284 
This hypothesis, which accounted for the formation of volcanic 
cones and eraters by a process of upheaval, or, as it was termed, 
the “ crater of elevation,” is here alluded to, only because it for 
a long time was accepted by many eminent men of science, until 
the subsequent researches, especially of Mr. Serope and Sir 
Charles Lyell, demonstrated conclusively that it is not confirmed 
when their actual structure is studied in the field, and explained 
their true formation, by what is now termed the “rater of 
eruption” theory, 
If we imagine’a yolcanic cone cut through its centre, so as to 
present us with a section of its entire mass, it will be seen that 
the mineral matter of which it is composed possesses in itself a 
sort of arrangement in layers, which at first sight somewhat 
resembles beds of ordinary sedimentary origin broken through and 
tilted up towards the centre; a closer examination, however, 
shows that these layers were never at any time horizontal, but 
that, on the contrary, they had from the very first been deposited 
in the same inclined position in which they are now seen, and 
that they must have been formed subsequently, not previous to 
the opening of the crater itself, since they are entirely composed 
of matter thrown up from its orifice, 
The commencement of an eruption !s known in most cases by 
certain preliminary symptoms indicative of great internal disturb- 
ance, such as rumbling noises, and sounds as if of explosions 
below, which have been likened to subterranean thunder. The 
surface waters, springs and wells in the vicinity generally acquire 
an unusually high temperature, diminish in volume or disappear 
altogether, and repeated earthquake shocks more or less severe 
are felt, which eventually culminate in a grand conyulsion, by 
which the surface is rent asunder with fearful viclence, alluwing 
immense yolumes of preyiously pent up vapour and gases to rush 
forth frem the fissure with such impetnosity as to hurl high into 
the air huge fragments of the shattered rocks, along with vast 
quantities of molten laya, in so liquid a condition that during its 
ascent it is seen to be sp'ashed about in the air like water, and to 
become separated into particles of all sizes. Vast quantities of 
these particles, to which the name of yo!canic ash or dust has 
been applied, are instantaneously reduced to so fine a state of 
division, literally ‘*bluwn to atoms,” as to become converted into 
ana!most impalpable powdcr, capable of being carried away by 
the winds prevailing during an eruption to distances of even 
hundreds of miles from the orifice from which they had been 
ejected, and ultimately settle down cn the land or in the sea to 
form deposits, whose nature woud ofien be a puzzle to 
gevlogists, did not the microscope at once reveal thcir true 
mineral chayactcr and volcanic origin, Other particles less finely 
divided become granulated and fall down from the air in the 
shaye of small back grains, known as voleanic sand ; whi'st still 
larger portions, owing to the Lubbles of vapour or gas entangled 
in their substance, descend is black porous or spongy stones, from 
the size of a pea to that of one’s head, or larger ; and have re- 
ceived the names of Lapilli, scurlae, or volcanic cinders, from 
their presenting much the a; pearance of an ordinary cinder from 
a coal fire. Although the scorize thiown up by volcanoes are in 
m: jor part of a dark colour, there are als» others (called trachy tic) 
much lighter both in coluur and weight, which are usually more 
common at the commencement of an eruption, the ordinary 
pumice stone which is imported in large quantities from the 
volcanoes in the Lipari Islands, for the use of the painters, &¢,, 
is an example of this variety familiar to you all, A peculiar form 
of lava is preduced by the currents of wind blowing oyer the 
surface of the molten matter in the crater, catching up portions 
of it and drawing them out into long slender filaments like hair 
or spun glass of all shades of black, brown, or yellow. In the 
Sandwich Islands, where this variety is very abundant, it is 
called Pele’s hair, from the name of one of their ancient goddesses. 
In the intervals of an eruption, or after the greatest force of the 
rush has spent itself, the yapouis often rise through the molten 
lava in the crater, in smart puffs which earry up with them 
portions of the fluid lava high into the air, whence they descend 
consolidated as spheres or somewhat elongated bodies consisting 
of an external shell of solid lava, hollow or only filled with 
vapour or gas inthe centre. From their resemblance to military 
projectiles, these bodies, which vary from tlie size of an orange to 
that of a pumpkin, have received the name of yolcanic bombs. 
