THE VOLCANIC DISTRICT OF THE RHINE VALLEY.
The region bordering the Rhine along both its banks above Bonn, and
extending thence along the valley of the Moselle and into the Eifel, has
been the theatre of active volcanic phenomena down into recent times,
but at the present day the volcanoes are dormant or extinct.
(_a._) _Geological Structure._--The fundamental rocks of this region
belong to the Silurian, Devonian, and Carboniferous systems, consisting
of schists, grits, and limestones, with occasional horizontal beds of
Miocene sandstone and shale with lignite, resting on the upturned edges
of the older rocks. Scattered over the greater part of the district here
referred to are a number of conical eminences, often with craters, the
bottoms of which are usually sunk much below the present level of the
country, and thus receiving the surface drainage, have been converted
into little lakes called "maars," differing from ordinary lakes by their
circular form and the absence of any _apparent_ outlet for their
waters.[1]
But before entering into details, it may be desirable to present the
reader with a short outline of the physical history of the region (which
has been ably done by Dr. Hibbert in his treatise, to which I have
already referred), so as to enable him better to understand the
succession of physical events in its volcanic history.
[Illustration: Fig. 20.--Sketch Map to show the physical condition of
the Rhenish area in the Miocene epoch.--(After Hibbert.)]
(_b._) _Physical History._--From the wide distribution of stratified
deposits of sand and clay at high levels on both banks of the Rhine
north of the Moselle, it would appear that an extensive fresh-water
basin, which Dr. Hibbert calls "The Basin of Neuwied," occupied a
considerable tract on both banks, in the centre of which the present
city of Neuwied stands. This basin was bounded towards the south by the
slopes of the Huendsruck and Taunus, which at the time here referred to
formed a continuous chain of mountains. (Fig. 20.) To the south of this
chain lay the Tertiary basin of Mayence, which was connected at an early
period--that of the Miocene--with the waters of the ocean, as shown by
the fact that the lower strata contain marine shells; these afterwards
gave place to fresh-water conditions. The basin of Neuwied was bounded
towards the north by a ridge of Devonian strata which extended across
the present gorge of the Rhine between Andernach and Linz, and to the
north of this barrier lay another more extensive fresh-water basin, that
of Cologne. From this it will be seen that the Rhine, as we now find it,
had then only an infantile existence; in fact, its waters to the south
of the Huendsruck ridge drained away towards the south. But towards the
commencement of the Pliocene period the barriers of the Huendsruck and
Taunus, as also that of the Linz, were broken through, and the course of
the waters was changed; and thus gradually, as the river deepened its
bed, the waters were drained off from the great lakes.[2] This rupture
of the barriers may have been due, in the first instance, to the
terrestrial disturbances accompanying the volcanic eruptions of the
Eifel and Siebengebirge, though the erosion of the gorges at Bingen and
at Linz to their present depth and dimensions is of course due to
prolonged river action. It was about the epoch we have now arrived
at--viz., the close of the Miocene--that volcanic action burst forth in
the region of the Lower Rhine. It is probable that this action commenced
in the district of the Siebengebirge, and afterwards extended into that
of the Moselle and the Eifel, the volcanoes of which bear evidence of
recent date. Layers of trachytic tuff are interstratified with the
deposits of sand, clay, and lignite of the formation known as that of
the Brown Coal--of Miocene age--which underlies nearly the whole of the
volcanic district on both sides of the Rhine near Bonn,[3] thus showing
that volcanic action had already commenced in that part to some extent;
but it does not appear from Dr. Hibbert's statement that any such
fragments of eruptive rock are to be found in the strata which were
deposited over the floor of the Neuwied basin.[4] It will be recollected
that the epoch assigned for the earliest volcanic eruptions of Auvergne
was that here inferred for those of the Lower Rhine--viz., the close of
the Miocene stage--and from evidence subsequently to be adduced from
other European districts, it will be found that there was a very widely
spread outburst of volcanic action at this epoch.
(_c._) _The Range of the Siebengebirge._--This range of hills--formed of
the older volcanic rocks of the Lower Rhine--rises along the right bank
of this noble river opposite Bonn, where it leaves the narrow gorge
which it traverses all the way from Bingen, and opens out on the broad
plain of Northern Germany. The range consists of a succession of conical
hills sometimes flat-topped--as in the case of Petersberg; and at the
Drachenfels, near the centre of the range it presents to the river a
bold front of precipitous cliffs of trachyte porphyry. The sketch (Fig.
21) here presented was taken by the author in 1857 from the old extinct
volcano of Roderberg, and will convey, perhaps, a better idea of the
character of this picturesque range than a description. The
Siebengebirge, although appearing as an isolated group of hills, is in
reality an offshoot from the range of the Westerwald, which is connected
with another volcanic district of Central Germany known as the
Vogelsgebirge. The highest point in the range is attained in the
Lohrberg, which rises 1355 feet above the sea; the next, the Great
Traenkeberg, 1330 feet; and the next, Great Oelberg, 1296 feet.
[Illustration: Fig. 21.--The Volcanic Range of the Siebengebirge, seen
from the left bank of the Rhine, above Bonn.--(Original.)]
The range consists mainly of trachytic rocks--namely,
trachyte-conglomerate, and solid trachyte, of which H. von Dechen makes
two varieties--that of the Drachenfels, and that of the Wolkenburg. But
associated with these highly-silicated varieties of lava--and generally,
if not always, of later date--are basaltic rocks which cap the hills of
Petersberg, Nonnenstrom, Gr. and Ll. Oelberg, Gr. Weilberg, and Ober
Dollendorfer Hardt. The question whether there is a transition from the
one variety of volcanic rock into the other, or whether each belongs to
a distinct and separate epoch of eruption, does not seem to be very
clearly determined. Mr. Leonard Horner states that it would be easy to
form a suite of specimens showing a gradation from a white trachyte to a
black basalt;[5] but we must recollect that when Mr. Horner wrote, the
microscopic examination of rocks by means of thin sections was not known
or practised, and an examination by this process might have proved that
this apparent transition is unreal. According to H. von Dechen, there
are sheets of basalt older than the greater mass of the brown coal
formation, and others newer than the trachyte;[6] while dykes of basalt
traversing the trachytic lavas are not uncommon.[7]
The trachyte-conglomerate--which seems to be associated with the upper
beds of the brown coal strata--is traversed by dykes of trachyte of
later date; and though it is difficult to trace the line between the two
varieties of this rock on the ground, Dr. von Rath has recognised the
general distinction between them, which consists in the greater
abundance of hornblende and mica in the trachyte of the Wolkenburg than
in that of the Drachenfels.
The trachyte of the Drachenfels was probably the neck of a volcano which
burst through the fundamental schists of the Devonian period. It is
remarkable for the large crystals of sanidine (glassy felspar) which it
contains, and has a rude columnar structure.
The absence of any clearly-defined craters of eruption, such as are to
be found in the Eifel district and on the left bank of the Rhine--as,
for example, in the case of the Roderberg--may be regarded as sufficient
evidence that this range is of comparatively high antiquity. It seems to
bear the same relation to the more modern craters of the Eifel and
Moselle that the Mont Dore and Cantal volcanoes do to those of the Puy
de Dome. In both cases, denudation carried on throughout perhaps the
Pliocene and Post-Pliocene periods down to the present day has had the
effect of demolishing the original craters; so that what we now observe
as forming these ranges are the consolidated columns of original molten
matter which filled the throats of the old volcanoes, or the sheets of
lava which were extruded from them, but are now probably much reduced in
size and extent.
Having thus given a description of the older volcanic range on the right
bank of the Rhine, we shall cross the river in search of some details
regarding the more recent group of Rhenish volcanoes, commencing with
that of the Roderberg, a remarkable hill a few miles south of Bonn, from
which the view of the Seven Mountains was taken.
[Illustration: Fig. 22.--Section of the extinct crater of the Roderberg
on the bank of the Rhine, above Bonn.--(Original.)]
(_d._) _The Roderberg._--This crater, which was visited by the author in
1857, is about one-fourth of a mile in diameter, and is in the form of a
cup with gentle slopes on all sides. In its centre is a farmhouse
surrounded by corn-fields. The general section through the hill is
represented above (Fig. 22).
The flanks on the north side are composed of loose quartzose gravel
(gerolle), a remnant of the deposits formed around the margin of the
"Basin of Neuwied" described above (p. 114). This gravel is found
covering the terraces of the brown coal formation several hundred feet
above the Rhine. Besides quartz-pebbles, the deposit contains others of
slate, grit, and volcanic rock. On reaching the edge of the crater we
find the gravel covered over by black and purple scoria or slag the
superposition of the scoria on the gravel being visible in several
places, showing that the former is of more recent origin. On the
opposite side of the crater, overlooking the Rhine, we find the cliff of
Rolandsec composed of hard vesicular lava, rudely prismatic, and
extending from the summit of the hill to its base, about 250 feet below.
This is the most northerly of the group of the Eifel volcanoes.
(_e._) _District of the Rivers Bruehl and Nette._--The volcanic region of
the Lower Eifel, drained by these two principal streams which flow into
the Rhine, will amply repay exploration by the student of volcanic
phenomena, owing to the variety of forms and conditions under which
these present themselves within a small space. The fundamental rock is
slate or grit of Devonian age, furrowed by numerous valleys, often
richly wooded, and diversified by conical hills of trachyte; or by
crater-cones, formed of basalt or ashes, sometimes ruptured on one side,
and occasionally sending forth streams of lava, as in the cases of the
Perlinkopf, the Bausenberg, and the Engelerkopf. The district attains
its greatest altitude in the High Acht (Der Hohe Acht), an isolated cone
of slate capped by basalt with olivine, and reaching a level of 2434
Rhenish feet.[8]
(_f._) _The Laacher See._--It would be impossible in a work of this kind
to attempt a detailed description of the Eifel volcanoes, often of a
very complex character and obscure physical history, as in the case of
the basin of Rieden, where tufaceous deposits, trachytic and basaltic
lavas and crater-cones, are confusedly intermingled, so that I shall
confine my remarks to the deservedly famous district of the Laacher
See, which I had an opportunity of personally visiting some years
since.[9]
[Illustration: Fig. 23.--Plan and Section of the Laacher See, a lake on
the borders of the Eifel, occupying the crater of an old volcano.--G.
