THE MODERN VOLCANOES OF ICELAND AS ILLUSTRATIVE OF THE
TERTIARY VOLCANIC HISTORY OF NORTH-WESTERN EUROPE
From the facts stated in the foregoing chapters concerning the
structure of the basalt-plateaux of North-Western Europe, it is evident
that in none of these areas have the eruptions come from one great
central volcano like Etna or Vesuvius. On the contrary, in every
instance there is abundant evidence that the basalt has flowed from
many scattered points of eruption. The uniformity of the lava-sheets in
petrographical characters, their continuity when viewed in mass, their
general horizontality, and their constant thinning away in different
directions, show that the eruptive vents must have been distributed
over the whole plateau-areas.
The conditions under which such eruptions took place can be most
readily understood by a comparison of the phenomena with those
observable in modern volcanic tracts where extensive outflows of lava
have taken place without the existence of any great central cones. Of
these regions the most instructive is undoubtedly to be found among the
recent lava-deserts of Iceland. There the parallels to the structures
described from the British and Faroe plateaux are so numerous and so
close that an account of the Icelandic region may appropriately be
inserted here.
The evidence furnished by Iceland is of special value in our present
enquiry, inasmuch as that island, besides its modern eruptions,
includes vast basaltic plateaux of Tertiary age. These areas of nearly
level sheets of basalt belong to the same geological period as those of
the British and Faroe Islands, and display the same internal structure
and external features. But they have this distinguishing peculiarity
that the volcanic fires beneath them are not yet extinguished. They
have been broken through again and again in recent times by volcanic
eruptions which have repeated many of the characteristics of their
Tertiary predecessors. The old and the new development of the same
volcanic type are thus visible side by side.
The Tertiary volcanic series of Iceland reaches a thickness of
upwards of 3000 metres, or nearly 10,000 English feet, but as its
base is nowhere seen, it may be still thicker. Its successive sheets,
piled over each other in parallel layers, form terraced hills and
bold escarpments along the coast, whence they slope gently inland.
The plateau, as in the Faroe Islands and in Scotland, has been
extensively eroded, and has been trenched by many long valleys and
fjords The composition of the basalts remains remarkably uniform over
the island. The lava sheets are often decomposing, amygdaloidal, and
filled with zeolites; while higher in the series compact basalts
abound, the uppermost fine-grained sheets being especially constant
in structure and composition. Numerous dykes traverse the plateau,
and some of them cut even its highest members. The parallel with the
geological structure of the Inner Hebrides is continued in Iceland by
the appearance of intrusive masses of gabbro and granophyre, which
represent the deeper parts of the Tertiary volcanic series, while the
basalts were poured out at the surface. Thus, at Papafjord, the gabbro
rises into mountainous peaks and, like the similar rock in Mull and
Skye, is intersected by dykes of a coarse-grained granitoid liparite or
granophyre. Large dykes and ramifying veins of the same acid material,
often with a thoroughly granitic aspect, extend into the basalts.[267]
[Footnote 267: Mr. Thoroddsen, _Dansk. Geografisk Tidsskrift_, vol. xiii.]
A long series of eruptions has taken place in Iceland since the Glacial
Period. There were likewise pre-glacial eruptions. The glaciated
lava-streams are found underneath the modern lavas. So far indeed as is
known, no evidence exists of any important cessation of subterranean
activity there since Tertiary time.[268] The existing volcanic phenomena
may with probability be regarded as the survival of those which were
so widely manifested over the Icelandic area and the north-west
of Europe in the older Tertiary ages. A careful study of them may
therefore be expected to throw light on the history of the Tertiary
basaltic plateaux; while, on the other hand, the thorough dissection of
these plateaux by the denuding agencies will not improbably be found
to explain some parts of the subterranean mechanism of the modern
Icelandic volcanoes.
[Footnote 268: See Dr. Johnston-Lavis, _Scottish Geographical Magazine_,
1895, p. 442.]
