Mr. Hull, “Physical Geography and Geology of Ireland,” p. 68, under the
head of “Volcanic Necks and Basaltic Dykes,” says that “although the
actual craters and cones of eruption have been swept from the surface
of the country by the ruthless hand of time, yet the old “necks” by
which the volcanic mouths were connected with the sources of eruption
can occasionally be recognized; they sometimes appear as masses of hard
trap, columnar or otherwise, projecting in knolls or hills above the
upper surface of the sheets through which they pierce.”
In other cases, the “neck” consists of a great pipe choked up by bombs
and blocks of trap, more or less consolidated, bombs which have been
shot into the air and have fallen back again. He then refers to one
of these near Portrush, and proceeds to state that the rock on which
stands the ruined Castle of Dunluce, “is formed of bombs of all sizes
up to six feet in diameter, of various kinds of basalt, dolerite, and
amygdaloid firmly cemented, and presenting a precipitous face to the
sea.”
In a note dated September, 1877, Mr. Hull states that subsequent
examination, since the above was written, of the rock of Dunluce Castle
and the cliffs adjoining, has led him “to suspect that we have here,
instead of old volcanic necks, simply pipes, formed by the filtration
out of the chalk into which the basaltic masses have fallen and slipped
down, thus giving rise to their fragmental appearance.”
Further on (page 146) he describes without any sceptical comment, “the
remarkable mass of agglomerate made up (as on the southern flanks of
Slieve Gullion) of bombs of granite, which have been torn up from the
granite mass of the hills below, and blown through the throat of an old
crater.” Other geologists still adhere firmly to the bomb theory, some
ascribing the bombs to subaqueous rather than subaerial ejection.
Immediately under Dunluce Castle is a sea-worn cavern or tunnel, which
is about 40 or 50 feet high at its mouth, affording a fine section of
this curious conglomerate. The floor of the cavern which slopes upwards
from the sea is strewn with a beach of boulders. The resemblance
of this beach to those I had recently examined at the foot of the
boulder-clay cliffs of Galway Bay (and described in a paper read to the
British Association), suggested the explanation of the origin of the
rock I am about to offer.
In shape and size they are exactly like the Galway shore boulders,
those nearest the sea being the most rounded; higher up the slope,
where less exposed to wave action, they are subangular. They differ
from the Galway boulders in being chiefly basaltic instead of being
mainly composed of carboniferous limestone. Some of these at Dunluce
are granitic, and a few, if I am not greatly mistaken, are of
carboniferous limestone. I had not at hand the means of positively
deciding this.
Neither could I find any unquestionable examples of glacial striation
among them, though at the upper part I saw some lines on boulders that
were very suggestive of partially obliterated scratches.
On looking at the cavern walls surrounding me the theory so obviously
suggested by the boulders on the floor was strikingly confirmed by
their structure and general appearance. The imbedded “bombs” are
subangular, and of irregular shape and varying composition, and the
matrix of the rock is a brick-like material just such as would be
formed by the baking of boulder clay; the inference that I was looking
upon a bank or deposit of glacier drift that had been baked by volcanic
agency was irresistible.
I was unable to see on any part of the extensive section, or among the
fragments below, a single specimen of an unequivocal volcanic bomb;
no approach to anything like those described by Sir Samuel Baker in
his “Nile Tributaries of Abyssinia,” the miniature representatives
of which, ejected from the Bessemer converter, I have figured and
described in _Nature_, vol. 3, pp. 389 and 410, where Sir Samuel
Baker’s description is quoted.
I have witnessed the fall of masses of lava during a minor eruption of
an inner crater of Mount Vesuvius. These as they fell upon the ground
around me were flattened out into thin cakes. There was no approach
to the formation of subangular masses, like those displayed upon the
Dunluce cavern walls.
Some years ago a project for melting the basaltic rock known as “Rowley
Rag,” and casting it into moulds for architectural purposes was carried
out near Oldbury, and I had an opportunity of watching the experiment,
which was conducted on a large scale at great expense by Messrs. Chance.
It was found that if the basalt cooled rapidly it became a black
obsidian, and to prevent the formation of such brittle material, the
castings, and the moulds, which enclosed them, had to be kept at a
red-heat for some days, and very gradually cooled.[29]
It is physically impossible that lava ejected under water, in lumps no
larger than these boulders, could have the granular structure which
they display.
