August 6, 1891] 



NATURE 



321 



mountain, and had the good fortune to be accompanied 

 by Messrs. H. Elliot, A. Green, Linden, Newstead, and 

 Treiber, several of whom are excellent photographers, 

 so that with two of my own cameras we were able to 

 make an extensive pictorial record of some very unique 

 formations. 



At the point of issue of the lava, at the junction of 

 the foot of the great Vesuvian cone and the Atrio del 

 Cavallo, the first lava had cooled sufficiently to walk over 

 it, but beneath our feet could still be seen in a few holes 

 the flowing lava. At the foot of the great cone, and 

 extending for half way across the Atrio along the radius 

 of the eruptive rent, as if this had continued so far, were 

 a series of driblet cone fumaroles. We counted seven 

 complete and well-formed examples, besides numerous 

 abortive ones. Most were giving out intensely heated 

 vapour, which was liberated from the lava flowing 

 beneath, and which soon carbonized a piece of wood 

 placed in it. Around the lips of the upper opening, 

 haematite with fused chlorides of potash, soda, iron. 



of scoria from the vapour that otherwise would escape 

 after its exit. Leucite I have also demonstrated to be 

 formed while the magma is simmering under low pressure 

 with free escape for vapour in the upper part of the 

 volcanic chimney.^ 



At the summit'of the great cone the crumbling in of the 

 edges was constantly going on, but the upper extremity 

 of the lateral rift at the foot of the cone of eruption and 

 at the summit of the great Vesuvian cone had nearly 

 ceased to give forth vapour. Along the line of rent on 

 the mountain side no fumaroles or other signs of activity 

 were visible except quite at the foot, where those com- 

 mence of which I have spoken. 



Up till June 26 there was a struggle to clear the upper 

 part of the volcanic chimney of the impeding materials, 

 which were constantly being added to by the slipsj from 

 the crater's edge ; but on that evening a dull red glow was 

 visible in the crater bottom, showing that a fairly clear 

 passage had been temporarily made for the continuous 

 escape of vapour, and also that the lava was at no very 



copper, &c., were being condensed from the vapour, 

 and trickling down the outer surface of the fumarole, 

 consolidated as curious vari-coloured stalactites of very 

 deliquescent nature. 



The lava had first flowed towards the escarpment of 

 Monte Somma in a fan-like manner, so that the eastern 

 extremity reached that great natural section just beneath 

 the Punta del Nasone. Still following the natural in- 

 clination of the ground, it turned to the west, and on 

 June 15 was opposite dyke 16 (as marked on my large 

 geological map just published, and on the dykes them- 

 selves), advancing at a very slow rate. 



The lava is a vitreous and coarse-grained rock, especially 

 in regard to the included leucite crystals, whilst the sur- 

 face is, with one exceptional tongue, of the corded or 

 " pahoehoe " type. This is due to the magma being one 

 that has been simmering since January in the chimney of 

 the volcano, so that most of its dissolved H^O has been 

 boiled off, and so allowing it to cool without the formation 



NO. I I 36, VOL. 44] 



great depth from the summit of the volcano. This 

 of course indicates that the lateral opening was in- 

 sufficient to drain off much of the lava which occupies 

 the chimney above the level of the lateral outlet. Had 

 such evacuation really taken place, the eruption would 

 have assumed enormous proportions, from the actual 

 amount of lava above the tap, but more from frothing up 

 of that below that level in consequence of the relief of 

 pressure that in that case would occur. (;f course, during 

 all these days the ejection of dust with the smoke 

 occurred, giving the latter that peculiar dark grey colour. 

 Further destruction of the crater edge took place, so as to 

 partly block the outlet, and it was not till our next visit 

 that it again cleared. 



On June 30 I again visited the crater, in company of my 

 friend Mr. A. Green. All the summit of the great cone 



• See H. J. J.L., "Geol. M. Somma and Vesuvius," Q.J.G.S., vol. xl. ; 

 and " Relationship of the Structure of Igneous Rocks to the Conditions of 

 their Formation," Scient. Proceed. R. Dublin Soc, vol. v., N.S. 



