1845.] LAVAS OF ETNA FRONTAL DIP. 89 



direction from left to right in Plate L, fig. 6. The lateral parts, 

 PP, QQ,* have been literally torn to pieces longitudinally (as I 

 wrote on my note-book on the spot) by the multiplied rents 

 which showed the dislocation of the quicker moving central 

 from the lateral parts, and these rents inclined towards the centre 

 of the stream in the direction in which it moved. The length of 

 the stream was divided by transverse rents strikingly convex 

 towards the origin of the stream, as shown in the same figure. 

 These cracks were marked by another peculiarity ; the cake of 

 floating scoria had not only been cracked across, but pushed 

 upwards, generally forwards and upwards, before it was finally 

 included in the cooling mass of the stream ; the result was the 

 arrangement shown in the longitudinal section, fig. 7, which it 

 will be seen resembles the tiling of a house, only that the frac- 

 tured parts do not always overlap, but the anterior edge is 

 tilted upwards. It will thus be seen that this tendency to 

 separation acts also in the vertical plane, and the dotted lines 

 aa', bb', etc., indicate the direction of its action, coinciding with 

 the surfaces of differential motion, which produce what I have 

 called the frontal dip of the veined structure of the ice of 

 glaciers. 



3. At no great distance from this lava, and near the foot 

 of the hillock called the Serra Pizzuta, between the last-named 

 point and the valley of Tripodo, I observed a transverse section 

 of a lava stream exhibiting an arrangement in bands or plates, 

 nearly parallel to the side of the current, but inclining towards 

 the centre. 



4. Between Zafarana and the Porta Calanna (Etna), a 

 remarkably pretty illustration occurs in the surface of an old 

 lava stream, worn and polished by the action of a brook. 

 Where the lava has had to turn an abrupt corner of a rock, A, fig. 

 8 (which represents a ground plan) , the progress of the lava being 



* [The lines PP, QQ do not (I believe) represent the converging banks of the 

 lava stream, which was probably of uniform breadth, but the converging lines of 

 structural dislocation merely. In the sketch done on the spot which is now before 

 me, the central and unstriated portion is marked as being 6 paces wide. Nov. 

 1858.J 



