TEANSACXIONS OF SECTION C. 667 



solidification as the result of that process. This would not be the case in the 

 lava in a volcanic chimney, in which we should expect that the temperature 

 would fall by steps, remaining fixed as long as any definite mineral species was 

 crystallising, and would then drop to the crystallising temperature of the next 

 species, again remaining for a certain time fixed. What these temperatures should 

 be we do not know until the crystallising temperature of each rock-forming species 

 is known. It must be remarked that the simple fusion point of auy mineral is no 

 indication of the temperature of the crystallising of a given mineral from a magma ; 

 for, in the former case we have to deal with a simple physical process, whereas in 

 the latter it is often a chemical one. This is illustrated well in the case of 

 orthoclase and augite, which both crystallise before leucite, which can only be 

 explained by a chemical reaction taking place in the magma. 



This process I take to be dependent upon a magma open to the atmosphere, by 

 which the alkaline chlorides break up, the HCl being liberated and the free alkalies 

 combining with the silica and alumina of the basic iron glass. Part of the iron 

 separates as magnetite and part combines with the HCl and escapes in the vapour 

 as chloride, always accompanied by some of the alkaline chlorides. 



In fact, it seems an almost impossible task, the determining theoretically or 

 even practically the temperature of solidification of a lava. 



However, a clear comprehension of what has been said demonstrates how the 

 supply of heat is kept up for a long time in the volcanic chimney, and the varying 

 activity resulting therefrom. "VVe have good reason to suppose that in our type 

 of volcanoes there comes to the surface a uniform quantity of magma in a given 

 time, although the extrusion may be more or less rhythmical, due to tidal and other 

 agencies. Let us suppose that the magma has been so long simmering in the 

 chimney that by volatihsation of steam, &c., it beL'ins to become pasty. In that 

 condition the escape of vapour from the lower part of the magma in the chimney 

 cannot go on, and consequently the fall of temperature is arrested, although 

 crystallisation may go on for some time, so that the temperature of the lava rises. 

 Meanwhile more water is being dissolved or taken up by the magma, and, in conse- 

 quence of this and the rise of temperature, the tension of the magma increases until 

 it overcomes the resistance of the pasty magma choking the upper part of the 

 chimney. There occurs a more or less strong paroxysmal eruption, in which, from 

 the excess of water and the higher temperature there will result a tendency to the 

 issue of the magma in a fragmentary condition, and this will approach "more or 

 less the scoriaceous or even pumiceous character. This is what is constantly 

 occurring at Stromboli and even at Vesuvius, but in the latter it is much modified 

 by the lateral oozing of lava, the effect of which I propose to treat of elsewhere. 



In fine, we must conclude that the calorific capacity of the original vitreous 

 magma must be very great, although its temperature may not be very high, and 

 that its heat energy under favourable conditions may keep up the temperature for 

 a long time. This would be the case iu a volcanic chimney when soliditication was 

 taking place by crystallisation instead of cooling as a glass — a similar condition, in 

 fact, to what we observe in the solidification of melted sulphur and other vitreous 

 materials.. 



5. Note on a Mass containing Metallic Iron found on Vesuvius. 

 By H. J. Johnston-Lavis, M.B., F.G.S. 



In the year 1882, during an excursion that I made on the slopes of Vesuvius 

 to the N.W. of Camaldoli della Torre, a rusty-looking block about the size of a 

 human head was met with loose on the surface. I was provided with a hammer 

 sufficiently powerful to break up the usual size of ejected blocks, but on striking 

 the mass in question, much to ray astonishment, it persistently refused to break. 

 A near examination revealed the fact that the spot where the blows had been 

 delivered was malleable, and had been beaten out into scales. By a little 

 manneuvring the block was eventually split up into various pieces. 



The surface was rough and irregular, showing no patina of any kind, and was 

 much rust-stained. The mass is principally composed of a dark grey vesicular 