Gravel and volcanic sand forming banks of the lake and rim of old
crater; L. Sheet of trachytic lava with columnar structure; B. Basaltic
dyke; S. Devonian slate, etc.]
The Laacher See is a lake of an oval form, over an English mile in the
shorter diameter, and surrounded by high banks of volcanic sand, gravel,
and scoriae, except on the east side, where cliffs of clay-slate, in a
nearly vertical position, and striking nearly E.W., may be observed. Its
depth from the surface of the water is 214 feet.[10] The ashes of the
encircling banks contain blocks of slate and lava which have been torn
from the sides of the orifice or neck of the volcano and blown into the
air; and there can be no doubt that the ashes and volcanic gravel is the
result of very recent eruptions.
At the east side of the lake we find a stream of scoriaceous lava of a
purple or reddish colour, highly vesicular, and containing crystals of
mica; but the most important lava-stream is that which has taken a
southerly direction from the crater of the Laacher See towards Nieder
Mendig and Mayen, for a distance of about six miles. This great stream
is covered throughout half its distance by beds of volcanic ash and
lapilli, but emerges into the air at a distance of about two miles from
the edge of the crater (see Fig. 23), and was formerly extensively
quarried in underground caverns for millstones. Here the rock is a
vesicular trachyte, of a greyish colour, solidified in vertical columns
of hexagonal form, about four feet in diameter, and traversed by
transverse joint planes. These quarries have been worked from the time
of the Roman occupation of the country; and, before the introduction of
iron or steel rollers for grinding corn, millstones were exported to all
parts of Europe and the British Isles from this quarry.[11]
The district around the Laacher See is covered by laminated _ejecta_ of
the old volcano, probably of subaerial origin, through which bosses of
the fundamental slate peer up at intervals, while the surface is
diversified by several truncated cones.
(_g._) _Trass of the Bruehl Valley._--The Bruehl Valley, which unites
with that of the Rhine at the town of that name, and drains the northern
side of the volcanic region, has always been regarded with much interest
by travellers for the presence of a deposit of "trass" with which it is
partially filled. The origin of this valley was pre-volcanic, as it is
hewn out of the slaty rocks of the district. But at a later period it
became filled with volcanic mud (tuffstein), out of which the stream has
made for itself a fresh channel. The source of this mud is considered by
Hibbert[12] to have been the old volcano of the Lummerfeld, which, after
becoming dormant, was filled with water, and thus became a lake. At a
subsequent period, however, a fresh eruption took place near the edge of
the lake, resulting in the remarkable ruptured crater known as the
Kunkskoepfe, which rises about four miles to the north of the Laacher
See. The eruptions of this volcano appear to have displaced the mud of
the Lummerfeld, causing it to flow down into the deep gorge of the
Bruehl, which it completely filled, as stated above.
On walking down the valley one may sometimes see the junction of the
tuff with the slate-rock which enfolds it. The tuff consists of white
felspathic mud, with fragments of slate and lava, reaching a depth in
some places of 150 feet. After it has been quarried it is ground in
mills, and used for cement stone under the name of _trass_. It is said
to resemble the volcanic mud by which Herculaneum was overwhelmed during
the first eruption of Vesuvius, and which was produced by the torrents
of rain mixing with the ashes as they were blown out of the volcano.
Sufficient has probably now been written regarding the dormant, or
recently extinct, volcanic districts of Europe to give the reader a
clear idea regarding their nature and physical structure. Other
districts might be added, such as those of Central Germany, Hungary,
Transylvania, and Styria; but to do so would be to exceed the proposed
limits of this work; and we may therefore pass on to the consideration
of the volcanic region of Syria and Palestine, which adjoins the
Mediterranean district we have considered in a former page.
[1] Daubeny, _loc. cit._, p. 71. The geology of this region has had many
investigators, of whom the chief are Steininger, _Erloschenen Vulkane in
der Eifel_ (1820); Hibbert, _Extinct Volcanoes of the Basin of Neuwied_,
1832; Noeggerath, _Das Gebirge im Rheinland_, etc., 4 vols.; Horner, "On
the Geology of Bonn," _Transactions of the Geological Society, London_,
vol. iv.
[2] The views of Dr. Hibbert are not inconsistent with those of the late
Sir A. Ramsay, on "The Physical History of the Valley of the Rhine,"
_Quart. Jour. Geol. Soc._, vol. xxx. (1874).
[3] Von Dechen, _Geog. Beschreib. des Siebengebirges am Rhein_ (Bonn,
1852).
[4] Hibbert, _loc. cit._, p. 18.
[5] Horner, "Geology of Environs of Bonn," _Transactions of the
Geological Society_, vol. iv., new series.
[6] H. von Dechen, _Geog. Fuehrer in das Siebengebirge am Rhein_ (Bonn,
1861).
[7] _Ibid._, p. 191.
[8] Dr. Hibbert's work is illustrated by very carefully drawn and
accurate views of some of the old cones and craters of this district,
accompanied by detailed descriptions.
[9] The lava of Schorenberg, near Rieden, is interesting from the fact,
stated by Zirkel, that it contains leucite, nosean, and nephelin.--_Die
Mikros. Beschaf. d. Miner. u. Gesteine_, p. 154 (1873).
[10] Hibbert, _loc. cit._, p. 23.
[11] At the time of the author's visit the underground caverns, which
are deliciously cool in summer, were used for the storage of the
celebrated beer brewed by the Moravians of Neuwied.
[12] Hibbert, _loc. cit._, p. 129.
PART III.
DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD.
CHAPTER I.
DORMANT VOLCANOES OF PALESTINE AND ARABIA.
(_a._) _Region east of the Jordan and Dead Sea._--The remarkable line of
country lying along the valley of the Jordan, and extending into the
great Arabian Desert, has been the seat of extensive volcanic action in
prehistoric times. The specially volcanic region seems to be bounded by
the depression of the Jordan, the Dead Sea, and the Arabah as far south
as the Gulf of Akabah; for, although Safed, lying at the head of the Sea
of Galilee on the west of the Jordan valley, is built on a basaltic
sheet, and is in proximity to an extinct crater, its position is
exceptional to the general arrangement of the volcanic products which
may be traced at intervals from the base of Hermon into Central Arabia,
a distance of about 1000 miles.[1]
The tract referred to has been described at intervals by several
authors, of whom G. Schumacher,[2] L. Lartet,[3] Canon Tristram,[4] M.
Niebuhr,[5] and C. M. Doughty[6] may be specially mentioned in this
connection.
The most extensive manifestations of volcanic energy throughout this
long tract of country appear to be concentrated at its extreme limits.
At the northern extremity the generally wild and rugged tract of the
Jaulan and Hauran, called in the Bible _Trachonitis_, and still farther
to the eastward the plateau of the Lejah, with its row of volcanic peaks
sloping down to the vast level of Bashan, is covered throughout nearly
its whole extent by great sheets of basaltic lava, above which rise at
intervals, and in very perfect form, the old crater-cones of eruption. A
similar group of extinct craters with lava-flows has been described and
figured by a recent traveller, Mr. C. M. Doughty, in parts of Central
Arabia. The general resemblance of these Arabian volcanoes to those of
the Jaulan is unquestionable; and as they are connected with each other
by sheets of basaltic lava at intervals throughout the land of Moab, it
is tolerably certain that the volcanoes lying at either end of the chain
belong to one system, and were contemporaneously in a state of activity.
(_b._) _Geological Conditions._--Before entering any further into
particulars regarding the volcanic phenomena of this region, it may be
desirable to give a short account of its geological structure, and the
physical conditions amongst which the igneous eruptions were developed.
Down to the close of the Eocene period the whole region now under
consideration was occupied by the waters of the ocean. The mountains of
Sinai were islands in this ocean, which had a very wide range over parts
of Asia, Africa, and Europe. But at the commencement of the succeeding
Miocene stage the crust was subjected to lateral contraction, owing to
which the ocean bed was upraised. The strata were flexured, folded, and
often faulted and fissured along lines ranging north and south, the
great fault of the Jordan-Arabah valley being the most important. At
this period the mountains of the Lebanon, the table-lands of Judaea and
of Arabia, formed of limestone, previously constituting the bed of the
ocean during the Eocene and Cretaceous periods, were converted into land
surfaces. Along with this upheaval of the sea-bed there was extensive
denudation and erosion of the strata, so that valleys were eroded over
the subaerial tracts, and the Jordan-Arabah valley received its primary
form and outline.
Up to this time there does not appear to have been any outbreak of
volcanic forces; but with the succeeding Pliocene period these came into
play, and eruptions of basaltic lava took place along rents and fissures
in the strata, while craters and cones of slag, scoriae, and ashes were
thrown up over the region lying to the east of the Sea of Galilee and
the sources of the Jordan on the one hand, and the central parts of the
great Arabian Desert on the other. These eruptions, probably
intermittent, continued into the succeeding Glacial or Pluvial period,
and only died out about the time that the earliest inhabitants appeared
on the scene.
(_c._) _The Jaulan and Hauran._--This tract is bounded by the valley of
the Jordan and the Sea of Galilee on the west, from which it rises by
steep and rocky declivities into an elevated table-land, drained by the
Yarmuk (Hieromax), the Nahr er Rukkad, and other streams, which flow
westwards into the Jordan along deep channels in which the basaltic
sheets and underlying limestone strata are well laid open to view.
On consideration it seems improbable that the great sheets of augitic
lava, such as cover the surface of the land of Bashan, are altogether
the product of the volcanic mountains which appear to be confined to
special districts in this wide area. Some of the craters do indeed send
forth visible lava-streams, but they are insignificant as compared with
the general mass of the plateau-basalts; and the crater-cones themselves
appear in some cases to be posterior to the platforms of basalt from
which they rise. It is very probable, therefore, that the lavas of this
region have, in the main, been extruded from fissures of eruption at an
early period, and spread over the surface of the country in the same
manner as those of the Snake River region, and the borders of the
Pacific Ocean of North America, and possibly of the Antrim Plateau in
Ireland, afterwards to be described.
