27G 
RECENT VOLCANIC ERUPTIONS IN THE VEST INDIES. 
and they are violently discharged from the crater. When the greater 
part of the steam and gases has been discharged, the lava, still rising, 
finds a vent either over the lip of the crater, or often through a lateral 
fissure, and flows quietly down the side of the mountain. 
It is quite recognised that these phenomena may occur in various 
relative proportions. The explosive phase may predominate, in which 
case only sand, pumice, and fragmentary material are discharged, with 
perhaps ejected blocks torn from the sides of the chimney, and in this 
case an ordinary ash or cinder cone is built up. On the other hand, the 
magma may contain little vapour, and the lava may be discharged 
quietly and spread out widely as a sheet over the surrounding country. 
The Snake river basalts in Western North America are of this class, 
and though they cover an area larger than England and France com¬ 
bined, no eruptive cones or craters have been found on them, and it is 
supposed that none ever existed, but that the lava welled out quietly 
through fissures. Such fissures I have seen in Iceland, studded with 
a row of quite small craters only. We believe that in these Pelean 
eruptions an intermediate phase occurs. The lava which rises in the 
chimney is charged with steam and gases, which explode as usual, but 
some of the explosions happen to have only just sufficient force to blow 
the mass to atoms and lift the greater part of it over the lip of the 
crater without distributing the whole widely in the air. The mixture 
of solid particles and incandescent gas behaves like a heavy liquid, and 
before these particles have time to subside the whole rolls down the 
side of the mountain under the influence of gravity, and consequently 
gathers speed and momentum as it goes. The heavy solid particles are 
gradually deposited, and the remaining steam and gases, thus relieved 
of their burden, are free to ascend, as v T as the case with the black cloud 
which rose over our heads on July 9. 
We had concluded, from our examination of the Soufriere, that some¬ 
thing of this sort must occur, but the explanation was obvious when we 
saw the eruption of Mount Pelee. Dr. Flett remembers my saying while 
the eruption continued, “That’s an avalanche,” and among my notes 
made while in the Indies are the following : “ The cloud of incandescent 
material, as we saw it welling out of the great fissure, reminded us of 
nothing so much as a snow-avalanche as seen in the Alps. It rolled 
rapidly from the mountain side in well-defined billows, giving the 
impression of a vast volume of separate small particles mixed with a 
certain quantity of air or vapour, and, as in the case of Alpine avalanches, 
entangling more air in its progress, and setting up a blast sufficient to 
overturn large objects in its course.” 
This effect of avalanches in compressing the air before them and 
setting up a powerful blast, the effects of which extend beyond the 
area covered by the fallen material, has long been recognized. Plate 
13 shows a group of large trees overthrown by the blast of the great 
