ON THE TOLCANIC PHENOMENA OF VESUVIUS. 



31' 



The lava liad first flowed towards the escarpment of Monte Somraa in 

 a fan like manner, so that the eastern extremity reached that prreat 

 natural section just beneath the Punta del Nasone Still following the 

 natural inclination of the ground, it turned to the west, and on June 15 

 was opposite dyke 16 (as marked on my large geological map just pub- 

 lished, and on the dykes themselves), advancing at a very slow rate. 



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

 the included leucite as well as augite crystals, whilst the surfacs is, with 

 one exceptional tongue, of the corded or ' pahoehoe ' type. This is due 

 to the magraa being one that has been simmering since January in the 

 chimney ot the volcano, so that most of its dissolved water has been boiled 

 off, and so allowing^ it to cool without the formation of scorite from the 



Ftg. 3. — Futnaroles formed on the new lava close to its exit at foot of great cone in 

 the Atrio, as seen on June 15, 1891. (Photographed by the auihor.) 



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 of which I have spoken commence. 



Up till June 26 there was an effort to clear the upper part of the 



' See H. J.J. L., 'Geol. M. Somma and Vesuvius,' Quart. Journ. Gcol. Soc. vol.xl. ; 

 and ' Relationship of the Structure of Igneous Eocks to the Conditions of their For- 

 mation,' Scientific Proceedings Roy. Dublin Soc. vol. v. N.S. 