The volcanic hills which rise above the plateau are described in detail
by Schumacher. Of these, Tell Abu Nedir is the largest in the Jaulan. It
reaches an elevation of 4132 feet above the Mediterranean Sea, and 1710
feet above the plain from which it rises; the circumference of its base
is three miles, and the rim of the crater itself, which is oval in form,
is 1331 yards in its larger diameter. The interior is cultivated by
Circassians, and is very fruitful; the walls descend at an angle of
about 30 deg. on the inside, the exterior slope of the mountain being about
22 deg.. The cone seems to be formed chiefly of scoriae, and the lava-stream,
which issues forth from the interior, forms a frightfully stony and
lacerated district.[7]
[Illustration: Fig. 24.--Extinct Craters in the Jaulan, north-east from
the Sea of Galilee, called Tell Abu en Neda and Tell el Uram, with a
central cone.--(After Schumacher.)]
Another remarkable volcano is the Tell Abu en Neda (Fig. 24). This is a
double crater, with a cone (probably of cinders) rising from the
interior of one of them. The highest point of the rim of one of the
craters reaches a level of 4042 feet above the sea. A lava-stream issues
forth from Abu en Neda, and unites with another from a neighbouring
volcano.
Tell el Ahmar is a ruptured crater of imposing aspect, reaching an
elevation of 4060 feet, and sending forth a lava-current, which falls in
regular terraces from the outlet towards the west and north.
The ruptured crater of Tell el Akkasheh, which reaches a height of 3400
feet, has a less forbidding aspect than the greater number of the
extinct volcanoes of this region, owing to the fact that its sides are
covered by oaks, which attain to magnificent proportions along the
summit. Numerous other volcanic hills occur in this district, but the
most remarkable is that called Tell el Farras (the Hill of the Horse).
It is an isolated mountain, visible from afar, and reaches an elevation
of 3110 feet, or nearly 800 feet above the surrounding plain. The oval
crater of this volcano opens towards the north, and has a depth of 108
feet below the edge, with moderately steep sloping sides (17 deg.-32 deg.),
while the slope of the exterior, at first steep, gradually lessens to
20 deg.-21 deg.. These slopes are covered with reddish or yellowish slag. The
above examples will probably suffice to afford the reader a general idea
of the size and form of the volcanoes in this little known region.
It has been stated above that the great lava-floods have probably been
poured forth intermittently. The statement receives confirmation from
the observations of Canon Tristram, made in the valley of the Yarmuk.[8]
This impetuous torrent rushes down a gorge, sometimes having limestone
on one side and a wall of basalt on the other. This is due to the fact
that the river channel had been eroded before the volcanic eruptions had
commenced; but on the lava-stream reaching the channel, it naturally
descended towards the valley of the Jordan along its bed, displacing the
river, or converting it into clouds of steam. Subsequently the river
again hewed out its channel, sometimes in the lava, sometimes between
this rock and the chalky limestone. But, in addition to this, it has
been observed that there is a bed of river gravel interposed between two
sheets of basalt in the Yarmuk ravine; showing that after the first flow
of that molten rock the river reoccupied its channel, which was
afterwards invaded by another molten lava-stream, into which the waters
have again furrowed the channel which they now occupy. The basaltic
sheets descend under the waters of the Sea of Galilee on the east side,
and were probably connected with those of Safed, crossing the Jordan
valley north of that lake; owing to this the waters of the Lake of Merom
(Huleh) were pent up, and formerly covered an extensive tract, now
formed of alluvial deposits.
(_d._) _Land of Moab._--Proceeding southwards into the Land of Moab, the
volcanic phenomena are here of great interest. Extensive sheets of
basaltic lava, described as far back as 1807 by Seetzen, and more
recently by Lartet and Tristram, are found at intervals between the
Wadies Mojib (Arnon) and Haidan. On either side of the Mojib, cliffs of
columnar basalt are seen capping the beds of white Cretaceous limestone,
while a large mass has descended into the W. Haidan between cliffs of
limestone and marl on either hand.
Around Jebel Attarus--a dome-shaped hill of limestone--a sheet of
basaltic lava has been poured, and has descended the deep gorge of the
Zerka Main, which enters the Dead Sea some 2000 feet below. This gorge
had been eroded before the basaltic eruption, so that the stream of
molten lava took its course down the bed of this stream to the water's
edge, and grand sections have been laid bare by subsequent erosion along
the banks. Pentagonal columns of black basalt form perpendicular walls,
first on one side, then on the other; while considerable masses of
scoriae, peperino, and breccia appear at the head of the glen, probably
marking the orifice of eruption. Other eruptions of basalt occur, one at
Mountar ez Zara, to the south of Zerka Main, and another at Wady
Ghuweir, near the north-eastern end of the Dead Sea. There are no
lava-streams on the western side of the Ghor, or of the Dead Sea.[9]
The outburst of the celebrated thermal springs of Callirrhoe, together
with nine or ten others, along the channel of the Zerka Main, is a
circumstance which cannot be dissociated from the occurrence of basaltic
lava at this spot. In a reach of three miles, according to Tristram,
there are ten principal springs, of which the fifth in descent is the
largest; but the seventh and eighth, about half a mile lower down, are
the most remarkable, giving forth large supplies of sulphurous water.
The tenth and last is the hottest of all, indicating a temperature of
143 deg. Fahr. Thus it would appear that the heat increases with the depth
from the upper surface of the table-land; a result which might be
expected, supposing the heated volcanic rocks to be themselves the
source of the high temperature. To a similar cause may be attributed the
hot-springs of Hammath, near Tiberias, and those of the Yarmuk near its
confluence with the Jordan. Some of these and other springs break out
along, or near, the line of the great Jordan-Arabah fault which ranges
throughout the whole extent of this depression, from the base of Hermon
to the Gulf of Akabah, generally keeping close to the eastern margin of
the valley.
(_e._) _The Arabian Desert._--The basaltic lava-floods occupy a very
large extent of the Arabian Desert, from El Hisma (lat. 27 deg. 35' N.) to
the neighbourhood of Mecca on the south, a distance of about 440 miles,
with occasional intervals. The lava-sheets are called "Harras" (or
"Harrat"), one of which, Harrat Sfeina, terminates about ten miles north
of Mecca. The lava-sheets rest sometimes on the red sandstone, at other
times, on the granite and other crystalline rocks of great geological
antiquity. In addition to the sheets of basalt, numerous crater-cones
rise from the basaltic platform at a level of 5000 feet above the sea,
and two volcanic mountains, rising far to the west of the principal
range, called respectively Harrat Jeheyma and H. Rodwa, almost overlook
the coast of the Red Sea.[10]
(_f._) _Age of the Volcanic Eruptions._--It is very clear that the first
eruptions, producing the great basaltic sheets of Moab and Arabia,
occurred after the principal features of the country had been developed.
The depression of the Jordan-Arabah valley, the elevation of the eastern
side of this valley along the great fault line, and the channels of the
principal tributary streams, such as those of the Yarmuk and Zerka Main,
all these had been eroded out before they were invaded by the molten
streams of lava. Now, as these physical features were developed and
sculptured out during the Miocene period, as I have elsewhere shown to
be the case,[11] we may with great probability refer the volcanic
eruptions to the geological epoch following--namely, the Pliocene. How
far downwards towards the historic period the eruptions continued is not
so certain. Dr. Daubeny, quoting several passages from the Old Testament
prophets,[12] says it might be inferred that volcanoes were in activity
even so late as to admit of their being included within the limits of
authentic history. The poetic language and imagery used in these
passages by the prophets certainly lends a probability to this view, but
nothing more. On the other hand, these regions have suffered through
many centuries from the secondary effects of seismic action and
subterranean forces, and earthquake shocks have laid in ruins the great
temples and palaces of Palmyra, Baalbec, and other cities of antiquity.
The same uncertainty regarding the time at which volcanic action died
out, with reference to the appearance of man on the scene, hangs over
the region of Arabia and Syria, as we have seen to be the case in
reference to the extinct volcanoes of Auvergne, the Eifel, and the Lower
Rhine. In all these cases the commencement and close of eruptive action
appear to have been very much about the same period--namely, the Miocene
period on the one hand, and that at which man entered upon the scene on
the other; but in the case of Syria and Western Palestine, the close of
the volcanic period may have been somewhat more than 2000 B.C.
[1] Lake Phiala, near the Lake of Huleh, is also situated to the west of
the Jordan valley. Its origin, according to Tristram, is volcanic.
[2] Schumacher, "The Jaulan," _Quarterly Statement of the Palestine
Exploration Fund_, 1886 and 1888; and _Across the Jordan_, London, 1886.
[3] Lartet, _Voyage d'Exploration de la mer Morte_ (Geologie), Paris,
1880.
[4] Tristram, _Land of Moab_, London, 1873; and _Land of Israel_, 1866.
[5] Niebuhr, _Beschreibung von Arabien_, 1773.
[6] C. M. Doughty, _Arabia Deserta_, 2 vols., 1888. A generalised
account of this volcanic region by the author will be found in the
"Memoir on the Physical Geology of Arabia Petraea, and Palestine,"
_Palestine Exploration Fund_, 1887.
[7] Schumacher, _loc. cit._, p. 248.
[8] _Land of Israel_, p. 461.
[9] "Geology of Arabia Petraea, and Palestine," _Memoirs of the Palestine
Exploration Fund_, p. 95.
[10] Doughty, _loc. cit._, vol. i., plate vi., p. 416. An excellent
geological sketch map accompanies this work.
[11] "Memoir of the Geology of Arabia Petraea, and Palestine," chap. vi.
p. 67.
[12] Nahum, i. 5, 6; Micah, i. 3, 4; Isaiah, lxiv. 1-3; Jeremiah, l. 25.
CHAPTER II.
THE VOLCANIC REGIONS OF NORTH AMERICA.