The fundamental idea upon which this bomb theory is based will not
bear examination. Such bombs could not have been shot into either air
or water and have fallen back again into the volcanic neck at any
other time than during an actual eruption; and at such time they could
not have remained where they fell, and have become embedded in any
such matrix as now contains them. True volcanic bombs and ordinary
spattering lumps of lava, are, as we know, flung obliquely out of
active craters, and distributed around, while those which are ejected
perpendicularly into the air and return are re-ejected, and finally
pulverized into volcanic dust if this perpendicular ejection and return
are continued long enough.
In the course of a rapid drive round the Antrim coast I observed other
examples of this peculiar conglomerate, and have reason to believe that
it is far more common than is generally supposed. I found it remarkably
well displayed at a place almost as largely visited as the Giant’s
Causeway, and where it nevertheless appears to have been hitherto
unnoticed, viz., Carrick-a-Rede, where the public car stops to afford
visitors an opportunity of examining or crossing the rope bridge, etc.
Here the whole formation is displayed in a manner that strikingly
illustrates my theory.
There is an overlying stream of basalt forming the surface of
the isolated rock, and this basalt rests directly upon a base of
conglomerate, having exactly the appearance that would result from the
slow baking of a mass of boulder clay.
The sea gully that separates the insular rock from the mainland
displays a fine section above eighty feet in thickness, and has the
advantage of full daylight as compared with Dunluce Cave. That this is
no mere neck or pipe is evident from its extent. Its position below the
basalt cap refutes the above quoted subsequent explanation, which Mr.
Hull and others have recently adopted.
The heterogeneous bomb-like character of the boulders is not so
strongly marked as in the Dunluce rock, and this may arise from the
closer proximity of the basalt, which, coming here in direct contact,
would be likely to heat the clay matrix (itself formed mainly of
ice-ground basalt) to incipient fusion, and thereby render it more like
the basalt boulders it contains than the other clay that had been less
intensely heated on account of greater distance from the lava-flow.
The path leading to the ladder by which the bridge is approached passes
over such conglomerate, and further extensions are seen in sections
around. I saw sufficient in the course of my hurried visit to indicate
the existence of a large area of this particular formation.
At a short distance from Carrick-a-Rede, on the way to Ballycastle, the
car passes in sight of considerable deposits of ordinary boulder clay
uncovered and unaltered.
The blocks of basalt, etc., embedded in this correspond in general size
and shape with the “bombs,” excepting that some of the latter have a
laminated, or shaly, character near their surfaces.
I regret my inability to do justice to this subject in consequence
of the fact that the above explanation of the origin of this curious
formation only suggested itself when hurrying homeward after a somewhat
protracted visit to Ireland. As I may not have an opportunity of
further investigation for some time to come, I offer the hypothesis in
this crude form in order that it may be discussed, and either confirmed
or refuted by the geologists of the Ordnance Survey, or others who have
better opportunities of observation than I can possibly command.
Should this conglomerate prove to be, as I suppose, a drift deposit
altered by a subsequent flow of lava, it will supply exceedingly
interesting data for the determination of the chronological relations
of the glacial epoch to that period of volcanic activity to which the
lavas of the N.E. of Ireland are due. Though it will nowise disturb
the general conclusion that the great eruptions that overspread the
cretaceous rocks of this region, and supplied the boulders of my
supposed metamorphosed drift, occurred during the Miocene period, it
will show that this volcanic epoch was of vastly greater duration than
is usually supposed; or that there must have been two or more volcanic
epochs—pre-glacial, as usually understood, and post-glacial, in order
to supply the lava overflowing the drift.
This post-glacial extension of the volcanic period has an especial
interest in Ireland, as the “Annals of the Four Masters,” and other
records of ancient Irish history and tradition, abound in accounts of
physical changes, many of which correspond remarkably with those of
recent occurrence in the neighborhood of active and extinct volcanoes.
In a paper read before the Royal Irish Academy, June 23, 1873, and
published in its “Proceedings,” Dr. Sigerson has collected some of the
best authenticated of these accounts, and compares them with similar
phenomena recently observed in Naples, Sicily, South America, Siberia,
etc. etc. The “great sobriety of diction, and circumstantial precision
of statement,” of names, dates, etc., which characterize these accounts
render them well worthy of the sort of comparison with strictly
scientific data which Dr. Sigerson has made.
As we now know that man existed in Britain during the inter-glacial, if
not the pre-glacial period, and as so violent a volcanic disturbance as
that which poured out the lavas of Antrim and the Mourne district could
scarcely have subsided suddenly, but was probably followed by ages of
declining activity, it is not at all surprising that this period of
minor activity should have extended into that of tradition and the
earliest of historical records.