In calling attention to some of the more obvious analogies which
may be traced between the modern and the ancient volcanoes, I am
more particularly indebted to the excellent memoirs of the resident
Icelandic geologist, Mr. Th. Thoroddsen, who has examined so large a
part of the island.[269] The account given by Mr. A. Holland of the Laki
craters has likewise been of much service to me.[270] Among other recent
observers I may cite Dr. Tempest Anderson,[271] who has made himself
familiar with extensive tracts of Iceland. He was accompanied one year
by Dr. Johnston-Lavis, who has published a narrative of the journey.[272]
[Footnote 269: See In particular his paper on the volcanoes of north-east
Iceland (_Bihang till. k. Svensk. Vet. Akad. Handl._ xiv. ii. No. 5,
1888) and that on Snaefell and Faxebugt in the south-west of the island
(_op. cit._ xvii. ii. No. 2, 1891); also papers in _Dansk. Geografisk
Tidsskrift_, vols. xii. xiii. (1893-95); _Verhand. Gesellsch. Erdkunde
zu Berlin_, 1894-95.]
[Footnote 270: "Lakis Kratere og Lavaströmme, Universitætsprogram,"
Christiania, 1885. See Mr. Thoroddsen's remarks on this paper,
_Verhand. Gesell. Erdkunde_, 1894, p. 289.]
[Footnote 271: _Brit. Assoc. Rep._ 1894, p. 650.]
[Footnote 272: Dr. Johnston-Lavis, _Scottish Geographical Magazine_,
September 1895.]
It is a mistake to suppose that the Icelandic volcanoes are generally
built on the plan of such mountains as Vesuvius or Etna. Mr. Thoroddsen
can evidently hardly repress his impatience to find these two Italian
cones cited in almost every handbook of geology as types of modern
volcanoes and their operations. The regular volcanic cone, composed
of alternations of lavas and tuffs, plays a very subordinate part in
Iceland.
[Illustration: Fig. 292.--Fissure (gjá) in a lava-field, Iceland. (From
a photograph by Dr. Tempest Anderson.)]
The fundamental feature in the Icelandic eruptions is the production
of fissures which reach the surface and discharge streams of lava
from many points. Two systems of such fissures appear to be specially
marked, one in southern Iceland running from south-west to north-east,
the other, in the north part of the island, stretching from south
to north.[273] Hekla and Laki belong to the former. The dislocations
have often followed the boundaries of the "horsts," or solid blocks
of country which have withstood terrestrial displacement. The vast
outbreaks of Odádahraun and Myvatn have almost all issued from fissures
of that nature.
[Footnote 273: In the Snaefell promontory they run nearly east and west.
Mr. Thoroddsen, _Bihang. Svensk. Akad._ xvii. (ii.) No. 2, p. 91.]
The violent eruption of 1875 in Askja found its exit at the
intersection of two lines of fissures. Many large fissures were opened
on the surface in a nearly north and south direction, which could be
followed for 80 kilometres or nearly 50 English miles. Some of them
became the theatre of intense volcanic activity.[274]
[Footnote 274: Mr. Thoroddsen, _op. cit._ xiv. ii. No. 5, p. 63.]
Many lines of fissure are traceable at the surface as clefts or "gjás,"
that run nearly straight for long distances, with a width of one to
three yards, and sometimes of unknown depth.[275] The most stupendous
example of the structure yet discovered is probably the Eldgjá found by
Dr. Thoroddsen in the year 1893, below the Mýrdalsjökull. This gigantic
chasm has a length of 30 kilometres (more than 18 English miles), and
a depth of 130 to 200 metres (426 to 656 feet). Over its vertical walls
lofty waterfalls plunge from the crest to the bottom.
[Footnote 275: On the various modes of origin of these chasms, see Dr.
Tempest Anderson, _Brit. Assoc. Rep._ p. 650. The gjá shown in Fig. 292
is not an eruptive fissure. For this and the following illustration
I am indebted to the kindness of Dr. Tempest Anderson, who himself
photographed the scenes.]
Occasionally a fissure has not been continuously opened to the surface.
An interesting example of such intermittent chasms is supplied by the
great rent which gave forth the enormous volume of lava in 1783. The
mountain of Laki, composed of palagonite tuff, stands on the line of
this dislocation, but has not been entirely ruptured. The fissure has
closed up beneath the mountain, a short distance above the bottom of
the slope, as is shown by the position of a couple of small craters.[276]
[Footnote 276: Mr. A. Helland, _op. cit._ p. 25.]
Some fissures have remained mere open chasms without any discharge of
volcanic material; others have served as passages for the escape of
lava and the ejection of loose slags and cinders.[277]
[Footnote 277: Mr. Thoroddsen has observed that in the Reykjanes
peninsula in the south-west of Iceland, by the subsidence of one side
of a fissure, a row of four craters has been cut through, leaving their
segments perched upon the upper side. _Globus._ vol. lxix. No. 5.]
[Illustration: Fig. 293.--Cones on the great Laki fissure, Iceland.