(_a._) _Contrast between the Eastern and Western Regions._--In no point
is there a more remarkable contrast between the physical structure of
Eastern and Western America than in the absence of volcanic phenomena in
the former and their prodigious development in the latter. The great
valley of the Mississippi and its tributaries forms the dividing
territory between the volcanic and non-volcanic areas; so that on
crossing the high ridges in which the western tributaries of America's
greatest river have their sources, and to which the name of the "Rocky
Mountains" more properly belongs, we find ourselves in a region which,
throughout the later Tertiary times down almost to the present day, has
been the scene of volcanic operations on the grandest scale; where
lava-floods have been poured over the country through thousands of
square miles, and where volcanic cones, vying in magnitude with those of
Etna, Vesuvius, or Hecla, have established themselves. This region,
generally known as "The Great Basin," is bounded on the west by the
"Pacific Range" of mountains, and includes portions of New Mexico,
Arizona, California, Nevada, Utah, Colorado, Idaho, Oregon, Wyoming,
Montana, and Washington. To the south it passes into the mountainous
region of Mexico, also highly volcanic; and thence into the ridge of
Panama and the Andes. It cannot be questioned but that the volcanic
nature of the Great Basin is due to the same causes which have
originated the volcanic outbursts of the Andes; but, from whatever
cause, the volcanic forces have here entered upon their secondary or
moribund stage. In the Yellowstone Valley, geysers, hot springs, and
fumaroles give evidence of this condition. In other districts the
lava-streams are so fresh and unweathered as to suggest that they had
been erupted only a few hundred years ago; but no active vent or crater
is to be found over the whole of this wide region. A few special
districts only can here be selected by way of illustration of its
special features in connection with its volcanic history.
(_b._) _The Plateau Country of Utah and Arizona._--This tract, which is
drained by the Colorado River and its tributaries, is bounded on the
north by the Wahsatch range, and extends eastwards to the base of the
Sierra Nevada. Round its margin extensive volcanic tracts are to be
found, with numerous peaks and truncated cones--the ancient craters of
eruption--of which Mount San Francisco is the culminating eminence.
South of the Wahsatch, and occupying the high plateaux of Utah, enormous
masses of volcanic products have been spread over an area of 9000 square
miles, attaining a thickness of between 3000 and 4000 feet. The earlier
of these great lava-floods appear to have been trachytic, but the later
basaltic; and in the opinion of Captain Dutton, who has described them,
they range in point of time from the Middle Tertiary (Miocene) down to
comparatively recent times.
(_c._) _The Grand Canyon._--To the south of the high plateaux of Utah
are many minor volcanic mountains, now extinct; and as we descend
towards the Grand Canyon of Colorado we find numerous cinder-cones
scattered about at intervals near the cliffs.[1] Extensive lava-fields,
surmounted by cinder-cones, occupy the plateau on the western side of
the Grand Canyon; and, according to Dutton, the great sheets of basaltic
lava, of very recent age, which occupy many hundred square miles of
desert, have had their sources in these cones of eruption.[2] Crossing
to the east of the Grand Canyon, we find other lava-floods poured over
the country at intervals, surmounted by San Francisco--a volcanic
mountain of the first magnitude--which reaches an elevation, according
to Wheeler, of 12,562 feet above the ocean. It has long been extinct,
and its summit and flanks are covered with snow-fields and glaciers.
Other parts of Arizona are overspread by sheets of basaltic lava,
through which old "necks" of eruption, formed of more solid lava than
the sheets, rise occasionally above the surface, and are prominent
features in the landscape.
Further to the eastward in New Mexico, and near the margin of the
volcanic region, is another volcanic mountain little less lofty than San
Francisco, called Mount Taylor, which, according to Dutton, rises to an
elevation of 11,390 feet above the ocean, and 8200 feet above the
general level of the surrounding plateau of lava. This mountain forms
the culminating point of a wide volcanic tract, over which are
distributed numberless vents of eruption. Scores of such
vents--generally cinder-cones--are visible in every part of the plateau,
and always in a more or less dilapidated condition.[3] Mount Taylor is a
volcano, with a central pipe terminating in a large crater, the wall of
which was broken down on the east side in the later stage of its
history.
[Illustration: Fig. 25.--Mount Shasta (14,511 feet), a snow-clad
volcanic cone in California, with Mount Shastina, a secondary crater, on
the right; the valley between is filled with glacier-ice.--(After
Dutton).]
(_d._) _California._--Proceeding westwards into California, we are
again confronted with volcanic phenomena on a stupendous scale. The
coast range of mountains, which branches off from the Sierra Nevada at
Mount Pinos, on the south, is terminated near the northern extremity of
the State by a very lofty mountain of volcanic origin, called Mount
Shasta, which attains an elevation of 14,511 feet (see Fig. 25). This
mountain was first ascended by Clarence King in 1870,[4] and although
forming, as it were, a portion of the Pacific Coast Range, it really
rises from the plain in solitary grandeur, its summit covered by snow,
and originating several fine glaciers.
The summit of Mount Shasta is a nearly perfect cone, but from its
north-west side there juts out a large crater-cone just below the
snow-line, between which and the main mass of the mountain there exists
a deep depression filled with glacier ice. This secondary crater-cone
has been named Mount Shastina, and round its inner side the stream of
glacier ice winds itself, sometimes surmounting the rim of the crater,
and shooting down masses of ice into the great caldron. The length of
this glacier is about three miles, and its breadth about 4000 feet.
Another very lofty volcanic mountain is Mount Rainier, in the Washington
territory, consisting of three peaks of which the eastern possesses a
crater very perfect throughout its entire circumference. This mountain
appears to be formed mainly of trachytic matter. Proceeding further
north into British territory, several volcanic mountains near the
Pacific Coast are said to exhibit evidence of activity. Of these may be
mentioned Mount Edgecombe, in lat. 57 deg..3; Mount Fairweather, lat. 57 deg..20
which rises to a height of 14,932 feet; and Mount St. Elias, lat. 60 deg..5,
just within the divisional line between British and Russian territory,
and reaching an altitude of 16,860 feet. This, the loftiest of all the
volcanoes of the North American continent, except those of Mexico, may
be considered as the connecting link in the volcanic chain between the
continent and the Aleutian Islands.[5]
(_e._) _Lake Bonneville._--Returning to Utah we are brought into contact
with phenomena of special interest, owing to the inter-relations of
volcanic and lacustrine conditions which once prevailed over large
tracts of that territory. The present Great Salt Lake, and the smaller
neighbouring lakes, those called Utah and Sevier, are but remnants of an
originally far greater expanse of inland water, the boundaries of which
have been traced out by Mr. C. K. Gilbert, and described under the name
of Lake Bonneville.[6] The waters of this lake appear to have reached
their highest level at the period of maximum cold of the Post-Pliocene
period, when the glaciers descended to its margin, and large streams of
glacier water were poured into it. Eruptions of basaltic lava from
successive craters appear to have gone on before, during, and after the
lacustrine epochs; and the drying up of the waters over the greater
extent of their original area, now converted into the Sevier Desert, and
their concentration into their present comparatively narrow basins,
appears to have proceeded _pari passu_ with the gradual extinction of
the volcanic outbursts. Two successive epochs of eruption of basalt
appear to have been clearly established--an earlier one of the "Provo
Age," when the lava was extruded from the Tabernacle craters, and a
later epoch, when the eruptions took place from the Ice Spring craters.
The oldest volcanic rock appears to be rhyolite, which peers up in two
small hills almost smothered beneath the lake deposits. Its eruption was
long anterior to the lake period. On the other hand, the cessation of
the eruptions of the later basaltic sheets is evidently an event of such
recent date that Mr. Gilbert is led to look forward to their resumption
at some future, but not distant, epoch. As he truly observes, we are not
to infer that, because the outward manifestations of volcanic action
have ceased, the internal causes of those manifestations have passed
away. These are still in operation, and must make themselves felt when
the internal forces have recovered their exhausted energies; but perhaps
not to the same extent as before.
(_f._) _Region of the Snake River._--The tract of country bordering the
Snake River in Idaho and Washington is remarkable for the vast sheets of
plateau-basalt with which it is overspread, extending sometimes in one
great flood farther than the eye can reach, and what is still more
remarkable, they are often unaccompanied by any visible craters or vents
of eruption. In Oregon the plateau-basalt is at least 2,000 feet in
thickness, and where traversed by the Columbia River it reaches a
thickness of about 3,000 feet. The Snake and Columbia rivers are lined
by walls of volcanic rock, basaltic above, trachytic below, for a
distance of, in the former, one hundred, in the latter, two hundred,
miles. Captain Dutton, in describing the High Plateau of Utah, observes
that the lavas appear to have welled up in mighty floods without any of
that explosive violence generally characteristic of volcanic action.
This extravasated matter has spread over wide fields, deluging the
surrounding country like a tide in a bay, and overflowing all
inequalities. Here also we have evidence of older volcanic cones buried
beneath seas of lava subsequently extruded.
(_g._) _Fissures of Eruption._--The absence, or rarity, of volcanic
craters or cones of eruption in the neighbourhood of these great sheets
has led American geologists to the conclusion that the lavas were in
many cases extruded from fissures in the earth's crust rather than from
ordinary craters.[7] This view is also urged by Sir A. Geikie, who
visited the Utah region of the Snake River in 1880, and has vividly
described the impression produced by the sight of these vast fields of
basaltic lava. He says, "We found that the older trachytic lavas of the
hills had been deeply trenched by the lateral valleys, and that all
these valleys had a floor of black basalt that had been poured out as
the last of the molten materials from the now extinct volcanoes. There
were no visible cones or vents from which these floods of basalt could
have proceeded. We rode for hours by the margin of a vast plain of
basalt stretching southward and westward as far as the eye could
reach.... I realised the truth of an assertion made first by
Richthofen,[8] that our modern volcanoes, such as Vesuvius and Etna,
present us with by no means the grandest type of volcanic action, but
rather belong to a time of failing activity. There have been periods of
tremendous volcanic energy, when instead of escaping from a local vent,
like a Vesuvian cone, the lava has found its way to the surface by
innumerable fissures opened for it in the solid crust of the globe over
thousands of square miles."[9]
(_h._) _Volcanic History of Western America._--The general succession of
volcanic events throughout the region of Western America appears to have
been somewhat as follows:--[10]
The earliest volcanic eruptions occurred in the later Eocene epoch and
were continued into the succeeding Miocene stage. These consisted of
rocks moderately rich in silica, and are grouped under the heads of
propylite and andesite. To these succeeded during the Pliocene epoch
still more highly silicated rocks of trachytic type, consisting of
sanidine and oligoclase trachytes. Then came eruptions of rhyolite
during the later Pliocene and Pleistocene epochs; and lastly, after a
period of cessation, during which the rocks just described were greatly
eroded, came the great eruptions of basaltic lava, deluging the plains,
winding round the cones or plateaux of the older lavas, descending into
the river valleys and flooding the lake beds, issuing forth from both
vents and fissures, and continuing intermittently down almost into the
present day--certainly into the period of man's appearance on the scene.
