TJIE MEDITERRANEAN NATURALIST 
mation of Mount Nuovo, or the same thing at 
Torre del Greco in the Yesuvian eruption of 1861. 
The actual rending and enlargement of the fissure 
will give rise to a series of vibrations of small 
amplitude, such as are first registered in an earth- 
quake (1). These will be immediately followed by 
the sudden arrest of expanding matter coming in 
contact with the walls of the fissure, which space 
it injects immediately. The effect is well imitated 
by allowing steam to escape from a boiler under 
high pressure, and suddenly closing the opening. 
Other examples are the sudden injection by water 
of a blind and collapsed hose, or the rapid closing 
of a tap from which was flowing a stream of water 
under pressure, conducted through a pipe of some 
length. This impact of the fluid matter against 
the solid fissure walls is followed by a series of 
diminishing oscillations or throbs This group of 
disturbances no doubt constitute the more power- 
ful or destructive portion of the earthquake, and 
the character of these vibrations, which we should 
deduce on the above theoretical grounds, comple- 
tely coincide with earthquake registration. 
The extension of the fissure may have been suf- 
ficient to allow of the formation of steam, which 
may collect together throughout the pasty mass as 
bubbles; and, should solidification soon follow, the 
resulting dyke-metal would present a vesicular or 
amygdaloidal structure. On the other hand, the 
expansion may only have taken place to such a 
point, that no conversion of liquid into gas has 
taken place, and as a result we should look for, in 
case of solidification, a dyke presenting no signs of 
vesicularity. The finding of a dyke-metal, in 
which no vesicularity is manifest, is no proof that 
at some time it may not have had such; for, were 
cooling not to follow soon on vesicularization, the 
renewed gradually increasing pressure would again 
compel the steam to redissolve in the magma. 
These facts probably account for the rarity of a 
vesicular state of granite, though even this is 
sometimes known to occur as in the island of Mull, 
and that of the plateau of the Palais du Roi, 
Lozere, described by Lecoq (1). 
Under the two former circumstances we should 
expect the first to end in solidification moi’e often 
than the second; for, by the conversion of the 
(I) •/. A. E winy, Earthquake Measurement, Mem. 
Sci. Depart. Univ. Tokio. x No. 0,p. 54, & following. 
213 
dissolved water into steam, a very much larger 
amount of heat would be used up, proportionally 
of course to the amount of conversion that took 
place. 
By the progressive extension of the fissure a lar- 
ger area of igneous rock surface will be exposed to 
the conditions which have been described, so that 
the tendency will be towards the more rapid ab- 
sorbtion of water, and consequent crisis between 
tension and resistance. Besides, from the larger 
amount of expansive matter capable of acting, the 
effects will be more violent each consecutive time. 
The factsare borne out in such examples as Jorul- 
loand Monte Nuovo, and are now probably in pro- 
gress under Casamicciola, in the island of Ischia. 
In such examples we find, that for a long period 
earthquakes occur at distant intervals, but that 
these have a gradual tendency to follow each other 
progressively more often, and often increasing in 
destructiveness at one spot, although the area 
affected may proportionally become more concen- 
trated. At last the frequency may become so 
great that the intervals are almost imperceptible 
until the fissure reaches the surface, and the 
igneous matter finds a vent for its expansion. 
(to be continued ) 
The Great African Cataract. 
A late visitor to the great Victoria Falls of the 
upper Zambesi River, which were first described 
by Livingstone, states that their grandeur is un- 
seen and for that reason it is impossible to com- 
pare them with Niagara. The immense river, 
about a mile wide at this place, suddenly con- 
tracts and disappears, apparently into the bowels 
of the earth, failing from a height estimated at 
about 400 feet into a gorge narrowing to about 500 
feet. A column of spray rises at least 300 feet 
above the level of the river, and may be seen 
seven miles away, ’.vhile the roar can be heard 
for many miles. But in no place can the bottom 
of the gorge be seen, and in only one place 
was it possible to survey as much as 600 feet 
in width of the falls at one time. 
(1) Les epoqv.es geologiqucs de V Auvergne, 
/., page 465. 