(From a photograph by Dr. Tempest Anderson.)]
In some instances, according to Mr. Thoroddsen, lava wells out from
the whole length of a fissure without giving rise to the formation of
cones, the molten material issuing either from one or from both sides
and flowing out tranquilly. Thus from three points on the great Eldgjá
chasm lava spread out quietly without giving rise to any craters,
though at the southern prolongation of the fissure, where it becomes
narrower, a row of low slag-cones was formed. The three lava-streams
flooded the low ground over an area of 693 square kilometres, or 270
English square miles. In the great majority of cases, however, the
lava as it ascends in the fissure gives rise to long ramparts of slags
and blocks of lava piled up on either side, or to a row of cones along
the line of the open chasm. Thus, on the Laki fissure, which runs for
about 20 miles in a north-east direction, the cones amount to some
hundreds in number.
[Illustration: Fig. 293_a_.--Plan of small craters along the line of
great Laki fissure, Iceland. (After Mr. Helland, reduced.)]
The cones consist generally of slags, cinders, and blocks of lava.
They are on the whole not quite circular but oblong, their major axis
coinciding with the line of the chasm on which they have been piled up,
as along the marvellous line of the Laki fissure. In many places they
are exceedingly irregular in form, changes in the direction of outflow
of lava or of escape of steam having caused the cones partially to
efface each other.
As regards their size, the cones present a wide range. Some of them
are only a few yards in diameter, others several hundred yards.
Generally they are comparatively low mounds. On a fissure hardly 30
feet long, Mr. Thoroddsen found a row of twelve small cones built
exactly like those of largest size, but with craters less than three
feet in diameter. On the Laki fissure some are only a couple of yards
high; the majority are much less than 50 yards in height, and hardly
one is as much as 100 yards.[278] And yet these little monticules, as
Mr. Helland remarks, represent the pipes from which milliards of
cubic metres of lava have issued. While other European volcanoes form
conspicuous features in the landscape, the Icelandic volcanoes of the
Laki district, from which the vastest floods of lava have issued in
modern times, are so low that they might escape notice unless they were
actually sought for.[279]
[Footnote 278: Mr. Thoroddsen, however, states that there are about 100
ranging between 20 and 100 metres in height.]
[Footnote 279: _Op. cit._ p. 27.]
As they have generally arisen along lines of fissure, the cones are,
for the most part, grouped in rows. The hundreds of cones that mark the
line of the Laki fissure present an extraordinary picture of volcanic
energy of this type. In other instances the cones occur in groups,
though this distribution may have arisen from the irregular uprise
of scattered vents along a series of parallel fissures. Thus to the
north-east of Laki a series of old cones entirely surrounded by the
lavas of 1783 lie in groups, the most northerly of which consists of
about 100 exceedingly small craters that have sent out streams of lava
towards the N.N.E.[280]
[Footnote 280: _Op. cit._ p. 25. The great lava-fields of Iceland are
likewise dotted over with secondary craters or "hornitos" which have
no direct connection with the magma below, but arise from local causes
affecting the outflowing lava. They are grouped in hundreds over a
small space.]
It would appear from Mr. Helland's observations that the same fissure
has sometimes been made use of at more than one period of eruption. He
describes some old craters on the line of the Laki fissure, which had
been active long before the outbreak of 1783.[281]
[Footnote 281: _Op. cit._ p. 26.]
When the lava issues from fissures it is in such a condition of
plasticity that it can be drawn out into threads and spun into
ropes. When the slope over which it flows is steep it often splits
up into blocks on the surface. Where the ground is flat the lava
spreads out uniformly on all sides, forming wide plains as level as
a floor. Thus the vast lava-desert of Odádahraun covers a plain 3640
square kilometres in area, or, if the small-lava-streams north from
Vatnajökull be included, 4390 square kilometres. This vast flood of
lava (about 1700 English square miles in extent) would, according to
Mr. Thoroddsen, cover Denmark to a depth of 16 feet. The whole of this
enormous discharge has been given forth from more than twenty vents
situated for the most part on parallel fissures.