Thus the volcanic history of Western America corresponds remarkably to
that of the European regions with which we have previously dealt, both
as regards the succession of the various lavas and the epochs of their
eruption.
(_i._) _The Yellowstone Park._--The geysers and hot springs of the
Yellowstone Park, like those in Iceland and New Zealand, are special
manifestations of volcanic action, generally in its secondary or
moribund stage. The geysers of the Yellowstone occur on a grand scale;
the eruptions are frequent, and the water is projected into the air to a
height of over 200 feet. Most of these are intermittent, like the
remarkable one known as Old Faithful, the Castle Geyser, and the
Giantess Geyser described by Dr. Hayden, which ejects the water to a
height of 250 feet. The geyser-waters hold large quantities of silica
and sulphur in solution, owing to their high temperature under great
pressure, and these minerals are precipitated upon the cooling of the
waters in the air, and form circular basins, often gorgeously tinted
with red and yellow colours.[11]
[1] J. W. Powell, _Exploration of the Canyons of the Colorado_, pp. 114,
196. Major Powell describes a fault or fissure through which floods of
lava have been forced up from beneath and have been poured over the
surface. Many cinder-cones are planted along the line of this fissure.
[2] Capt. C. E. Dutton. _Sixth Ann. Rep. U.S. Geol. Survey_, 1884-85.
[3] Dutton, _loc. cit._, chap. iv. p. 165.
[4] _Amer. Jour. Science_, vol. 3., ser. (1871). A beautiful map of this
mountain is given in the _Fifth Annual Report, U.S. Geol. Survey_,
1883-84. Plate 44.
[5] Daubeny, _loc. cit._, p. 474.
[6] Gilbert, _Monograph U.S. Geol. Survey_, vol. i. (1890).
[7] Powell, _Exploration of the Colorado River_, p. 177, etc. (1875).
Hayden, _Rep. U.S. Geol. Survey of the Colorado, etc._ (1871-80).
[8] Richthofen, _Natural System of Volcanic Rocks_, Mem. California
Acad. Sciences, vol. i. (1868).
[9] Geikie, _Geological Sketches at Home and Abroad_, p. 271 (1882).
[10] Prestwich, _Geology_, vol. i. p. 370, quoting from Richthofen.
[11] The origin of geysers is variously explained; see Prestwich,
_Geology_, vol. i. p. 170. They are probably due to heated waters
suddenly converted into steam by contact with rock at a high
temperature.
CHAPTER III.
VOLCANOES OF NEW ZEALAND.
One other region of volcanic action remains to be noticed before passing
on to the consideration of those of less recent age. New Zealand is an
island wherein seem to be concentrated all the phenomena of volcanic
action of past and present time. Though it is doubtful if the term
"active," in its full sense, can be applied to any of the existing
craters (with two or three exceptions, such as Tongariro and Whakari
Island), we find craters and cones in great numbers in perfectly fresh
condition, extensive sheets of trachytic and basaltic lavas, ashes, and
agglomerates; lava-floods descending from the ruptured craters of ashes
and scoriae; old crater-basins converted into lakes; geysers, hot springs
and fumaroles which may be counted by hundreds, and cataracts breaking
over barriers of siliceous sinter; and, lastly, lofty volcanic mountains
vying in magnitude with Vesuvius and Etna. All these wonderful
exhibitions of moribund volcanic action seem to be concentrated in the
northern island of Auckland. The southern island, which is the larger,
also has its natural attractions, but they are of a different kind;
chief of all is the grand range of mountains called, not
inappropriately, the "Southern Alps," vying with its European
representative in the loftiness of its peaks and the splendour of its
snowfields and glaciers, but formed of more ancient and solid rocks than
those of the northern island.
(_a._) _Auckland District._--We are indebted to several naturalists for
our knowledge of the volcanic regions of New Zealand, but chiefly to
Ferdinand von Hochstetter, whose beautiful maps and graphic descriptions
leave nothing to be desired.[1] In this work Hochstetter was assisted by
Julius Haast and Sir J. Hector. From their account we learn that the
Isthmus of Auckland is one of the most remarkable volcanic districts in
the world. It is characterised by a large number of extinct
cinder-cones, in a greater or less perfect state of preservation, and
giving origin to lava-streams which have poured down the sides of the
hills on to the plains. Besides these are others formed of stratified
tuff, with interior craters, surrounding in mural cliffs eruptive cones
of scoriae, ashes, and lapilli; these cones are scattered over the
isthmus and shores of Waitemata and Manukau. The tuff cones and craters
rise from a floor of Tertiary sandstone and shale, the horizontal strata
of which are laid open in the precipitous bluffs of Waitemata and
Manukau harbours; they sometimes contain fossil shells of the genera
_Pecten_, _Nucula_, _Cardium_, _Turbo_, and _Neritae_. As the volcanic
tuff-beds are intermingled with the Upper Tertiary strata, it is
inferred that the first outbursts of volcanic forces occurred when the
region was still beneath the waters of the ocean. Cross-sections show
that the different layers slope both outwards (parallel to the sides)
and inwards towards the bottom of the craters. Sometimes these craters
have been converted into lakes, as in the case of those of the Eifel;
but generally they are dry or have a floor of morass. Of the
crater-lakes, those of Kohuora, five in number, are perhaps the most
remarkable; and in the case of two of these the central cones of slag
appear as islets rising from the surface of the waters. The fresh-water
lake Pupuka has a depth of twenty-eight fathoms. To the north of
Auckland Harbour rises out of the waters of the Hauraki Gulf the cone of
Rangitoto, 920 feet high, the flanks formed of rugged streams of basalt,
and the summit crowned by a circular crater of slag and ash, out of the
centre of which rises a second cone with the vent of eruption. This is
the largest and newest of the Auckland volcanoes, and appears to have
been built up by successive outpourings of basaltic lava from the
central orifice, after the general elevation of the island.
[Illustration: Fig. 26.--Forms of volcanic tuff cones, with their
cross-sections, in the Province of Auckland.--No. 1. Simple tuff cone
with central crater; No. 2. Outer tuff cone with interior cinder cone
and crater; No. 3. The same with lava-stream issuing from the interior
cone.--(After Hochstetter.)]
Before leaving the description of the tuff-cones, which are a peculiar
feature in the volcanic phenomena of New Zealand, and are of many forms
and varieties, we must refer to that of Mount Wellington (Maunga Rei).
This is a compound volcano, in which the oldest and smallest of the
group is a tuff-crater-cone, exhibiting very beautifully the outward
slope of its beds. Within this crater arise two cones of cinders, each
with small craters. It would appear that after a long interval the
larger of the two principal cones, formed of cinders and known as Mount
Wellington, burst forth from the southern margin of the older tuff-cone,
and, being built up to a height of 850 feet, gradually overspread the
sides of its older neighbour. Mount Wellington itself has three craters,
and from these large streams of basaltic lava have issued forth in a
westerly direction, while a branch entered and partially filled the old
tuff-crater to the northwards.
Southwards from Manukau Harbour, and extending a short distance from
the coast-line to Taranaki Point, there occurs a plateau of
basalt-conglomerate (_Basaltkonglomerat_), with sheets of basaltic lava
overspreading the Tertiary strata. These plateau-basalts are intersected
by eruptive masses in the form of dykes, but still there are no craters
or cones of eruption to be seen; so that we may infer that the sheets,
at least, were extruded from fissures in the manner of those of the
Colorado or Idaho regions of America. Proceeding still further south
into the interior of the island, we here find a lofty plateau of an
average elevation of 2,000 feet, interposed between the Tertiary beds of
the Upper and Middle Waikato, and formed of trachytic and pitch-stone
tuff, amongst which arise old extinct volcanic cones, such as those of
Karioi, Pirongia, Kakepuku, Maunga Tautari, Aroha, and many others.
These trachytic lavas would seem to be more ancient than the basaltic,
previously described.
(_b._) _Taupo Lake, and surrounding district._--But of all these
volcanic districts, none is more remarkable than that surrounding the
Taupo Lake, which lies amidst the Tertiary strata of the Upper Waikato
Basin. The surface of this lake is 1,250 feet above that of the ocean,
and its margin is enclosed within a border of rhyolite and
pitchstone--rising into a mass of the same material 1,800 feet high on
the eastern side. The form of the lake does not suggest that it is
itself the crater of a volcano, but rather that it was originated by
subsidence. On all sides, however, trachytic cones arise, of which the
most remarkable group lies to the south of the lake, just in front of
the two giant trachytic cones, the loftiest in New Zealand, one called
Tongariro, rising about 6,500 feet, and the other Ruapahu, which attains
an elevation of over 9,000 feet, with the summit capped by snow. These
two lofty cones, standing side by side, are supposed by the Maoris to be
the husband and wife to whom were born the group of smaller cones above
referred to as occupying the southern shore of Taupo Lake. The volcano
of Tongariro may still be considered as in a state of activity, as its
two craters (Ngauruhoe and Ketetahi) constantly emit steam, and several
solfataras break out on its flanks.[2]
(_c._) _Roto Mahana._--In a northerly direction from Tongariro, and
distant from the coast by a few miles, lies in the Bay of Plenty the
second of the active volcanoes of New Zealand, the volcanic island of
Whakari (White Island), from the crater of which are constantly erupted
vast masses of steam clouds. The distance between these two active
craters is 120 nautical miles; and along the tract joining them
steam-jets and geysers issue forth from the deep fissures through which
the lava sheets have formerly been extruded. Numerous lakes also occupy
the larger cavities in the ground; and hot-springs, steam-fumaroles and
solfataras burst out in great numbers along the banks of the Roto Mahana
Lake and the Kaiwaka River by which it is drained. Amongst such
eruptions of hot-water and steam we might expect the formation of
siliceous sinter, and the deposition of sulphur and other minerals; nor
will our expectations be disappointed. For here we have the wonderful
terraces of siliceous sinter deposited by the waters entering Roto
Mahana as they descend from the numerous hot-springs or pools near its
margin. All travellers concur in describing these terraces as the most
wonderful of all the wonders of the Lake district of New Zealand--so
great is their extent, and so rich and varied is their colouring.
