SOUFPJEEE, AXD ON A VISIT TO MONTAGNE PELEE, IN 1902. 
507 
wind, when its upward energy is spent it sinks to the ground, owing to its own 
Aveight, and then floAA^s down the hill. But if we consider the great eruption of 
the Soufriere and the enormous amount of hot dust AAdiich was then shot into the 
Amlleys, it seems quite unlikely that more than a small fraction of this AAms elevated 
to any great height above the crater. The bulk of the material must have swept 
down in a river of hot sand, though over it a lighter cloud Avould form, consisting 
mostly of hot gases densely chai'ged Avith dust. 
All Avitnesses, however, agree that the outburst appears in the form of a cloud, 
which rises to a certain height, and then flows over the surface of the ground. None 
have seen it in the form of a fluid molten laAm. Professor Hill describes the black 
cloud of May 26th as floating out horizontally in the air, but in this respect that 
eruption differs entirely from all the others. 
The mixture of dust and gases is so heavy that it courses doAvn the slopes like 
a torrent in a rNer, clinging to all the Amlley bottoms, ever availing itself of the 
steepest descents, and deflected by the projections and irregularities of the ground. 
That it does so we are convinced, not only from our oAvn observations of its effects 
on the Soufriere and in St. Pierre, but also from what we saAV on the night of 
July 9th. The black cloud poured doAvn the hill like a torrent of inky AAmter, 
except that it spread out, expanded, and its upper surface rose slowly in the air 
as it advanced. This explains also AApy the great mass of the ejecta at the Soufriere 
came doAAui the south side of the hill, where the crater rim is loAA^est, Avhile the north 
side was spared, as the Somma Avail protected it. 
That a mixture of gases and dust should behaA’e in this AA^ay is certainly remarkable, 
but similar phenomena are to be obserA^ed in connection Avith even the minor steam 
jets emitted by Montagne Pelee. Professor Lacroix and his colleagues describe : 
“ Waves of dense A^apour, heavy, dark coloured, often coppery, Avhich roll over the 
external taluses of the crater, and doAAm to the bottom of the fissures in the region of 
the Riviere Blanche. They are probably puffs of gas and steam very richly charged 
Avith ashes.” It is, moreover, not uncommon, Avhen a steam cloud larger than usual 
has been ejected, to see its loAver part dark, heaAy, and laden Avith dust, rolling oA^er 
the surface of the ground, Avhile its summit of pure white steam steadily mounts into 
the air. 
In considering this property of perfect fluidity, Avhich the black cloud possesses, Ave 
must remember its origin. Within the crater it AA^as a molten magma, in Avhich a 
considerable number of small crystals floated in a liquid Avhich contained enormous 
quantities of occluded steam. As it rose in the throat of the volcano the relief of 
pressure alloAA^'ed the gases to expand, and to free themselves from the liquid in which 
they were held. Sooner or later the cohesion of the licjuid AA^as overcome, and from 
a spongy froth the mass changed to a cloud of particles, mostly solid, but, perhaps, in 
* MjM. a. Lacroix, Roller de lTsle & Giraud, “ Sur rRruption de la IMartiiiique,” ‘ Comptes 
Rendus,’ \'ol. cxxxal, p. 380. See also vol. cxxxv., p. 426. 
3 T 2 