Not less striking is the picture of fissure-eruption to be met with at
Laki--the scene of the great lava-floods of 1783. "Conceive now," says
Mr. Helland, "these hundreds of craters, or, as they are called by the
Icelanders, 'borge,' lying one behind another in a long row; every one
of them having sent out two or more streams of lava, now to the one
side, now to the other. Understand further that these streams merge
into each other, so as to flow wholly round the cones and form fields
of lava miles in width, which, like vast frozen floods, flow down to
the country districts, and you may form some idea of this remarkable
region."[282]
[Footnote 282: _Op. cit._ p. 24. Mr. Helland allows an average thickness
of 30 metres for the mass of lava which issued in two streams, one
80 kilometres (nearly 50 miles), the other 45 kilometres (about 28
miles) long. He estimates the total volume of lava discharged in
the 1783 eruption at 27 milliards of cubic metres, equal to a block
10 kilometres (6 miles 376 yards) long, 5 kilometres (3 miles 188
yards) broad, and 540 metres (1771 feet) high; _op. cit._ p. 31. Mr.
Thoroddsen remarks that the older estimates of the volume of lava
discharged by this eruption have been greatly exaggerated. He puts
the area covered by lava at 565 square kilometres and the contents at
12-1/3 cubic kilometres. Verhand. _Gesell. Erdkunde Berlin_, 1894, p.
296.]
The basaltic lavas have issued in a comparatively liquid state, form
thin sheets and reach to great distances. The western stream from the
Laki eruption of 1783 flowed for upwards of 40 miles; a prehistoric
lava from Trölladyngjá in Odádahraun flowed for more than 60 miles.
In the course of time the successive streams of lava poured out upon
one of these wide volcanic plains gradually increase the height of
the ground, while preserving its generally level aspect. The loose
slag-cones of earlier eruptions are effaced or swallowed up, as one
lava-stream follows another. Eventually, when, by the operation of
running water or by fissure and subsidence, transverse sections are
cut through these lava-sheets, the observer can generally notice only
horizontal beds of lava piled one above another, including the dykes
connected with them and intercalated masses of loose slag, that remain
as relics of the old craters.
In some places the lava has gradually built up enormous domes, like
those of Hawaii, having a gentle inclination in every direction, as may
be seen especially in the district between Floderne Skjalfanafljot and
Jökulsà Most of the large volcanic piles of North Iceland are of this
nature. The highest of them are 1209 and 1491 metres high by from 6
to 15 kilometres in diameter. The elliptical crater of the highest of
these eminences measures 1100 by 380 metres.[283]
[Footnote 283: Mr. Thoroddsen, _op. cit._ xiv. ii. No. 5, pp. 10, 23.]
Large conical volcanoes of the Vesuvian type built up of alternating
lavas and tuffs are not common in Iceland, but some occur and rise
into lofty glacier-covered mountains, such as Öræfajökull (6241 feet),
Eyjafjallajökull (5432), and Snaefellsjökull (4577). Hekla (4961) also
is similarly composed of sheets of lava and tuffs, but has not been
built as a cone. It forms an oblong ridge which has been fissured
in the direction of its length and bears a row of craters along the
fissure.[284]
[Footnote 284: Mr. Thoroddsen, _Dansk. Geograf. Tidsskrift_, vol. xiii.]
Explosion-craters likewise occur among the modern volcanic phenomena
of Iceland. One of these was formed by a violent explosion at Askja
on 29th March 1875. It has a diameter of only about 280 feet, yet so
great was the vigour of the outburst that pumiceous stones were spread
over an area of more than 100 Danish (468 English) square miles, and
the dust was carried as far as Norway and Sweden. Nine years later Mr.
Thoroddsen found the bottom of this crater filled with bluish-green
boiling mud, which will probably in the end become a sheet of still
water. The borders of these Icelandic explosion-craters seem to be very
little higher than the ground around them. Most of the ejected material
is expelled with such force and to such a distance that only a small
fraction of it falls down around the orifice of eruption.[285]
[Footnote 285: Mr. Thoroddsen, _op. cit._]
There is still another feature of the Icelandic volcanic regions which
may be cited as an interesting parallel to the sequence of eruptive
discharges among the Inner Hebrides. While the lavas are as a rule
more or less basic--many of them being true basalts--they have been
at different times pierced by much more acid liparites and obsidians.
Examples of these rocks of post-Glacial age have recently been traced
on the ground by Mr. Thoroddsen,[286] and their petrographical characters
have been studied by Mr. Bäckström.[287] The wide distribution of such
rocks all over the island, their occurrence in isolated bosses among
the more basic lavas, and their remarkable internal structures have
been noted by several observers.[288] The liparites and obsidians are
contrasted with the basalt by the colours and forms of their streams.