The beautiful map of Roto Mahana on an enlarged scale by Hochstetter
shows no fewer than ten large sinter terraces descending towards the
margin of this lake, besides several mud-springs, fumaroles, and
solfataras. But the largest and most celebrated of all the sinter
terraces has within the last few years been buried from view beneath a
flood of volcanic trass, or mud, an event which was as unexpected as it
was unwelcome. In May, 1887, the mountain of Tarawera, which rises to
the north-east of Roto Mahana, and on the line of eruption above
described, suddenly burst forth into violent activity, covering the
country for miles around with clouds of ashes, and, pouring down
torrents of mud, completely enveloped the beautiful terrace of sinter
which had previously been one of the wonders of New Zealand. By the same
eruption several human beings were entombed, and their residences
destroyed.
The waters of Roto Mahana, together with the hot-springs and fountains
are fed from rain, and from the waters of Taupo Lake, which, sinking
through fissures in the ground, come in contact with the interior heated
matter, and thus steam at high temperature and pressure is generated.[3]
(_d._) _Moribund condition of New Zealand Volcanoes._--From what has
been said, it will be inferred that in the case of New Zealand, as in
those of Auvergne, the Eifel and Lower Rhine, Arabia, and Western
America, we have an example of a region wherein the volcanic forces are
well-nigh spent, but in which they were in a state of extraordinary
activity throughout the later Tertiary, down to the commencement of the
present epoch. In most of these cases the secondary phenomena of
vulcanicity are abundantly manifest; but the great exhibitions of
igneous action, when the plains were devastated by sheets of lava, and
cones and craters were piled up through hundreds and thousands of feet,
have for the present, at least, passed away.
[1] _Geol.-topographischer Atlas von Neu-Seeland_, von Dr. Ferd. von
Hochstetter und Dr. A. Petermann. Gotha: Justus Perthes (1863). Also
_New Zealand_, trans. by E. Sauter, Stuttgart (1867).
[2] Tongariro was visited in 1851 by Mr. H. Dyson, who describes the
eruption of steam.
[3] Mr. Froude figures and describes the two terraces, the "White" and
"Pink," in _Oceana_, 2nd edition, pp. 285-291.
PART IV.
TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES.
CHAPTER I.
ANTRIM.
It is an easy transition to pass from the consideration of European and
other dormant, or extinct, volcanic regions to those of the British
Isles, though the volcanic forces may have become in this latter
instance quiescent for a somewhat longer period. In all the cases we
have been considering, whether those of Central Italy, of the Rhine and
Moselle, of Auvergne, or of Syria and Arabia, the cones and craters of
eruption are generally present entire, or but slightly modified in form
and size by the effects of time. But in the case of the Tertiary
volcanic districts of the British Isles this is not so. On the contrary,
these more prominent features of vulcanicity over the surface of the
ground have been removed by the agents of denudation, and our
observations are confined to the phenomena presented by extensive sheets
of lava and beds of ash, or the stumps and necks of former vents of
eruption, together with dykes of trap by which the plateau-lavas are
everywhere traversed or intersected.
The volcanic region of the British Isles extends at intervals from the
North-east of Ireland through the Island of Mull and adjoining districts
on the mainland of Morvern and Ardnamurchan into the Isle of Skye, and
comprises several smaller islets; the whole being included in the
general name of the Inner Hebrides. It is doubtful if the volcanic lavas
of Co. Antrim were ever physically connected with those of the west of
Scotland, though they may be considered as contemporary with them; and
in all cases the existing tracts of volcanic rock are mere fragments of
those originally formed by the extrusion of lavas from vents of
eruption. In addition to these, there are large areas of volcanic rock
overspread by the waters of the ocean.
(_a._) _Geological Age._--The British volcanic eruptions now under
consideration are all later than the Cretaceous period. Throughout
Antrim, and in parts of Mull, the lavas are found resting on highly
eroded faces either of the Upper Chalk (Fig. 27), or, where it has been
altogether denuded away, on still older Mesozoic strata. From the
relations of the basaltic sheets of Antrim to the Upper Chalk, it is
clear that the latter formation, after its deposition beneath the waters
of the Cretaceous seas, was elevated into dry land and exposed to a long
period of subaerial erosion before the first sheets of lava invaded the
surface of the ground. We are, therefore, tolerably safe in considering
the first eruptions to belong to the Tertiary period; but the evidence,
derived as it is exclusively from plant remains, is somewhat conflicting
as to the precise epoch to which the lavas and beds of tuff containing
the plant-remains are to be referred. The probabilities appear to be
that they are of Miocene age; and if so, the trachytic lavas, which in
Antrim are older than those containing plants, may be referred to a
still earlier epoch--namely, that of the Eocene.[1] As plant remains are
not very distinctive, the question regarding the exact time of the first
volcanic eruptions will probably remain for ever undecided; but we are
not likely to be much in error if we consider the entire volcanic period
to range from the close of the Eocene to that of the Miocene; by far the
greater mass of the volcanic rocks being referable to the latter epoch.
In describing the British volcanic districts it will be most convenient
to deal with them in three divisions--viz., those of Antrim, Mull, and
Skye, commencing with Antrim.[2]
(_b._) _Volcanic Area._--The great sheets of basalt and other volcanic
products of the North-east of Ireland overspread almost the whole of the
County Antrim, and adjoining districts of Londonderry and Tyrone,
breaking off in a fine mural escarpment along the northern shore of
Belfast Lough and the sea coast throughout the whole of its range from
Larne Harbour to Lough Foyle; the only direction in which these features
subside into the general level of the country being around the shores of
Lough Neagh. Several outliers of the volcanic sheets are to be found at
intervals around the great central plateau; such as those of Rathlin
Island, Island Magee, and Scrabo Hill in Co. Down. The area of the
basaltic plateau may be roughly estimated at 2,000 square miles.
[Illustration: Fig. 27.--"The White Rocks," Portrush, Co. Antrim,
showing the plateau-basalt resting on an eroded surface of the Upper
Chalk, with bands of flint.--(From a photograph.)]
The truncated edges of this marginal escarpment rising to levels of
1,000 to 1,260 feet, as in the case of Benevenagh in Co. Derry, and
1,825 feet at Mullaghmore, attest an originally greatly more extended
range of the basaltic sheets; and it is not improbable that at the close
of the Miocene epoch they extended right across the present estuary of
Lough Foyle to the flanks of the mountains of Inishowen in Donegal in
one direction, and to those of Slieve Croob in the other. In the
direction of Scotland the promontories of Kintyre and Islay doubtless
formed a part of the original margin. Throughout this vast area the
volcanic lavas rest on an exceedingly varied rocky floor, both as
regards composition and geological age. (See Fig. 28.) Throughout the
central, southern, eastern, and northern parts of their extent, the
Chalk formation may be considered to form this floor; but in the
direction of Armagh and Tyrone, towards the southwestern margin, the
basaltic sheets are found resting indiscriminately on Silurian,
Carboniferous, and Triassic strata. The general relations of the
plateau-basalts to the underlying formations show, that at the close of
the Cretaceous period there had been considerable terrestrial
disturbances and great subaerial denudation, resulting in some cases in
the complete destruction of the whole of the Cretaceous strata, before
the lava floods were poured out; owing to which, these latter are found
resting on formations of older date than the Cretaceous.[3]
[Illustration: Fig. 28.--Section across the volcanic plateau of Antrim,
from the Highlands of Inishowen, Co. Donegal, on the N.W., to Belfast
Lough on the S.E., to show the relations of the volcanic rocks to the
older formations.--B. Basaltic sheets breaking off in high escarpments;
T. Trachyte porphyry of Tardree mountain rising from below the newer
plateau-basalts; C. Upper Chalk with flints; N.R. New Red marl and
sandstone (Trias); M. Metamorphic beds of quartzite, various schists and
crystalline limestone; F. Large fault.]
[1] Mr. J. Starkie Gardner, from a recent comparison of the
plant-remains of Antrim and Mull, concludes that "that they might belong
to any age between the beginning and the end of the warmer Eocene
period; and that they cannot be of earlier, and are unlikely to be of
later, date."--_Trans. Palaeont. Soc._, vol. xxxvii. (1883).
[2] Having dealt with this district rather fully in _The Physical
Geology and Geography of Ireland_ (Edit. 1891, p. 81), and also in my
Presidential Address (Section C.) at the meeting of the British
Association, 1874, a brief review of the subject will be sufficient
here, the reader being referred to the former treatises for fuller
details. The following should also be consulted: Gen. Portlock, _Geology
of Londonderry and Tyrone_ (1843); Sir A. Geikie, "History of Volcanic
Action during the Tertiary Period in the British Isles," _Trans. Roy.
Soc. Edinburgh_, 1888; and the _Descriptive Memoirs_ of the Geological
Survey relating to this tract of country.
[3] Owing to the superposition of the basaltic masses on beds of chalk
throughout a long line of coast, we are presented with the curious
spectacle of the whitest rocks in nature overlain by the blackest, as
may be seen in the cliffs at Larne, Glenarm, Kinbane and Portrush. (See
Fig. 27.)
CHAPTER II.
SUCCESSION OF VOLCANIC ERUPTIONS.
(_c._) _First Stage._--The earliest eruptions of lava in the North-east
of Ireland belonged to the highly acid varieties, consisting of
quartz-trachyte with tridymite.[1] This rock rises to the surface at
Tardree and Brown Dod hills and Templepatrick. It consists of a
light-greyish felsitic paste enclosing grains of smoke-quartz, crystals
of sanidine, plagioclase and biotite, with a little magnetite and
apatite. It is a rock of peculiar interest from the fact that it is
almost unique in the British Islands, and has its petrological
counterpart rather amongst the volcanic hills of the Siebengebirge than
elsewhere. It is generally consolidated with the columnar structure.
