1845.] INSTANCES FROM VESUVIUS AND ETNA. 87 



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 

 1'avoit deversee a 1'exterieur et qui etaient croisees par de nom- 

 breuses gerqures transver sales," f 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 admis- 

 sion 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 conclusive as to the 

 manner in which a mass partially solidified, yet moving as a 

 fluid, is torn up by the interior forces which act upon it. 



1. At Vesuvius, the Fossa della Vetrana, between the Her- 

 mitage and Monte Somma, is a valley lined with the lava of 

 1751. I here observed that the lava was in some places de- 

 tached from the wall of the valley, leaving a cavity on the shel- 

 tered side of a projecting elbow of rock, just as a glacier does 



* Dufrenoy sur les Environs des Naples, p. 324. 

 fE.de Beaumont, p. 38. 



