318 The American Geologist. November, 1905 
prin, who believes that the spine, or tower, consists of an ancient 
core or plug remaining since the last eruption, now dislodged and 
thrust upward by renewal of volcanic activity. Lacroix, however, 
in his resume et conclusions, (p. 643) details more specifically his 
theory of the origination of the celebrated spine. Thus: 
"The mechanism of the production of domes, which are com- 
posed principally of rocks but little fusible (rhyolytes, trachytes, 
andesytes, phonolytes) in so many regions, remained to the pres- 
ent very obscure, no direct observation having as yet permitted 
the following of the steps of growth of this kind of volcanic moun- 
tain. We know now, by, a concrete example, how such a dome is 
produced. The molten magma, reaching the surface, by 
means of some fissure, forms there a mass which is quickly 
surrounded by a solid shell. This carapace not only pro- 
tects the interior against too rapid congelation but through the 
fissures formed in its surface by contraction exudes a frothy lava 
which adds, according to the internal pressure, intermittently new 
quantities of molten matter to the exterior. Thus it increases in 
hight and in size, a constant rocky mass bristling with asperities, 
bounded by abrupt walls which rise in the midst of the talus of 
fallen pieces which are constantly increased by superficial crumb- 
ling. Materials ejected by the paroxysms of activity play a very 
insignificant role and often take no part in the constitution of such 
a dome. 
"The dome thus constructed is not pierced by any yawning 
opening, permanent as a crater, but the paroxysmal explosions 
produce in it temporary openings which are again rapidly sealed. 
When the external shell becomes so firm that it cannot longer yield 
to expansion in all directions the internal pressure is locally effec- 
tive at a limited number of points, producing then the extrusion of 
solid rocky masses which may rise as if drawn through framed 
orifices, forming needles which may reach the hight of several hun- 
dred feet, in the manner of that whose birth and growth I wit- 
nessed and examined day by day during several months. In the 
course of a single eruption the point of concentration of effective 
pressure may be displaced, forming thus successive spines, varying 
in form, dimensions and position." 
The destructive agent of the eruption, the nuees ardentes, aie 
regarded as the result of explosion issuing from the flanks of the 
dome, at Mont Pelee, but from the depth of the open crater at Saint 
Vincent. The explosion furnished an enormous mass of gas and 
water vapor, released rapidly on reaching the atmosphere, carrying 
along a considerable quantity of solid matter of all forms. Instead 
of being always projected vertically these clouds roll with great 
swiftness down the slopes of the mountain. In the case of feeble 
eruptions gravity has a determining influence upon their course, 
but in great paroxysms they are moved more rapidly, due to the 
accumulated action of the initial projection and of gravity acting 