[Illustration: Fig. 29.--Part of the section shown in the quarry at
Templepatrick, showing the superposition of the basalt (_d_) to the
trachyte (_b_), with the intervening bed of flint gravel (_c_). All
these rocks are seen to rest upon an eroded surface of the Chalk
formation (_a_).]
The trachyte appears to have been extruded from one or more vents in a
viscous condition, the principal vent being probably situated under
Tardree mountain, where the rock occurs in greatest mass, and it
probably arose as a dome-shaped mass, with a somewhat extended margin,
above the floor of Chalk which formed the surface of the ground.[2]
(Fig. 27.) At Templepatrick the columnar trachyte may be observed
resting on the Chalk, or upon a layer of flint gravel interposed between
the two rocks, and which has been thrust out of position by a later
intrusion of basalt coming in from the side.[3] It is to be observed,
however, that the trachytic lavas nowhere appear cropping out along with
the sheets of basalt around the escarpments overlooking the sea, or
inland; showing that they did not spread very far from their vents of
eruption; a fact illustrating the lower viscosity, or fluidity, of the
acid lavas as compared with those of the basic type.
(_d._) _Second Stage._--After an interval, probably of long duration, a
second eruption of volcanic matter took place over the entire area; but
now the acid lavas of the first stage are replaced by basic lavas. Now,
for the first time, vast masses of basalt and dolerite are extruded both
from vents of eruption and fissures; and, owing to their extreme
viscosity, spread themselves far and wide until they reach the margin of
some uprising ground of old Palaeozoic or Metamorphic rocks by which the
volcanic plain is almost surrounded. The great lava sheets thus produced
are generally more or less amorphous, vesicular and amygdaloidal, often
exhibiting the globular concentric structure, and weathering rapidly to
a kind of ferruginous sand or clay under the influence of the
atmosphere. Successive extrusions of these lavas produce successive
beds, which are piled one over the other in some places to a depth of
600 feet; and at the close of the stage, when the volcanic forces had
for the time exhausted themselves, the whole of the North-east of
Ireland must have presented an aspect not unlike that of one of those
great tracts of similar lava in the region of Idaho and the Snake River
in Western America, described in a previous chapter.
(_e._) _Third Stage (Inter-volcanic)._--The third stage may be described
as inter-volcanic. Owing to the formation of a basin, probably not deep,
and with gently sloping sides, a large lake was formed over the centre
of the area above described. Its floor was basalt, and the streams from
the surrounding uplands carried down leaves and stems of trees, strewing
them over its bed. Occasionally eruptions of ash took place from small
vents, forming the ash-beds with plants found at Ballypallidy, Glenarm,
and along the coast as at Carrick-a-raide. The streams also brought down
sand and gravel from the uprising domes of trachyte, and deposited them
over the lake-bed along with the erupted ashes.[4] The epoch we are now
referring to was one of economic importance; as, towards its close,
there was an extensive deposition of pisolitic iron-ore over the floor
of the lake, sometimes to the depth of two or three feet. This ore has
been extensively worked in recent years.
[Illustration: Fig. 30.--Cliff section above the Giant's Causeway, coast
of Co. Antrim, showing successive tiers of basaltic lava, with
intervening bands of bole.]
(_f._) _Fourth Stage (Volcanic)._--The last stage described was brought
to a termination by a second outburst of basic lavas on a scale probably
even grander than the preceding. These lavas consisting of basalt and
dolerite, with their varieties, and extruded from vents and fissures,
spread themselves in all directions over the pre-existing lake deposits
or the older sheets of augitic lava, and probably entirely buried the
trachytic hills. These later sheets solidified into more solid masses
than those of the second stage. They form successive terraces with
columnar structure, each terrace differing from that above and below it
in the size and length of the columns, and separated by thin bands of
"bole" (decomposed lava), often reddish in colour, clearly defining the
limits of the successive lava-flows. Nowhere throughout the entire
volcanic area are these successive terraces so finely laid open to view
as along the north coast of Antrim, where the lofty mural cliffs, worn
back into successive bays with intervening headlands by the
irresistible force of the Atlantic waves, present to the spectator a
vertical section from 300 to 400 feet in height, in which the successive
tiers of columnar basalt, separated by thin bands of bole, are seen to
rise one above the other from the water's edge to the summit of the
cliff, as shown in Fig. 30. Here, also, at the western extremity of the
line of cliffs we find that remarkable group of vertical basaltic
columns, stretching from the base of the cliff into the Atlantic, and
known far and wide by the name of "The Giant's Causeway," the upper ends
of the columns forming a tolerably level surface, gently sloping
seawards, and having very much the aspect of an artificial tesselated
pavement on a huge scale. A portion of the Causeway, with the cliff in
the background, is shown in the figure (Fig. 31). The columns are
remarkable for their symmetry, being generally hexagonal, though
occasionally they are pentagons, and each column is horizontally
traversed by joints of the ball-and-socket form, thus dividing them into
distinct courses of natural masonry. These are very well shown in the
accompanying view of the remarkable basaltic pillars known as "The
Chimneys," which stand up from the margin of the headland adjoining the
Causeway, monuments of past denudation, as they originally formed
individuals amongst the group belonging to one of the terraces in the
adjoining coast.[5] (Fig. 32).
[Illustration: Fig. 31.--The Giant's Causeway, formed of basaltic
columns in a vertical position, and of pentagonal or hexagonal section;
above the Causeway is seen a portion of the cliff composed of tiers of
lava with intervening bands of bole, etc.--(From a photograph.)]
[Illustration: Fig. 32.--"The Chimneys," columns of basalt on slope of
cliff overlooking the Atlantic, north coast of Co. Antrim. The
horizontal segments, or cup-and-ball joints, of the columns are well
shown in this figure. (From a photograph.)]
(_g._) _Original Thickness of the Antrim Lavas._--It is impossible to
determine with certainty what may have been the original thickness of
the accumulated sheets of basic lavas with their associated beds of ash
and bole. The greatest known thickness of the lower zone of lavas is, as
I have already stated, about 600 feet. The intermediate beds of ash and
bole sometimes attain a thickness of 40 feet, and the upper group of
basalt about 400 feet; these together would constitute a series of over
1,000 feet in thickness. But this amount, great as it is, is undoubtedly
below the original maximum, as the uppermost sheets have been removed by
denuding agencies, we know not to what extent. Nor is it of any great
importance. Sufficient remains to enable us to form a just conception of
the magnitude both as regards thickness and extent of the erupted matter
of the Miocene period over the North-east of Ireland and adjoining
submerged tracts, and of the magnitude of the volcanic operations
necessary for the production of such masses.
(_h._) _Volcanic Necks._--As already remarked, no craters of eruption
survive throughout the volcanic region of the North-east of Ireland,
owing to the enormous extent of the denudation which this region has
undergone since the Miocene Epoch; but the old "necks" of such
craters--in other words, the pipes filled with either solid basalt, or
basalt and ashes--are still to be found at intervals over the whole
area. Owing to the greater solidity of the lava which filled up these
"necks" over the plateau-basaltic sheets which surround them, they
appear as bosses or hills rising above the general level of the ground.
One of these bosses of highly columnar basalt occurs between Portrush
and Bushmills, not far from Dunluce Castle, another at Scawt Hill, near
Glenarm, and a third at Carmoney Hill above Belfast Lough. But by far
the most prominent of these old solidified vents of eruption is that of
Sleamish, a conspicuous mountain which rises above the general level of
the plateau near Ballymena, and attains an elevation of 1,437 feet above
the sea. Seen from the west, the mountain has the appearance of a
round-topped cone; but on examination it is found to be in reality a
huge dyke, breaking off abruptly towards the north-west, in which
direction it reaches its greatest height, then sloping downwards towards
the east. This form suggests that Sleamish is in reality one of the
fissure-vents of eruption rather than the neck of an old volcano. The
rock of which it is formed consists of exceedingly massive,
coarsely-crystalline dolerite, rich in olivine, and divided into large
quadrangular blocks by parallel joint planes. Its junction with the
plateau-basalt from which it rises can nowhere be seen; but at the
nearest point where the two rocks are traceable the plateau-basalt
appears to be somewhat indurated; breaking with a splintery fracture and
a sharp ring under the hammer, suggesting that the lava of Sleamish had
been extruded through the horizontal sheets, and had considerably
indurated the portions in contact with, or in proximity to, it.[6]
Amongst the vents filled with ash and agglomerate, the most remarkable
is that of Carrick-a-raide, near Ballycastle. It forms this rocky island
and a portion of the adjoining coast, where the beds of ash are finely
displayed; consisting of fragments and bombs of basalt, with pieces of
chalk, flint, and peperino, which is irregularly bedded. These ash-beds
attain a thickness of about 120 feet just below the road to Ballycastle,
but rapidly tail out in both directions from the locality of the vent.
Just below the ash-beds, the white chalk with flints may be seen
extending down into the sea-bed. Nowhere in Antrim is there such a
display of volcanic ash and agglomerate as at this spot.[7]
(_i._) _Dykes: Conditions under which they were Erupted._--No one can
visit the geological sections in Co. Antrim and the adjoining districts
of Down, Armagh, Derry, and Tyrone, without being struck by the great
number and variety of the igneous dykes by which the rocks are
traversed. The great majority of these dykes are basaltic, and they are
found traversing all the formations, including the Cretaceous and
Tertiary basaltic sheets. The Carlingford and Mourne Mountains are
seamed with such dykes, and they are splendidly laid open to view along
the coast south of Newcastle in Co. Down, as also along the Antrim coast
from Belfast to Larne. The fine old castle of Carrickfergus has its
foundations on one of those dyke-like intrusions, but one of greater
size than ordinary. All the dykes here referred to are not, however, of
the same age, as is conclusively proved by sections amongst the Mourne
Mountains where cliffs of Lower Silurian strata, superimposed on the
intrusive granite of the district, exhibit two sets of basaltic
dykes--one (the older) abruptly terminated at the granite margin, the
other and newer penetrating the granite and Silurian rocks alike. It is
not improbable that the older dykes belong to the Carboniferous or
Permian age, while the newer are with equal probability of Tertiary age.
