150 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
which gravity acted, and this is a sufficient answer to any attempt to maintain that the 
plasticity of a glacier is a collateral but not a primary cause of motion, — a distinction 
surely without a difference. 
As in the case of all imperfect fluids, the central and superficial particles move 
faster than the lateral and inferior ones ; and when the fluidity is exceedingly imper- 
fect, as in those long-flowing lavas, there must be a rupture of continuity between 
the parts to permit them to slide and jostle past one another. This is evidently the 
cause of the noise referred to by Mr. Scrope and other writers. This tearing up of 
the stream into longitudinal stripes, occasioned by the varying velocity of the parts, 
is thus described by M. Dufrenoy in his account of Vesuvius : “ La plupart des 
coulees presentent des bandes longitudinales assez paralleles entre elles : ces larges 
stries saillantes sur la surface sont les traces du mouvement de la lave qui ne s’avance 
pas d’une seule piece, mais par bandes paralleles*.” 
And M. Elie de Beaumont describes a lava stream at Etna in these terms : “ La 
surface offrait de profondes cannelures paralleles entre elles, dirigees dans le sens du 
mouvement qui l’avoit deversee a l’exterieur et qui etaient croisees par de nombreuses 
gerqures transversales\ ” Here then is evidently the twofold system of rents and 
perpendicular fissures described in the commencement of this paper as being found 
in the models, and as being conformable to the phenomena of glaciers. 
During the winter 1843-44 which I spent in Italy, I had an opportunity of testing 
these resemblances, and tracing others to glaciers in the lavas of Vesuvius and Etna. 
I entered on the inquiry with a very jealous care of being drawn into the admission 
of fanciful or imperfect analogies ; and I shall confine myself to the statement of one 
or two most plain and undeniable confirmations, selected from the results of many 
fatiguing rambles. 
The plastic nature of the viscous lavas of Vesuvius and Etna is such as well might 
obliterate any internal traces of rents due to differential velocity, which, in the mass, 
are speedily closed and reunited as in a stream of treacle, or in the plaster models 
before explained, where the interior is homogeneous and the superficial coating above 
is permanently dislocated. 
In lavas the indescribable ruggedness of the surface very generally prevents any 
record of the gentler play of forces. The following facts appear to me quite conclu- 
sive as to the manner in which a mass partially solidified, yet moving as a fluid, is 
torn up by the interior forties which act upon it. 
1 . At Vesuvius, the Fossa della V etrana between the Hermitage and Monte Somma, 
is a valley lined with the lava of 1751. I here observed that the lava was in some 
places detached from the wall of the valley, leaving a cavity on the sheltered side of 
a projecting elbow of rock, just as a glacier does in similar circumstances, showing 
the considerable consistence which the lava possessed. 
* Dufrenoy sur les Environs des Naples, p. 324. 
t E. de Beaumont, p. 38. 
