ST. VINCENT IN 1902, AND ON A VISIT TO MONTAGNE PELEE. 
209 
quietly, while other parts exploded into minute fragments with sudden escape of 
vapour and descended the mountain as “ nuees ardentes,” accompanied often with 
a mixture of the larger fragments. It is impossible to say exactly the depth to 
which this breaking up and liberation of the vapours extended. As the whole 
gradually cooled and became consistent, and as further material was forced up from 
below, the upper portion no longer spread out into a dome, but was forced up 
“ en masse” as a spine, though it still retained veins of pasty or liquid material 
spreading through it. This forcing up from below by fluid pressure was clearly the 
main mechanism of the ascent, but Lacroix thinks that the pressure of the veins of 
semi-fluid lava might be an accessory cause of the ascent and especially of the lateral 
swelling and flaking off of the crust. It is impossible to deny the existence of this 
cause, since it has been postulated that the dome was formed in this way in the early 
stages of the eruption, but it clearly became much less important later on. Some 
authors have suggested that the spine was an old plug formed in the chimney, 
by the materials left there at the close of an earlier eruption. There is no proof 
whatever of this theory and it does not accord well with the transition from the 
formation of the dome to that of the spine, nor does it account for the high tempera¬ 
ture of the central parts of the spine with its veins of still semi-fluid lava, or for the 
flaking off of the crust and emission of “ nuees ardentes ” as the veins of lava came 
to the surface. 
I made two ascents of the mountain in March, 1907, and on the second occasion the 
cloud lifted for a few moments and enabled me to get a photograph which showed the 
stump of the spine rising out of a cone of talus surrounding it, and obviously formed 
of its ruins (Plate 24). At the line of junction of the spine and the talus was a ring 
of very active fumaroles from which steam and other vapour was escaping with a loud 
roar, obviously from under considerable pressure. The clouds closed in again before 
it was possible to complete the examination, but the photograph shows the spine to 
consist of a sort of volcanic agglomerate of blocks of various sizes, similar to what I 
imagine the structure of the dome to have been. 
The Wall of the Crater .—The talus extended in every direction up to the walls of 
the'crater, and had in a great degree filled it up. On the south-west side was the 
above-mentioned V-shaped gap where the talus had overtopped the former crater 
ring and extended down to the valley of Riviere Blanche. On the east and south¬ 
east the crater was least filled. Its wall continued uncovered from the side of the 
gap to beyond the remains of Morne Lacroix, to an average depth of probably above 
100 feet. It was almost vertical throughout all this extent. The valley formed 
between it and the talus appeared to extend, but at a decreasing depth, round the 
north of the dome. 
The resemblances between the valleys of the Wallibu in St. Vincent and the Riviere 
Blanche in Martinique, and the phenomena observed in those valleys respectively, are 
summed up in Part I., p. 489, and the changes which have since occurred serve chiefly 
2 Q 2 