Some of them are so black as to look like heaps of coal, though their
surfaces pass into grey pumice. They have flowed out in a much less
liquid condition than the basalts, and have consequently formed short,
thick and irregular sheets. The liparites and basalts appear to have
been nearly contemporaneous. They certainly belong to the same volcanic
cycle and their vents lie close to each other. Though none of the
acid eruptions are known to have occurred in modern times, some of the
liparites are crusted with sulphur and from the connected fissures
steam still rises.
[Footnote 286: _Geol. Fören. Stockholm Förhandl._ xiii. (1891), p. 609;
_Bihang. Svensk. Vet. Akad. Handl._ xvii. ii. p. 21 (1891); _Dansk.
Geograf. Tidsskrift_, xiii. (1895).]
[Footnote 287: _Geol. Fören. Stockholm Förhandl._ xiii. (1891), p. 637.]
[Footnote 288: See in particular C. W. Schmidt, _Zeitsch. Deutsch. Geol.
Gesellsch._ xxxvii. (1885), p. 737.]
It will thus be seen how entirely the modern volcanic eruptions
of Iceland agree with the phenomena presented by our Tertiary
basalt-plateaux. It is, therefore, to the Icelandic type of
fissure-eruptions, and not to great central composite cones like
Vesuvius or Etna that we must look for the modern analogies that will
best serve as commentary and explanation for the latest chapter in the
long volcanic history of the British Isles.[289]
[Footnote 289: In his memoir of 1874, Professor Judd announced his
conclusion that there were formerly five great volcanoes amongst
the Western Isles, and that the lavas of the plateaux had issued
from these. He subsequently reiterated this view (_Quart. Journ.
Geol. Soc._ xlv., 1890, p. 187), and ridiculed the explanation of
fissure-eruptions. The evidence adduced by me in a paper published in
1896 (same journal, vol. lii. p. 331) and reprinted with additions
in this chapter, will, I trust, be regarded by geologists as having
finally settled this question.]
As a further but more ancient illustration of the type of volcanic
action which appears to have been prevalent during the formation of the
Tertiary volcanic plateaux of Britain, I may again refer to the vast
basalt-fields of Western America. The basalt of Idaho stretches out as
an apparently limitless plain. Along its northern boundary, this sea
of black lava runs up the valleys and round the promontories of the
older trachytic hills with almost the flatness of a sheet of water.
It has been deeply trenched, however, by the streams that wind across
it, and especially by the Snake River, which has cut out a gorge some
700 feet deep, on the walls of which the successive beds of basalt lie
horizontally one upon another, winding along the curving face of the
precipice exactly as those of Antrim and the Inner Hebrides do along
their sea-worn escarpments. Here and there, a low cinder-cone on the
surface of the plain marks the site of a late outflow. One is struck,
however, with the singular absence of tuffs and volcanic conglomerates.
The basalts appear to have flowed out stream after stream with few
fragmentary discharges.
These characteristic features of one distinctive type of volcanic
action have been repeated over a vast region, or rather a whole series
of regions, in Western America, the united area of which must equal
that of a considerable part of Europe. From Idaho, the basalt-fields
may be followed southwards interruptedly into Utah and Nevada, and
across the great plateau-country of the cañons into Arizona and New
Mexico, northwards into Montana, and westwards into Oregon. The tract
which has as yet been most carefully traversed and described is
probably that of the high plateaux of Utah and Arizona. Thus on the
Uinkaret plateau, which measures some 45 to 50 miles in length by 8 to
12 in breadth, a thick covering of basalt has been spread composed of
many successive flows. Between 160 and 170 separate cones have been
counted on this area, most of them quite small, mere low mounds of
scoriæ, though a few reach a height of 700 or 800 feet, with a diameter
of a mile. From three to seven or eight may be found in a row, as if
springing from a single line of fissure. But generally the grouping
is quite irregular.[290] My friend Captain C. E. Dutton, from whose
admirable memoir these details are quoted, remarks further that among
the Utah plateaux no trace of a cone is to be found at or near some of
the most recent basalt-fields, and that the most extensive outpours are
most frequently without cones. "The lavas," he adds, "appear to have
reached the surface and overflowed like water from a spring, spreading
out immediately and deluging a broad surface around the orifice."[291]
The deep gorges cut by the rivers through these thick accumulations of
horizontal or nearly horizontal basalts, have here and there revealed
parallel dykes that traverse the rocks, and in at least one case
have shown the dyke running for half a mile up a cliff and actually
communicating with a crater of scoriæ at the top.[292] Again, in New
Mexico, Captain Dutton noticed vast tracts of younger basalt, about
which "a striking fact is the entire absence of all distinguishable
traces of the vents from which they came. Some of them, however,
indicate unmistakably their sources in small depressed cones of very
flat profiles. No fragmental ejecta (scoriæ, lapilli, etc.) have been
found in connection with these young eruptions."[293] Such I believe
to have been the general conditions under which the basalts of the
Tertiary plateaux of the British Isles were also erupted.[294]
[Footnote 290: Captain C. E. Dutton, "Tertiary History of the Grand Cañon
District," _U.S. Geol. Survey_ (1882), p. 104.]
