Those who have been accustomed to regard volcanoes such as Vesuvius or
Etna as the one form of volcanic action, may be somewhat startled by
the statement that lavas have sometimes been poured forth from fissures
hundreds of miles in length, and have deluged vast tracts of country
beneath sheets of molten rock, compared with which the puny lava-fields
of Italy sink into insignificance. History, romance, and legend have
been so long associated with the group of volcanoes overlooking the
quiet Tyrrhenian Sea, that from the time when Pindar sung of the
fire-floods of Etna, and Pliny died, too rashly investigating the
great eruption of Vesuvius, till Scrope, Lyell, Von Buch, and Palmieri
made them the centre of their researches, they have occupied too
large a share of attention, and have been thus regarded as the full
normal development of that volcanic activity of which they are but
a phase, and only a minor phase. Hence, when, eighteen years ago,
Richthofen described the great lava-plains of Western America, and
attributed their origin to ejection from fissures, and not from vents,
so firm a hold had been taken of the minds of geologists by nearly
twenty centuries of observation of Vesuvius and its fellows, that his
arguments were received with incredulity; and though they have been
amply verified by subsequent investigations, and have afforded the clue
to the interpretation of the vast series of volcanic rocks in other
quarters of the globe, they have not been generally circulated, and
few, outside the circle of geologists, are acquainted with them.
In this paper, we propose briefly to describe some of the most noted
of these ‘fissure’—or as Richthofen called them—‘massive’ eruptions,
selecting as types that on the Snake River in the United States, and
those in India, Abyssinia, and the north-west of Europe; and finally,
to glance at their possible connection with the form of volcanic
excitement more frequently displayed.
The one which has attracted most attention is that which formed the
plateau of the Snake River, and which covers altogether, in Idaho,
Oregon, and Washington, an extent of country equal to France and Great
Britain combined. The district is one wide verdureless waste of black
basalt, stretching westwards from the mountains by which it is bounded
on the north and east, as an apparently boundless desert, black and
bare, as though it had but recently cooled. Except for the shifting
sand-dunes and slight ridges of basalt extending like long low waves or
ground swells—to which Professor Geikie compares them—the surface would
be quite level, the lava having either been poured over a plain, or
having buried the undulations of the country beneath hundreds of feet
of stone. The columnar structure, so often adopted by basalt, is here
largely developed, and to it is due the tesselated appearance of the
rock, which adds another to the many striking features in the scenery
of the district. The only river in the district is the Snake River,
which winds its way to the Pacific through a cañon seven hundred feet
deep, and which is joined, through underground courses, by the few
streams that flow on to the basalt from the neighbouring mountains, and
soon sink beneath the surface.
The Director-general of the Geological Survey visited the district five
years ago, and his graceful pen has thus described his first view of
this great lava desert: ‘We had been riding for two days over fields of
basalt level as lake bottoms among the valleys, and on the morning of
the last day we emerged from the mountains upon the great sea of black
lava which seems to stretch illimitably westwards. With minds keenly
excited by the incidents of the journey, we rode for hours by the side
of that apparently boundless plain. Here and there, a trachytic spur
projected from the hills, succeeded now and then by a valley, up which
the black flood of lava would stretch away into the high grounds. It
was as if the great plain had been filled with molten rock, which had
kept its level, and wound in and out along the bays and promontories of
the mountain slopes, as a sheet of water would have done.’
The feature, however, that most struck Professor Geikie, as it had
done previous observers, was the absence of ash and scoria, and of
any crater where the eruption could have occurred. There are indeed
a few cinder cones, but they are analogous—as he says—to the smaller
cones on the flanks of a volcano, or more so to those around the
vapour-vents on the surface of lava-streams. Such vast masses of lava
were certainly not ejected from these, nor in the ordinary method of
volcanic emission. We are therefore forced to accept Richthofen’s
theory, that they are due to a series of eruptions from fissures which
stretched across the country for several hundreds of miles, but are now
hidden by the sheets of stone in which, since no very remote period,
the district has been enwrapped. Geologically speaking, this must have
been recent, as is evidenced by the lava-floods having covered the
present valleys, and having sealed up the gravel and silt of their
lakes and rivers; but sufficient time has elapsed to have allowed of
the erosion of the picturesque ravine of the Snake River; and in so dry
a climate and on so hard a rock, this must have been slow work, though
in all probability it does not carry back the date of the event beyond
the human occupation of the continent. It is by no means impossible
that man may have witnessed the last of these eruptions from the summit
of the trachyte hills at the base of which was eddying this ocean of
molten rock.
