Ch. XXX.] THE POST-PLIOCENE PERIOD. 663 



M. Necker refers us to Sir "W. Hamilton's account of an eruption of 

 Vesuvius in the year 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 six 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 channel. 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 violence of the currents of the red-hot lavas which they 

 had conveyed for many weeks successively." * 



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 

 erosion as the sides of the channels before adverted to. The pro- 

 longed and uniform friction of the heavy fluid, as it is forced and 

 made to flow upwards, 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 sometimes vitreous, and always 

 finer grained. A thin parting band, approaching in its character to 

 pitchstone, occasionally intervenes, at the contact of the vertical dike 

 and intersected beds. M. Necker mentions one of these at the place 

 called Primo Monte, in the Atrio del Cavallo ; and when I examined 

 Somma, in 1828, I saw three or four others in different parts of the 

 great escarpment. These phenomena are in perfect harmony with the 

 results of the experiments of Sir James Hall and Mr. Gregory Watt, 

 which have shown that a glassy texture is the effect of sudden cooling, 

 while, on the contrary, a crystalline grain is produced where fused 

 minerals are allowed to consolidate slowly and tranquilly under high 

 pressure. 



It is evident that the central portion of the lava in a fissure would, 

 during consolidation, part with its heat more slowly than the sides, 

 although the contrast of circumstances would not be so great as when 

 we compare the lava near the bottom and at the surface of a current 

 flowing in the open air. In this case the uppermost part, where it 



* Phil. Trans., vol. Ixx., 1780. 