Sir A. Geikie has shown that the Tertiary dykes of the North of Ireland
are representatives of others occurring at intervals over the North of
England, and Central and Western Scotland, all pointing towards the
central region of volcanic activity; or in a parallel direction thereto,
approximating to the N.W. in Ireland, the Island of Islay, and East
Argyleshire, but in the centre of Scotland generally ranging from east
to west.[8] The area affected by the dykes of undoubted Tertiary age
Geikie estimates at no less than 40,000 square miles--a territory
greater than either Scotland or Ireland, and equal to more than a third
of the total land-surface of the British Isles;[9] and he regards them
as posterior "to the rest of the geological structures of the regions
which they traverse." It is clear that the dykes referred to belong to
one great system of eruption or intrusion; and they may be regarded as
the manifestation of the final effort of internal forces over this
region of the British Isles. They testify to the existence of a
continuous _magma_ (or shell) of augitic lava beneath the crust; and as
the aggregate horizontal extent of all these dykes, or of the fissures
which they fill, must be very considerable, it is clear that the crust
through which they have been extruded has received an accession of
horizontal space, and has been fissured by forces acting from beneath,
as the late Mr. Hopkins, of Cambridge, had explained on mechanical
grounds in his elaborate essay many years ago.[10] This view occurred to
myself when examining the region of the North-east of Ireland, but I was
not then aware that it had been dealt with on mathematical principles by
so eminent a mathematician. The bulging of the crust is a necessary
consequence of the absence of plication of the strata due to the
extrusion of this enormous quantity of molten lava; and the intrusion of
thousands of dykes over the North-east of Ireland, unaccompanied by
foldings of the strata, must have added a horizontal space of several
thousand feet to that region.[11]
[1] A peculiar form of crystalline quartz first recognized in this rock
by a distinguished German petrologist, the late Prof. A. von Lasaulx,
who visited the district in 1876.
[2] Sir A. Geikie has disputed the correctness of the view, which I
advocated as far back as 1874, that the trachytic lavas of Antrim are
the earliest products of volcanic action; but at the time he wrote his
paper on the volcanic history of these islands, it was not known that
pebbles of this trachyte are largely distributed amongst the ash-beds
which occur in the very midst of the overlying basaltic sheets, as I
shall have to explain later on. This discovery puts the question at rest
as regards the relations of the two sets of rocks.
[3] This remarkable section at the chalk quarries of Templepatrick the
author has figured and described in the _Physical Geology and Geography
of Ireland_, p. 99, 2nd edit. (1891), where the reader will find the
subject discussed more fully than can be done here.
[4] These pebbles were first noticed by Mr. McHenry, of the Irish
Geological Survey, in 1890.
[5] The vertical position of the columns of the Giant's Causeway is
rather enigmatical. The Causeway cannot be a dyke, as has often been
supposed, otherwise the columns would have been horizontal, _i.e._, at
right angles to the sides of the dyke. Mr. R. G. Symes, of the
Geological Survey, has suggested that the Causeway columns have been
vertically lowered between two lines of fault, and that originally they
formed a portion of the tier of beautiful columns seen in the cliff
above, and known as "The Organ."
[6] Sleamish and several other of the Antrim vents are described by Sir
A. Geikie in the monograph already referred to, _loc. cit._, p. 101, _et
seq._ Also in the _Expl. Memoirs of the Geological Survey of Ireland_.
[7] A diagrammatised section of the Carrick-a-raide volcanic neck is
given by Sir A. Geikie, _loc. cit._, p. 105.
[8] Geikie, _loc. cit._, p. 29, _et seq._
[9] P. 32. The view that the crust of the earth has been horizontally
extended by the intrusion of dykes is noticed by McCulloch in reference
to the dykes of Skye.
[10] Hopkins, _Cambridge Phil. Trans._, vol. vi. p. 1 (1836).
[11] As suggested in my Presidential Address to Section C. of the
British Association at Belfast, 1874.
CHAPTER III.
ISLAND OF MULL AND ADJOINING COAST.
The Island of Mull, with the adjoining districts of Morvern and
Ardnamurchan, forms the more southern of the two chief centres of
Tertiary volcanic eruptions in the West of Scotland, that of Skye being
the more northern. These districts have been the subject of critical and
detailed study by several geologists, from McCulloch down to the present
day; and amongst the more recent, Sir Archibald Geikie and Professor
Judd hold the chief place. Unfortunately, the interpretation of the
volcanic phenomena by these two accomplished observers has led them to
very different conclusions as regards several important points in the
volcanic history of these groups of islands; as, for example, regarding
the relative ages of the plateau-basalts and the acid rocks, such as the
trachytes and granophyres; again as regards the presence of distinct
centres of eruption; and also as regards the relations of the gabbros of
Skye to the basaltic sheets. Such being the case, it would appear the
height of rashness on the part of the writer, especially in the absence
of a detailed examination of the sections over the whole region, to
venture on a statement of opinion regarding the points at issue; and he
must, therefore, content himself with a brief account of the phenomena
as gathered from a perusal of the writings of these and other
observers,[1] guided also to some extent by the analogous phenomena
presented by the volcanic region of the North-east of Ireland.
(_a._) _General Features._--As in the case of the Antrim district, the
Island of Mull and adjoining tracts present us with the spectacle of a
vast accumulation of basaltic lava-flows, piled layer upon layer, with
intervening beds of bole and tuff, up to a thickness, according to
Geikie, of about 3,500 feet. At the grand headland of Gribon, on the
west coast, the basaltic sheets are seen to rise in one sheer sweep to a
height of 1,600 feet, and then to stretch away with a slight easterly
dip under Ben More at a distance of some eight miles. This mountain, the
upper part of which is formed of beds of ashes, reaches an elevation of
3,169 feet, so that the accumulated thickness of the beds of basalt
under the higher part of the mountain must be at least equal to the
amount stated above--that is, twice as great as the representative
masses of Antrim. The base of the volcanic series is seen at Carsaig and
Gribon to rest on Cretaceous and Jurassic rocks, like those of Antrim;
hence the Tertiary age is fully established by the evidence of
superposition. This was further confirmed by the discovery by the Duke
of Argyll,[2] some years ago (1850), of bands of flint-gravel and tuff,
with dicotyledonous leaves amongst the basalts of Ardtun Head. The
basement beds of tuff and gravel contain, besides pebbles of flint and
chalk, others of sanidine trachyte, showing that highly acid lavas had
been extruded and consolidated before the first eruption of the
plateau-basalts; another point of analogy between the volcanic
phenomenon of Antrim and the Inner Hebrides. These great sheets of
augitic lava extend over the whole of the northern tract of Mull, the
Isles of Ulva and Staffa, and for a distance of several miles inwards
from the northern shore of the Sound of Mull, covering the wild
moorlands of Morvern and Ardnamurchan, where they terminate in
escarpments and outlying masses, indicating an originally much more
extended range than at the present day. The summits of Ben More and its
neighbouring height, Ben Buy, are formed of beds of ash and tuff. The
volcanic plateau is, according to Judd, abruptly terminated along the
southern side by a large vault, bringing the basalt in contact with
Palaeozoic rocks.[3]
(_b._) _Granophyres._--The greater part of the tract lying to the south
of Loch na Keal, which almost divides Mull into two islands, and
extending southwards and eastwards to the shores of the Firth of Lorn
and the Sound of Mull, is formed of a peculiar group of acid (or highly
silicated) rocks, classed under the general term of "Granophyres." These
rocks approach towards true granites in one direction, and through
quartz-porphyry and felsite to rhyolite in another--probably depending
upon the conditions of cooling and consolidation. In their mode of
weathering and general appearance on a large scale, they present a
marked contrast to the basic lavas with which they are in contact from
the coast of L. na Keal to that of L. Buy. The nature of this contact,
whether indicating the priority of the granophyres to the
plateau-basalts or otherwise, is a matter of dispute between the two
observers above named; but the circumstantial account given by Sir A.
Geikie,[4] accompanied by drawings of special sections showing this
contact, appears to prove that the granophyre is the newer of the two
masses of volcanic rock, and that it has been intruded amongst the
basaltic-lavas at a late period in the volcanic history of these
islands. A copy of one of these sketches is here given (Fig. 33),
according to which the felsite is shown to penetrate the basaltic sheets
at Alt na Searmoin in Mull; other sections seen at Cruach Torr an
Lochain, and on the south side of Beinn Fada, appear to lead to similar
conclusions. These rocks are penetrated by numerous basaltic dykes.
[Illustration: Fig. 33.--Section at Alt na Searmoin, Mull, to show the
intrusion of felsite (or granophyre) (_b_) into basalt and dolerite
(_a_) of the plateau-basalt series.--(Geikie.)]
(_c._) _Representative Rocks of Mourne and Carlingford,
Ireland._--Assuming Sir A. Geikie's view to be correct, it is possible
that we may have in the granite and quartz-porphyries of Mourne and
Carlingford representatives of the granites, granophyres, and other acid
rocks of the later period of Mull. The granite of Mourne is peculiar in
structure, and differs from the ordinary type of that rock in which the
silica forms the ground mass. In the case of the granite of the Mourne
Mountains, the rock consists of a crystalline granular aggregate of
orthoclase, albite, smoke-quartz, and mica; it is also full of drusy
cavities, in which the various minerals crystallise out in very perfect
form. As far as regards direct evidence, the age of this rock can only
be stated to be post-Carboniferous, and earlier than certain Tertiary
basaltic dykes by which it is traversed. The granophyres of Mull are
traversed by similar dykes, which are representatives of the very latest
stage of volcanic action in the British Islands. The author is therefore
inclined to concur with Sir A. Geikie in assigning to the granite of the
Mourne Mountains, and the representative felsitic rocks of the
Carlingford Mountains, a Tertiary age--in which case the analogy between
the volcanic phenomena of the Inner Hebrides and of the North-east of
Ireland would seem to be complete.[5]