Another series of volcanic rocks that has long been a source of
perplexity to geologists is that which, covering two hundred thousand
square miles of India, is known as the Deccan Traps. Though the
separate lava-flows are of no great thickness, they attain a total
of six thousand feet, exclusive of the ‘intertrappean’ fresh-water
deposits with which they are associated. The rock is mainly dolerite
or basalt, but is very variable, and in many places it exhibits
spheroidal or columnar structure; unlike, however, that of the Snake
River, volcanic ash is common. The plateau formed by these deposits
consists of a vast undulating plain, and of flat-topped hills with
occasional ‘scarps’ or cliffs, which in the Sahyadri range are four
thousand feet high, the whole being marked by terraces along the
outcrop of the horizontal layers of basalt. In many points, the scenery
of this district is much like that of the Snake River; but, owing
to the greater age of the beds—belonging to the Cretaceous or chalk
group—they are more weathered, and covered by a thin soil formed by the
disintegration of the rock beneath, bear a slight vegetation. This,
however, heightens the monotony, as it consists of a simple covering of
straw-coloured grass; though, from March, when the grass is burnt, till
the commencement of the rainy season in June, the black soil, the black
rocks, and the black ashes of the vegetation, combine to produce a
scene of the most solemn desolation. The scene can be well viewed from
the railway between Bombay and Nagpur, which traverses this plateau for
five hundred and nineteen miles without once leaving the lava.
Many ingenious theories have been started to explain the origin of
these lava-fields. Some writers, as Newbold, hold that the beds were
ejected from submarine volcanoes; but this is conclusively disproved,
since no marine fossils are associated with them, and as the minute
dust—due to the shattering of the ash and ejected masses by the sudden
cooling—so characteristic a feature in subaqueous eruptions, is wholly
absent. According to another school, of which Hislop and Carter were
the leaders, the lavas were poured over the bottom of an enormous lake,
in places ‘so shallow as to allow the igneous rock to rise above its
surface into the atmosphere,’ thus giving rise to beds of ash; but as
this assumes the existence of a vast fresh-water lake hundreds of miles
long and broad and shallow throughout, for which no evidence has been
adduced, this theory is discredited. One of the latest writers upon the
subject—Mr W. T. Blanford of the Indian Geological Survey—rejecting the
former hypotheses, argues for the former existence of volcanic foci
in Cutch, in the lower Narbada valley and near the Sahyadri range,
to the east and north-east of Bombay, though he admits that if his
theory be true, the lava must have flowed for immense distances, and
hence postulates its excessive fluidity. The possibility, however,
of the rock having done so on a surface quite horizontal, and in the
semi-fluid viscous condition in which most basic lavas are erupted,
presents insuperable difficulties, and there is now hardly any doubt
that these Deccan Traps were ejected in the same manner as were those
of the Snake River.
For our knowledge of the series of volcanic rocks that covers the
greatest part of Abyssinia, we are also largely indebted to Mr
Blanford, who explored the district during the expedition of 1867.
These rocks, widespread though they be, are but the remnants, as are
also those of Arabia, of a fissure eruption that inundated Abyssinia
and Southern Arabia to a depth of two or three thousand feet.
Nearer home, in North-western Europe, are the relics of the same form
of volcanic activity, as evidenced by those disconnected patches
of lava-streams and trap dikes, which, scattered over the north of
Ireland, the Highlands of Scotland, and the northern counties of
England, form such prominent features in the landscapes of those
districts, as in the columnar basalts of Fingal’s Cave and the Giants’
Causeway, and the igneous dikes that cross, like walls of rock, the
hills of our northern counties. The area that this eruption covered
was at least one hundred thousand square miles; while, as it probably
extended to the Faroes and Iceland, it may have been much larger. We
are not left, as in the case of the Snake River, without evidence as
to whence this mass of rock has come, for, since the Miocene Age,
when it was ejected, denudation has attacked the district, and sea,
stream, and ice have carried off most of the three thousand feet of
lava that then covered the land, and have left but a few scattered
fragments on which to reconstruct the record of the event. In doing
so, they have bared the roots of the old fissures whence this mighty
flood must once have welled; and we thus learn that we must trace its
source to the long dikes that now stretch over the district, crossing
from formation to formation, and traversing dislocations of thousands
of feet without any break or change. Such dikes can be traced from
end to end of the region, from Donegal to Fife, and from Yorkshire to
the Faroes, increasing in numbers as we approach the volcanic regions
of Antrim and the Hebrides. They did not all reach and overflow the
surface, as is conclusively proved by Scotch mining operations, and
by the fact that they sometimes disappear, to rise again elsewhere
on the same line. Such may have been the case with all those of
Yorkshire, as the evidence by which we might decide has all been swept
away. Nevertheless, we know that a vast district was covered by the
great fire-flood which was poured over the tropical forest that then
flourished on the site of the Scottish Highlands.
From this brief description of the most important of these old
fissure eruptions, we see that there is another and a grander phase
of vulcanism than that now displayed either by Vesuvius or Hecla.
This is unquestioned, and the sphere of speculation is removed to the
relation between the two classes. It is to Richthofen that we owe the
most plausible theory: he considers these massive eruptions as the
fundamental development, and ‘modern volcanic cones as merely parasitic
excrescences on the subterranean lava reservoirs, very much in the
relation of minor cinder cones to their parent volcano.’ Thus the form
regarded till recently as the one method of volcanic ejection, appears
to be of but secondary importance, being merely a safety-valve to
relieve the pressure from the lava-sources below; or may represent but
a feebler and waning condition of that volcanic excitement of which
they have so long been regarded as the type.