88 VISCOUS THEORY OF GLACIER MOTION. [1845. 



in similar circumstances, showing the considerable consistence 

 which the lava possessed. 



In the upper part of this Fossa the lava has a distinct linear 

 structure where broken, in shells parallel to the sides, whose thick- 

 ness varies from one-third of an inch upwards. The position of 

 these surfac.es of dislocation is indicated (for illustration) in 

 fig. 5 of Plate I. 



2. In the vast lava wastes of Etna we encounter not only 

 a greater extent of surface, but a greater variety of condition 

 as to cohesion of the lava streams, and the slope down which 

 it has descended, and thus we have a better chance of meeting 

 with specimens of the manner in which the semi-solid crust of 

 a lava stream is torn up and crevassed by the effect of gravity 

 compelling it into the circumstances of fluid motion. From this 

 tendency of all lavas to form slags, and of these slags to be 

 splintered, tossed, and remoulded by the action of the still 

 liquid portion of the stream below or around, not one-thousandth 

 of the surface bears marks of the simple condition of fluidity 

 under which it was originally moulded ; and though, when viewed 

 from a distance, and in connexion with the form of the ground 

 over which it has passed, we see plainly enough that it has 

 flowed like a stream, the absence of any trace of easy undulating 

 forms which characterise fluids or plastic masses, gives to the 

 sciarre of Etna (the cheires of Auvergne) an appearance far 

 more removed from pristine fluidity than the glacier masses of 

 Switzerland. 



In traversing many miles of lava wastes between Nicolosi 

 and Zafarana, on the eastern slope of Etna, I met with one 

 singularly favourable specimen of a branch of a stream consoli- 

 dated exactly as it had moved, and undisturbed afterwards. It 

 is the part of the current of 1763, called Lava delle Cerve. The 

 branch stream in question may be ten yards wide, and presents 

 a thin crust, which has floated on the viscid lava below, and 

 which, while yet imperfectly solidified, has been urged to move 

 with the rest of the stream, and has undergone a process of 

 division and rending accordingly. The stream has flowed in the 