The mineral matter thrown up jnto the air from a yolcanic vent 
necessarily descends again by virtue of its own weight, a portion 
dreps back into the crater, but the major part failing beyond it, 
accumulates around its brink to form a mound, which, since the 
larger and heavier pieces are not projected to so great a distance 
NATURE 
[Aug. 4, 1870 
as the others, keeps, as it increases in size, raising itself more 
rapidly in height nearest around the vent, then farther off, and 
thus builds up a hollow cone, the throat or chimney of which is 
kept open, at least during the continuance of an eruption, by the 
upward rush of the gases and vapours forced through it by the 
pressure below. Theaction of the heat being of course much more 
intense in the chimney or throat of the crater, now causes, the 
at first comparatively loose materials which formed its walls, to 
soften and cement themselves together on the inside into a sort 
of compact stony tube of communication with the lower regions, 
much more solid and resistant than the rest of the mass of which 
as before deseribed the entire cone had been built up. Once this 
is the case, the molten lava, forced up by the gaseous pressure 
below, frequently ascends into the crater itself, and overflowing 
its brim, pours down the outside of the cone, just like water 
when placed aver too rapid a fire is seen to boil over the edge of 
the pot in which it is heated, These occasional overflows of 
lava explain how in tke section of a volcanic cone layers of mere 
compact lava are so frequently seen alternating with those of the 
porous scoria and volcanic sand before deseribed. In more rare 
instances, as for example in the eruption of Mauna Loa, in the 
Sandwich Islands, in February 1859, the lava is ejected in so 
wonderfully liquid a condition, and in such enormous yolumes, 
as to present the appearance of a red-hot fountain ; the jet of 
molten lava thrown up from the crater on that occasion is de- 
scribed as about 250 feet in diameter, and as rising some 500 fect 
above the level of the brim of the erater itself, Occasionally, 
during an eruption, the rim of the crater, unable to support the 
weight of the molten lava which fills it, gives way at its weakcst 
point, the lava bursting out and earrying away one side of the 
cone itself; at other times the lava, after having risen some height 
up the crater, finds out a point of weakn:ss and breaks through, 
discharging itself by a fissure some way up the side of the cone, 
as was the case with the volcano of Sajama, in Bolivia, in 1850, 
and with Etna in 1865. In many eruptions the lava does not 
ascend at all into the crater, but hreaks out at the very hase of 
the cone, or eyen at some considerable distance from it, through 
a subterranean passage, This took place in the eruj tion of 
Kilauea, in the Sandwich Islands, in June 1840, when tle lava 
first showed itself at the surface at Arare, some six miles east- 
ward of the crater which supplied it. In fact, most vo!canoes 
will, upon examination, be found at one or other per od in their 
history to have presented examples of more than one, if not cf 
all, these d ffcrent modes of discharging their molten produets, 
The eruption of Etnain 1865, which I witnessed, did not pro- 
ceed from the st:mmit or main erater, but broke out on the side 
of the mountain, about 5,000 feet above the level of the sea. 
Along the fissure or rent formed by this convulsion, no less than 
seyen distinct cones rose up at interyals, building themselves up 
very rapidly from the enormous quantities of scorize which were 
thrown up from their rents ; as they became larger the bases of 
several of these cones extended until they item and so formed 
a range of hills, the summits of which in but a few weeks reached 
the height of several hundred fect, and entirely changed tLe 
charactcr of the scenery of this part of the island, The’ four 
lowest cones were the most active, but from none of their craters 
was there any overflow of lava, which, however, poured out from 
the very base of the cones, forming a fiery river apparently about 
three miles aeross, which destroyed all before it, eutting through 
a large pine forest, and at one place leaping like a cascade uf 
liquid fire over a precipice some 150 feet in height, 
The formation of a new or re-opening of an old volcanic vent 
is usually accompanied by a terrific explosion, often to be heard 
at immense distances ; thus, in 1812, the outburst of the velearo 
of San Vineent was heard in the north of South America some 
700 miles distant, The enormous force developed by the rush 
of gases and vapours from the fissure may be imagined when it 
is known that in the eruption of Mount Ararat, in 1840, huge 
masses of rock weighing as much as 25 tens were thrown out of 
the crater ; Cotopaxi is said to have even hurled a 200-ton rock 
to a distance of nine miles ; whilst the volcano of Antuco, in 
Chili, in 1828, sent stones flying to a distance of 36 miles, 
The issue of gaseous matter from the ¢rater of a volcano is 
often described as a column of flame ; this is incorrect, for 
although possibly a litthe burning hydrogen or sulphuretted 
hydrogen might be present, especially on the outer edye of the 
column, the appearance of a column or fountain of flame is in 
reality due to the gaseous matter of which it consists being 
illuminated by the fragments of red-hot rock and molten lava 
thrown up along with it (like sparks in fireworks), assisted by the 
