Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 141 
resistance. The lava was, in fact, obliged to take a crooked course, and so in 
part disintegrated the cone at this unsupported spot. 
In like manner, the great thick buttress on the south-east slope of the 
Vesuvian cone very likely prevented the radial dyke from reaching the surface 
beneath it, or the thick lava masses on the Pedimentina farther down. The 
fluid rock no doubt found the direction of least resistance in the old incoherent 
materials at a lower level, 600 metres, at the Bosco dei Cognoli, on the still only 
partially covered flanks of the prehistoric Monte Somma. 
During the next few years, the height of the mountain steadily rose by 
additions of lava and scoria, on the lava-plain of what remained of the 1872 
crater. Fluid rock in small quantities issued high up on different sides of the 
great cone; and when the magma-level in the chimney fell, owing to one of 
these lateral drains, some of the cone of eruption crumbled in, but was soon 
again repaired and added to. 
On June 7th, 1891, the whole north side of the great cone was rent from 
the top down to the Atrio---that is 300 metres. Lava issued in fountains on the 
floor of the Atrio, inundating many hundreds of square metres of the almost 
level plain. Dr. L. Sambon, who was then my assistant, and myself, were nearly 
overwhelmed; but we escaped and clambered up the face of the Somma 
escarpment, and witnessed during the rest of the night one of the finest spectacles 
one could be present at with any chance of safety. This outflow continued with 
hardly any intermission till February, 1894. The rift in the side of the great 
cone became sealed up on the surface from the first days of the 1891 outburst, 
and a sort of permanent canal was established at its lower extremity, by which 
the lava issued during all these three years. The exit of this drain was at the 
base of the great cone in the Atrio.* 
The flow was slow, so that by the cooling it soon raised the level of exit in 
that direction, and had to find a new path in another one ; thus, by the constant 
increasing height of obstructing cooled lava at different points, the level of 
exit was raised by the growing pile of consolidated rock around it. The result 
of this was that a great lava-cone was built up in the Atrio, in which fragmentary 
ejecta took no part, and the summit of which stood away from the great Vesuvian 
cone. ‘The apex of this lava-cone, now known as the Colle Margherita, was just 
short of 100 metres above the original point of lava issue that laid down its 
foundations three years before. ‘he Colle Margherita slopes away in all 
directions, as sometimes the lava flowed far along the Atrio (3,000 metres 
maximum), which has been to a considerable extent filled up. In 1894 the 
deep notch in the outline of Somma and Vesuvius, so well known in the view 
* It is not unlikely that one of the hollow dykes (fig. B, p. 185) may have been the feeder of this 
issue of laya. 
2A 2 