[Footnote 291: Captain C. E. Dutton, "Geology of the High Plateaux
of Utah," _U.S. Geol. Survey of the Rocky Mountain Region_ (1880),
pp. 198, 200. See also pp. 232, 234, 276 of the same Monograph for
additional examples.]
[Footnote 292: _Tertiary History of the Grand Cañon_, etc., p. 95.]
[Footnote 293: _Nature_, xxxi. (1884), p. 49.]
[Footnote 294: I may again refer to Hopkins's _Researches in Physical
Geology_, where the conditions of the problem here discussed have
been distinctly realized. Speaking of the ejection of lava from a
number of fissures, he remarks that the imperfect fluidity of the
melted material "would seem to require a number of points or lines
of ejection as a necessary condition." "If there were only a single
centre of eruption, a bed of such matter approximating to uniformity
of thickness, could only be produced on a surface of a conical form."
"Where no such tendency to this conical structure can be traced, it
would probably be in vain to look for any single centre of eruption.
On the supposition, too, of ejection through continued fissures, or
from a number of points, that minor unevenness of surface which must
probably have existed under all circumstances during the formation of
the earth's crust, would not necessarily destroy the continuity of a
comparatively thin extensive bed of the ejected matter, in the same
degree in which it would inevitably produce that effect in the case of
central ejection" (_Cambridge Phil. Trans._ vi. 1835, p. 71).]
Although we may be convinced, from their general structure and
relations, that the stratified lavas of these plateaux have been poured
out from fissures and not from great central cones, it must obviously
be difficult to obtain demonstrative evidence of this origin from any
single section. Of the thousands of dykes which traverse the British
plateaux and the ground around them, I am not aware of a single one
which can be actually seen to have ever communicated with the surface.
The very process of denudation which has revealed these dykes has at
the same time removed all trace of any former connection they may have
had with the surface. The only places where we may hopefully search
for the missing evidence are the fronts of the escarpments. On these
precipices dykes may sometimes be seen to end off at some particular
platform among the basalt-sheets, but I have never found a case which
could be confidently cited as an example of lava rising in a fissure
and spreading out as a superficial sheet. That this connection may
eventually be found when a more detailed survey is made of these great
sea-walls I fully anticipate.
In recently mapping the basalt-plateau of Strathaird in Skye, Mr.
Harker has made some interesting observations regarding the probable
connection of the dykes with the plateau basalts. He has noticed that
the flanks of Slat Bheinn, a portion of the plateau, are abundantly
traversed by dykes containing numerous enclosed pieces of gabbro,
while the basalt on the summit of the plateau is full of similar
fragments--an occurrence not observed elsewhere. It is conceivable that
the gabbro-bearing basalt-sheets are sills, but Mr. Harker has found no
proof that they are so, the evidence so far as it has been collected
being rather in favour of the view that these sheets are superficial
lavas, and that they have been supplied from the dyke-fissures.
Various considerations suffice to assure us that actual instances of
the outflow of the basalt from its parent fissures should be expected
to be exceptional. The absence or scarcity of beds of scoriæ among the
basalt-plateaux may be taken as an indication that the lava as a rule
flowed out without the formation of cinder-cones, and therefore that
these conspicuous monuments of the eruptive vents were probably always
rare in Britain. If the lava was poured out tranquilly from one or two
points along a fissure which were subsequently buried under floods of
similar lava issuing from other fissures, the chances that such points
of emission should be laid open along the front of any escarpment are
small. And, even when so exposed, it might be difficult to feel sure
that the dyke below was really the feeder of the basalt above, unless
the cliff were accessible and the rocks could be scrutinized foot by
foot. These elements of uncertainty are happily removed where the
volcanic energy has drilled well-marked funnels of discharge and left
them filled with the erupted materials, as will be narrated in the next
chapter.