Ch. XXX.] THE POST- PLIOCENE PERIOD. 527 



Fig. 663. 



Dikes or yeins at the Punto del Nasone on Somma. (Necker.*) 



1779, who records the following facts : — " The lavas, when they either 

 boiled over the crater, or broke out from the conical parts of the volcano, 

 constantly formed channels as regular as if they had been cut by art 

 down the steep part of the mountain ; and, whilst in a state of perfect 

 fusion, continued their course in those channels, which were sometimes 

 full to the brim, and at other times more or less so, according to the 

 quantity of matter in motion. 



"These channels, upon examination after an eruption, I have found 

 to be in general from two to five or sis feet wide, and seven or eight 

 feet deep. They were often hid from the sight by a quantity of scoriae 

 that had formed a crust over them ; and the lava, having been conveyed 

 in a covered way for some yards, came out fresh again into an open chan- 

 nel. After an eruption, I have walked in some of those subterraneous 

 or covered galleries, which were exceedingly curious, the sides, top, and 

 bottom being worn perfectly smooth and even in most parts, by the vio- 

 lence of the currents of the red-hot lavas which they had conveyed for 

 many weeks successively, "f 



Now, the walls of a vertical fissure, through which lava has ascended 

 in its way to a volcanic vent, must have been exposed to the same ero- 

 sion as the sides of the channels before adverted to. The prolonged and 

 uniform friction of the heavy fluid, as it is forced and made to flow up- 

 wards, cannot fail to wear and smooth down the surfaces on which it 

 rubs, and the intense heat must melt all such masses as project and 

 obstruct the passage of the incandescent fluid. 



The texture of the Vesuvian dikes is different at the edges and in the 

 middle. Towards the centre, observes M. Necker, the rock is larger 

 grained, the component elements being in a far more crystalline state ; 

 while at the edge the lava is somewhat vitreous, and always finer grained. 

 A thin parting band, approaching in its character to pitchstone, occasion- 



* From a drawing of M. Necker, in Mem. above cited, 

 f PhiL Trans, vol. Isx. 1780. 



