenty 
days, or at the rate of 162 feet per hour, but required twenty-three days for the last 
two miles, giving a velocity of twenty-two feet per hour^[ ; and we learn from Dolo- 
* Darwin on Volcanic Islands, 1844. The whole passage, pp. 65-72, illustrates this analogy. 
t Bibliotheque Britanique, vol. xxx. The vertical height of the cone proper is 700 or 800 feet; the length , 
of the slope may therefore be 1300 feet. 
1 Institutions Geologiques, iii. 142. 
§ Nicholson’s Journal, vol. xii. || Breislak, Campanie, i. 203. 
Ferrara, Descr. del Etna, p. 105. This appears from the dates, though at variance with one assertion of 
the author. 
