88 



KNOWLEDGE 



[May 1, 1891. 



bably be aware, many of the lunar volcanic craters also 

 possess these little minor cones, which are well seen in 

 some of the larger photographs of the moon's surface. A 

 number of cracks, or fissures, radiating from the central 

 orifice, intersect the volcano. These get filled with lava 

 welling up from below, and from what are called " dykes," 

 which may be regarded as so many sheets of igneous rock 

 — basalt or felsite, as the case may be — that have, while 

 in a molten condition, forced their way in among the 

 layers of ash and lava. The word " ash " is used by 

 geologists in a special sense; and volcanic ash is not, as 

 might be supposed, a deposit of cinders, but mostly of 

 dust of various degrees of fineness ; and sometimes it is 

 very fine indeed. It is synonymous with the word 

 "tuff." Pieces of pumice-stone may be embedded in a 

 volcanic tuff, but they only form a small part. How 

 these volcanic tuffs are formed we shall explain pre- 

 sently. 



Dykes strengthen the mountain and tend to hold it 

 together when violently shaken during an eruption. But 

 notwithstanding, it sometimes happens that the whole 

 structure is blown to pieces by some unusually violent out- 

 burst. 



The shape and steepness of a volcano vary with the 

 nature of the materials ejected. The finer tije volcanic 

 tuff' the steeper and more conical is the mountain. The 

 formation of a volcano may not be inaptly illustrated by 

 the little cone of sand formed in an hour-glass as the 

 sand-grains fall. The latter settle down to a certain 

 slope, or angle, at which they can remain in their place. 

 This is known as the " angle of repose." When the 

 materials are coarse the angle is less. When they are 

 fine the angle is greater. The district of Auvergne in 

 France contains a number of very interesting extinct 

 volcanoes, some of which were formed principally of a 

 thick and viscous lava which slowly welled up from below, 

 and in so doing formed round and dome-shaped little hills 

 such as the " Puy de Dome." Vesuvius, Teneriffc, .Jorullo 

 in Mexico, and Cotopaxi in the Andes, are examples of 

 steep volcanoes built up principally of volcanic tuff. 

 Others, more irregular in shape, such as Kilauea in the 

 Sandwich Islands, are largely built uj) by successive lava- 

 Hows. Little minor cones are frequently developed on the 

 fiauks of a volcano, which during eruptions give rise to 

 small outbursts on their own account. They are easily 

 accounted for by the dykes wliich we mentioned just now ; 

 for when the molten rock forces its way through the 

 fissures, it sometimes finds an outlet at the surface, and, 

 being full of steam, as soda-water is fidl of gas, it gives 

 rise to an eruption. The central orifice, with its molten 

 lava, is, as it were, a great dyke which has reached the 

 surface and so succeeded in producing an eruption. The 

 opening of a soda-water bottle not infrequently illustrates 

 a volcanic eruption ; for when the pent-up carbonic acid 

 cannot escape fast enough it forces out some of the water, 

 even when the bottle is held upright. 



Lastly, every volcano is built up on a platform of 

 stratified rocks, or strata, laid down in the usual way 

 under water, and at some period subsequent to their 

 formation molten matter came up from below, and found 

 its way through them to the old land-surface which 

 they formed. Earthquake shocks preceding the first 

 eruption probably cracked up the strata, and so facilitated 

 the uprise of the lava with its imprisoned steam. 



The main point which we wish to emphasize is that 

 volcanoes are never formed by upheaval. They must not 

 be regarded as blisters due to the swelling or upheaval of 

 strata, but, as we have endeavoured to explain, they are 

 gradually built up from below, and may be compared to 



rubbish heaps, which grow by gradual accumulation. But 

 in the case of volcanoes, the rubbish comes from below. 

 It is not necessary to suppose that the subterranean reser- 

 voirs from which the molten rock is supplied, exist at any 

 very great depth below the original land-surface on which 

 the volcano grows up. Indeed, the eddence we at present 

 possess, from the denuded areas of volcanic action, goes 

 to show that this is not the case. 



The old "upheaval theory" of the formation of vol- 

 canoes, once advocated by certain geologists, instead of 

 being based upon actual evidence, or reasoning from 

 facts, as modern scientific theories are, was a mere guess. 

 Moreover, if the explanation we have given should not be 

 sufficiently convincing, thei-e is the proof furnished by the 

 case of a small volcano near Vesuvius, whose formation 

 was actually witnessed. It is called Monte Nuovo, or the 

 New Mountain. This mountain is a little tufl'-cone, 430 

 feet high, on the bank of Lake Avernus, with a crater 

 more than a mile and a half wide at the base. It was 

 mostly formed in a single night, in the year 1538 A.n. 

 We have two accounts of the eruption to which it owes its 

 existence, and each writer says distinctly that the moun- 

 tain was formed by the falling of stones and ashes. 



One witness says : " Stones and ashes were thrown up 

 with a noise like the discharge of great artillery, in quan- 

 tities which seemed as if they would cover the whole 

 earth ; and in four days their fall had formed a mountain 

 in the valley between Monte Barbara and Lake Averno of 

 not less than three miles in circumference, and almost as 

 high as Monte Barbaro itself — a thing incredible to those 

 who have not seen it, that in so short a time so consider- 

 able a mountain should have been formed." Another 

 says : " Some of the stones were larger than an ox. Tlie 

 mud [ashes mixed with water] was at first very liquid, 

 then less so, and in such quantities that, with the help of 

 the afore-mentioned stones, a moimtain was raised, 1 ,000 

 paces in height." These accounts are important as 

 showing how, in a much longer time, a big volcano may 

 bo built up. They are examples, or little epitomes, of 

 slow and vast processes which Nature vouchsafes to us, 

 and which enable us to comprehend her actions when they 

 are on a larger scale. 



We must now consider briefly the second question. The 

 following are the chief phenomena of a great eruption : — • 

 Its advent is heralded by earthquakes, afl'ecting the moun- 

 tain and the whole country round ; loud subterranean 

 explosions are heard, resembling the fire of distant artil- 

 lery. The vibrations are chiefly transmitted through the 

 ground ; tue mountain seems convulsed by internal throes 

 due, no doubt, to the efforts of imprisoned vapours and 

 liquid rock to find an opening. These idications are 

 accompanied by the drying up of wells and disappearance 

 of springs, since the water finds its way down new cracks 

 resulting from the explosions. When at last an opening 

 has been effected, the eruption begins, generally with one 

 tremendous burst, shaking the whole mountain down to 

 its foundations. Frequent explosions follow with great 

 rapidity and increasing violence, generally from the crater. 

 These are indicated by the globular masses of steam which 

 are to be seen rising up in a tall column like that which 

 issues from the funnel of a locomotive. The elastic gases 

 in their violent ascent hurl up into the air a great deal of 

 solid rock from the sides of the crater, after first blowing 

 out the stones which previously stopped up the orifice. 



Blocks of stone falling down meet with others coming 

 up, and so a tremendous pounding action takes place, the 

 result of which is that great quantities of volcanic dust 

 are produced, generally of extreme fineness. Winds and 

 ocean currents transport these light materials for long 



